WO2016038327A1

WO2016038327A1 - Apparatus and methods

Info

Publication number: WO2016038327A1
Application number: PCT/GB2015/050853
Authority: WO
Inventors: Daniel ELFORD; Luke Chalmers; Richard Wilson
Original assignee: SONOBEX Ltd
Current assignee: SONOBEX Ltd
Priority date: 2014-09-08
Filing date: 2015-03-23
Publication date: 2016-03-17
Anticipated expiration: 2017-03-08
Also published as: GB201415873D0

Abstract

The invention provides apparatus comprising: transformer apparatus which emits acoustic waves, the transformer apparatus comprising a transformer; and one or more acoustic attenuators provided in an acoustic wave propagation path of the said acoustic waves emitted by the transformer apparatus, each of the said one or more acoustic attenuators comprising a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates at least a portion of the said acoustic waves emitted by the transformer apparatus.

Description

APPARATUS AND METHODS Field of the invention The invention relates to apparatus comprising a transformer and one or more acoustic attenuators; apparatus comprising a transformer and a transformer cooling system; a transformer; and methods of attenuating acoustic waves emitted by a transformer. Background to the invention Power transformers are used in the transmission and distribution of AC electrical power to step electrical voltages both up and down. Such power transformers are often found in electrical substations, which may be situated in either rural environments or near residential areas as required for efficient power distribution to homes and businesses. Noise generated by such power transformers, particularly what is often described as a low-frequency hum, can be disturbing and even distressing to any nearby residents. Such noise pollution is accordingly regulated under environmental legislation in many countries. Acoustic waves emitted by transformers with frequencies between 20 Hz and 250 Hz lie in an audible range found particularly irritating by humans. Beyond psychological or subjective complaints, such low-frequency vibrations can also cause known physiological reactions in humans, including effects on blood pressure, heart rate, the prevalence of headaches and other issues such as sleep disturbance. It is, therefore, important to minimise the impact of noise pollution generated by power transformers. There are several known mechanisms which lead to the production of noise in an electrical transformer substation. An initial source of noise is magnetostriction within the transformer's core, caused by the repetitive expansion and contraction of the ferromagnetic steel core within a constantly changing magnetic field. A second source of noise is termed load noise, caused primarily by the electromagnetic force on the current in the transformer windings by a direct interaction with the leakage magnetic flux generated by said current. Load noise is also generated by the same leakage flux inducing vibrations in the metallic transformer housing. Transformers generally require cooling and may comprise air, water or oil-based cooling systems. All such cooling systems typically generate noise (from vibrations of the fans in an air-based system, for example). Such vibrations may be transferred to the transformer housing, which can even reach a state of mechanical resonance unless the structure is modified to control this behaviour. Current steps taken to reduce the noise output by electrical transformers (such as the use of higher permeability steel cores, the reduction in air gaps, or the use of slower fans) also tend to increase system costs or reduce efficiency. Such steps are not particularly effective at reducing noise levels and so the use of acoustic barriers or enclosures has become necessary to comply with environmental legislation. Known acoustic barriers suitable for use with transformers generally comprise passive elements such as concrete walls. While useful for blocking higher frequencies of sound, such barriers are unable to effectively prevent escape of low- frequency vibrations from the transformer housing into the surrounding environment, because the acoustic wavelengths become longer than the typical barrier thicknesses and so are not absorbed or attenuated. A new way of attenuating noise generated by transformers is, therefore, required. Summary of the invention A first aspect of the invention provides apparatus comprising: transformer apparatus which emits acoustic (typically sound) waves (in use), the transformer apparatus comprising a transformer; and one or more acoustic attenuators provided in an acoustic wave propagation path of the said acoustic (typically sound) waves emitted by the transformer apparatus (in use), each of the said one or more acoustic attenuators comprising a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates at least a portion of the said acoustic waves emitted by the transformer apparatus (in use). By "transformer", we include any type of (typically electrical) transformer, including power transformers, distribution transformers, generator step-up transformers, step- down transformers, system intertie transformers, industrial transformers, phase shifting transformers, reactor transformers, reactors, shunt reactors, current limiting reactors, neutral earthing reactors, capacitor damping reactors, tuning (filter) reactors, earthing transformers (neutral couplers), arc suppression reactors, smoothing reactors, traction transformers and similar. The transformer may for example be provided as part of an electrical substation. The transformer is typically a high or medium voltage transformer configured to convert a voltage to or from a voltage greater than mains voltage (e.g. to or from a voltage exceeding 120V or 230V). Typically the transformer of the transformer apparatus emits acoustic waves (in use). Typically one or more acoustic attenuators are provided in an acoustic wave propagation path of the said acoustic waves emitted by the transformer (in use). Typically the body of each of the one or more attenuators is configured such that the acoustic attenuator attenuates at least a portion of the acoustic waves emitted by the transformer (in use). Additionally or alternatively it may be that the transformer apparatus comprises one or more inverters (and/or other associated components and/or circuitry) which emit at least a portion of the said acoustic waves attenuated by the attenuator(s). It may be that the transformer amplifies said acoustic waves emitted by the said inverters (and/or other associated components and/or circuitry), e.g. prior to attenuation of the amplified acoustic waves by the attenuator(s). The acoustic waves (typically sound waves) emitted by the transformer apparatus are typically considered to be unwanted acoustic noise. For example, the most troublesome acoustic noise generated by transformers in normal use is typically low frequency acoustic noise (e.g. acoustic noise in the frequency range 20Hz to 500Hz). Acoustic attenuators of this type are particularly suitable for attenuating such low frequency acoustic waves. Typically, the apparatus comprises one or more acoustic attenuators provided in each of a plurality of acoustic wave propagation paths of acoustic waves emitted by the transformer apparatus. Typically the one or more acoustic attenuators are provided within a radius of 100 metres of the transformer apparatus, more preferably within a radius of 50 metres of the transformer apparatus, even more preferably within a radius of 10 metres of the transformer apparatus, and most preferably within a radius of 1 metre of the transformer apparatus. Typically the attenuation of acoustic waves is more effective the closer the acoustic attenuators are to the transformer apparatus. Typically the cavities of the (first) bodies of the one or more acoustic attenuators are in fluid communication with the transformer apparatus. Typically the one or more acoustic attenuators comprises a plurality of acoustic attenuators, each of the said plurality of acoustic attenuators comprising a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the (first) body being configured such that the acoustic attenuator attenuates at least a portion of the said acoustic waves emitted by the transformer apparatus. Typically the cavities of the (first) bodies of each of the said plurality of acoustic attenuators are in fluid communication with the transformer apparatus (e.g. through the apertures of the said attenuators). Typically the (first) bodies of the plurality of acoustic attenuators are discrete from each other. It may be that a plurality of the said attenuators are arranged together (e.g. periodically, e.g. in a row) to form an acoustic barrier. The acoustic barrier may comprise a single layer of the said attenuators, or multiple layers of said attenuators (e.g. first and second rows may be provided, the second row being provided downstream of the first row with respect to acoustic waves emitted by the transformer apparatus, the first and second rows being arranged (e.g. periodically) so as to attenuate acoustic waves over a further (e.g. resonant) frequency band). It may be that a plurality of the said acoustic attenuators are arranged together to form an enclosure. The enclosure may comprise a single layer of the said attenuators. The enclosure formed by the said attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. Typically the (first) body of each of the said one or more acoustic attenuators is configured to attenuate acoustic (typically sound) waves emitted by the transformer apparatus (in use) of one or more acoustic frequencies. Typically the body of each of the said one or more acoustic attenuators is configured to attenuate acoustic (typically sound) waves emitted by the transformer apparatus (in use) having acoustic frequencies across an acoustic frequency band. The acoustic attenuator(s) (e.g. the (first) bodies of the one or more acoustic attenuators) are typically provided in a fluidic host medium. For example, the fluidic host medium may comprise air. However, the fluidic host medium may comprise any suitable gas or liquid or mixture of gas and liquid. In some embodiments, one or more (or each of the) acoustic attenuators are monolithic. For example, one or more (or each of the) acoustic attenuators may be formed monolithically from a single sheet of material (e.g. from a single sheet of metal). Typically the (first) body of each of the acoustic attenuators comprises an outer surface and an inner surface opposite the outer surface, the inner surface defining the cavity. Typically the open aperture extends through the (first) body to fluidly connect the outer surface of the (first) body and the cavity. Fluid can typically flow into and out of the cavity through the open aperture without obstruction. Typically the (first) body of each of the said one or more (or each of the said plurality of) acoustic attenuators is elongate. It may be that the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators is substantially cylindrical. For example it may be that the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators comprises a (typically hollow) cylinder. It may be that the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators is tubular. It may be that the cavity defined by the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators is (at least substantially) cylindrical. Typically the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators has a first end and a second end (which is typically opposite the first end). It may be that the first end is open such that air can flow into and out of the cavity through the first end (typically without obstruction). It may be that the first end is closed (e.g. such that air cannot flow into or out of the cavity through the first end). It may be that the second end is open such that air can flow into and out of the cavity through the second end (typically without obstruction). It may be that the second end is closed (e.g. such that air cannot flow into or out of the cavity through the second end). Typically the open aperture of the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators is elongate. Typically the said open aperture has a longitudinal axis which extends parallel to a longitudinal axis of the acoustic attenuator. Typically the said open aperture extends along at least a portion of (e.g. at least 30% of, at least 40% of, at least 50% of, at least 90% of) the, or along the entire, length of the (first) body (parallel to the longitudinal axis). It may be that the aperture does not extend to the first and/or second ends of the (first) body. In other cases, the aperture extends to the first and second ends of the (first) body. The open aperture may be provided in any orientation. Typically, however, the open aperture is provided facing towards, or at least with a direct line of sight with, at least part of the transformer apparatus which emits acoustic waves. It may be that a plurality of discrete (typically elongate) open apertures is provided along the length of the (first) body of each of the one or more (or each of the said plurality of) acoustic attenuators, each of the open apertures (typically comprising the said open aperture) being in fluid communication with the cavity. Said plurality of apertures may comprise a combined aperture length extending along, for example, at least 30% of, at least 40% of, at least 50% of or at least 90% of the length of the (first) body (parallel to the longitudinal axis of the (first) body). The apertures of the said plurality of apertures are typically spaced from each other, for example by (solid) portions of the (first) body. It may be that a line extending in a direction parallel to a longitudinal axis of the (first) body extends across each of the said plurality of open apertures. It may be that the apertures of the said plurality of open apertures are aligned with each other along the length of the (first) body. Typically the said plurality of apertures together form an elongate aperture area. In some embodiments, the (first) bodies of one or more (or each) of the acoustic attenuators define resonant frequency bands across which they attenuate acoustic waves. The resonant frequency band of each said acoustic attenuator is typically at least partly defined by the open aperture and the cavity of that (first) body. Typically the resonant frequency band at least partly defined by the cavity and the open aperture is a band of frequencies of acoustic waves which stimulate resonance of the fluidic host medium (e.g. air) provided in the cavity of that body. That is, incident acoustic waves having frequencies within the resonant frequency band cause the fluid (e.g. air) within the cavity to resonate at that frequency. Accordingly, acoustic energy from the incident acoustic waves is transferred to the air within the cavity, and the acoustic waves are thereby attenuated. The resonant frequencies of the said (first) bodies are typically dependent on their size and shape. The (first) bodies of each of the said acoustic attenuators typically comprise one or more walls. The walls typically have thicknesses. The resonant frequencies of the cavities of the said acoustic attenuators are typically dependent on the thicknesses of the walls of the (first) bodies. The resonant frequency of each said (first) body is typically dependent on the width and length of its open aperture. The resonant frequency of the said cavity is typically dependent on its volume. It may be that the (first) body of one or more (or each) of the attenuator(s) further comprises a neck. Typically the neck extends from the edge(s) of the open aperture into and/or away from the cavity. It may be that the said resonant frequency band of the said attenuator is at least partly defined by the length of the neck. It may be that each of the plurality of acoustic attenuators is provided with substantially the same resonant frequency band (e.g. at least 50%, preferably at least 80% of each resonant frequency band is common to the other resonant frequency bands). Alternatively, it may be that a first group of the said plurality of acoustic attenuators is provided with a first resonant frequency band and a second group of the said plurality of acoustic attenuators is provided with a second resonant frequency band different from the first resonant frequency band. Alternatively, each of the plurality of (first) bodies is provided with a different resonant frequency band. The resonant frequency bands of one or more or each of the said attenuators typically comprise one or more frequencies of acoustic wave emitted by the said transformer apparatus along the said acoustic propagation path. It may be that one or more (or each) of the said acoustic attenuators comprises: opposing first and second walls, the second wall being substantially parallel to the first wall, the said body of the or each of the said acoustic attenuators comprising at least one of the first and second walls, wherein the aperture and the cavity of the said body at least partly define a resonant frequency band across which the said body attenuates incident acoustic waves, and wherein the first and second walls are separated by a gap, wherein the transformer apparatus emits acoustic waves having a frequency within the said resonant frequency band, and acoustic waves which are (typically multiply) scattered by the first and second walls, the said scattered waves (typically destructively) interfering with each other such that the said incident acoustic waves are thereby attenuated. For example, it may be that the first and second walls are parallel to each other, and that the transformer apparatus emits acoustic waves having a frequency and being incident on the first and second walls with an angle of incidence satisfying the Bragg condition defined by the gap between the first and second walls. The first and second walls, and the gap extending between them, thus provide a finite one dimensional sonic crystal to incident acoustic waves having particular angles of incidence and frequencies (e.g. where the first and second walls are parallel, acoustic waves having angles of incidence and frequencies satisfying the Bragg condition defined by the gap between them). Typically one of the first and second walls is downstream from the other of the first and second walls with respect to the said acoustic waves emitted by the transformer apparatus. By providing the acoustic attenuator with a body having a cavity and an open aperture at least partly defining a resonant frequency band across which the attenuator attenuates acoustic waves, and providing a gap between the first and second walls (such that incident acoustic waves (typically multiply) scattered by the first and second walls (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated), two different mechanisms for acoustic attenuation (from the point of view of an observer on the opposite side of the acoustic attenuator from the part of transformer apparatus emitting the acoustic waves) are provided by the same acoustic attenuator. Synergy is achieved by virtue of the fact that the same (first) body defines the cavity and comprises the aperture and at least one of the first and second walls (and in some cases both the first and second walls - see below). The provision of first and second (substantially parallel) walls in the acoustic attenuator therefore removes the need to have a plurality of rows of acoustic attenuators in order to achieve the sonic crystal attenuation effect. This allows a single layer of acoustic attenuators to be provided which achieves both a local resonance-based acoustic wave attenuation effect and a one dimensional sonic crystal attenuation effect of incident acoustic waves. It may be that the frequencies of acoustic waves attenuated by the two mechanisms are the same, but more typically the frequencies of acoustic waves attenuated by the two mechanisms are different. Nevertheless, it may be that there is some overlap between the said resonant frequency band and the frequencies of acoustic waves which are scattered by the first and second walls such that they (typically destructively) interfere with each other and are thereby attenuated. Accordingly, the acoustic attenuator can provide stronger acoustic attenuation (where the frequencies of acoustic waves attenuated by the two mechanisms are the same, or where there is some overlap between the resonant frequency band and the frequencies of acoustic waves which are scattered by the first and second walls such that they (typically destructively) interfere with each other and are thereby attenuated) of acoustic waves of a given frequency, or attenuation of acoustic waves of different frequencies (where the frequencies of acoustic waves attenuated by the two mechanisms are different). This provides the acoustic attenuator with greater flexibility, allowing better performance to be achieved. It may be that two (or three) or more of the said plurality of attenuators is arranged in a row and that one or more of the said plurality of attenuators of the row are provided with opposing first and second walls which are spaced apart by a first distance and one or more attenuators of the row are provided with opposing first and second walls which are spaced apart by a second distance different from the first distance, the first and second distances defining different frequencies at (or frequency bands across) which the first and second walls of those attenuators scatter incident acoustic waves (e.g. emitted by the transformer apparatus) such that the said scattered acoustic waves interfere with each other and are thereby attenuated. It may be that first and second rows are provided, each comprising two or more of the said plurality of attenuators, the second row being provided downstream from the first row with respect to said acoustic waves emitted by the transformer apparatus. It may be that one or more (or each) of the attenuators of the first row are provided with opposing first and second walls which are spaced apart by a first distance and one or more (or each) of the attenuators of the second row are provided with opposing first and second walls which are spaced apart by a second distance different from the first distance, the first and second distances defining different frequencies at (or frequency bands across) which the first and second walls of those attenuators scatter incident acoustic waves (e.g. emitted by the transformer apparatus) such that the said scattered acoustic waves interfere with each other and are thereby attenuated. It will be understood that the Bragg condition (in respect of the one dimensional sonic crystal effect provided by the first and second walls), which applies when the first and second walls are parallel to each other, is: ηλ = 2d sin(a) where: n is an integer or a half-integer;

λ is the wavelength of the acoustic waves;

d is the shortest distance between the first and second walls;

and a is the angle of incidence of the incident acoustic waves on the first and second walls. It will also be understood that the gap between the first and second walls defines the Bragg condition by way of parameter d. It will be understood that, although the first and second walls are typically required to be parallel to each other for the Bragg condition to be satisfied, significant attenuation effects are still achieved when the first and second walls are not quite parallel to each other. It may be that a transversal line extending between the first and second walls intersects the first and second walls with corresponding angles between the said transversal and the respective first and second walls differing from each other by 20° or less. Preferably, the corresponding angles between the said transversal and the respective first and second walls differ from each other by 10° or less, more preferably by 5° or less, more preferably by 2.5° or less, more preferably by or less, even more preferably the corresponding angles between the said transversal and the respective first and second walls are the same. Typically the sonic crystal attenuation effect (multiple scattering, interference and resultant attenuation of incident acoustic waves) provided by the first and second walls (and the said gap between them) attenuates incident acoustic waves over a band of frequencies and harmonics and sub-harmonics of the said band of frequencies. Typically the gap between the first and second walls is sized such that acoustic waves having a frequency within the range 20Hz to 20kHz can have an angle of incidence on the first and second walls such that said acoustic waves which are (typically multiply) scattered by the first and second walls can (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated. Typically the gap between the first and second walls is sized such that acoustic waves having a frequency within the range 20Hz to 1 kHz can have an angle of incidence on the first and second walls such that said acoustic waves which are (typically multiply) scattered by the first and second walls can (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated. Typically the gap between the first and second walls is sized such that acoustic waves having a frequency within the range 20Hz to 500Hz can have an angle of incidence on the first and second walls such that said acoustic waves which are (typically multiply) scattered by the first and second walls can (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated. Typically the first and second walls are (substantially) planar. Typically the first and second walls have widths (perpendicular to the longitudinal axis of the (first) body and to the line of shortest distance between the first and second walls) which are greater than the width (or greater than twice the width) of the open aperture. It will be understood that the first and second walls typically (multiply) scatter incident acoustic waves propagating in a direction having a component parallel to the line of shortest distance extending between the first and second walls such that said (multiply) scattered incident acoustic waves (e.g. where the first and second walls are parallel to each other, having a frequency and angle of incidence on the first and second walls satisfying the Bragg condition defined by the distance between them) (typically destructively) interfere with each other such that said incident acoustic waves are attenuated. A line of shortest distance extending between the first and second walls is typically substantially perpendicular to the first and second walls. A line of shortest distance extending between the first and second walls typically intersects the first and second walls. Typically the said body of the or each of the said acoustic attenuators comprises the first wall and the first wall comprises the open aperture. However, it will be understood that either the first or second walls may comprise the (indeed each of the first and second walls may comprise a respective) open aperture. It may be that the first wall comprises first and second co-planar wall portions separated by the open aperture. It may be that the said body of the or each of the said acoustic attenuators comprises the first and second walls. It may be that the body (and typically the cavity) of the or each of the said acoustic attenuators has a cross section perpendicular to its longitudinal axis which is trapezoidal. The (first) body (and typically the cavity) may have a cross section perpendicular to its longitudinal axis which is quadrilateral (for example it is parallelogrammatical, square or rectangular), hexagonal or octogonal. Indeed the (first) body may have a cross section perpendicular to its longitudinal axis of any other suitable shape. It may be that the shortest distance between the first and second walls of one or more or each of the said acoustic attenuators is equal to a wavelength of acoustic waves emitted by the transformer apparatus. It may be that the first and second walls are parallel to each other in this case. It may be that the shortest distance between the first and second walls of one or more or each of the said acoustic attenuators is substantially equal to an integer or a half- integer number of wavelengths of acoustic waves emitted by the transformer apparatus. It may be that the first and second walls are parallel to each other in this case. It may be that one or more or each of the said acoustic attenuator(s) comprises a second body. It may be that the second body comprises the second wall. It may be that the said second body is provided next to the first body. Typically the first and second bodies are discrete from each other. Typically the second body is provided next to the first body. For example, the first and second bodies may be arranged together in a row. Typically the first and second bodies are spaced apart from each other. For example, the first and second bodies may be arranged in a row with a gap between them. Typically the first and second bodies are provided opposite each other. Typically the first and second bodies are provided outside each other. The second body is typically elongate. Typically the second body is tubular. Typically the second body is monolithic. The second body typically defines a cavity and comprises an open aperture in fluid communication with the cavity, the aperture and cavity at least partly defining a resonant frequency band across which the second body attenuates acoustic waves. Typically the second body comprises an outer surface and an inner surface opposite the outer surface, the inner surface defining the cavity. Typically the open aperture of the second body extends through the second body to fluidly connect the outer surface of the second body and the cavity. Typically the second body is (at least substantially) hollow. Typically fluid can flow into and out of the cavity of the second body through the open aperture of the second body (typically without obstruction). The second body typically has first and second ends. It may be that the first and second ends are closed but typically the first and second ends are open. The first and second bodies may be provided with triangular cross sections perpendicular to their longitudinal axes. It may be that each of a plurality of the said acoustic attenuators comprises opposing first and second walls, the second wall being substantially parallel to the first wall, the body of the said acoustic attenuator comprising at least one of the first and second walls, wherein the aperture and the cavity at least partly define a resonant frequency band across which the said body attenuates incident acoustic waves, and wherein the first and second walls are separated by a gap, wherein the transformer apparatus emits acoustic waves having a frequency within the said resonant frequency band, and acoustic waves which are (typically multiply) scattered by the first and second walls, the said scattered waves (typically destructively) interfering with each other such that the said incident acoustic waves are thereby attenuated. For example, it may be that the first and second walls of one or more or each of the said acoustic attenuators are parallel to each other, and that the transformer apparatus emits acoustic waves having a frequency and being incident on the first and second walls with an angle of incidence satisfying the Bragg condition defined by the gap between the first and second walls. It may be that the gaps between the first and second walls of each of the plurality of (first) bodies are the same (e.g. when the first and second walls are parallel to each other, the gaps between the first and second walls of each of the plurality of (first) bodies may be sized to provide substantially the same Bragg condition). Alternatively, it may be that the gaps between the first and second walls of a first group of the said plurality of (first) bodies are different from the gaps between the first and second walls of a second group of the said plurality of (first) bodies different from the first group (e.g. when the first and second walls of each of the first group of (first) bodies are parallel to each other, the gaps between the first and second walls of each of the said first group of (first) bodies may be sized to provide a first Bragg condition, and when the first and second walls of each of the second group of (first) bodies are parallel to each other, the gaps between the first and second walls of each of the said second group of (first) bodies may be sized to provide a second Bragg condition different from the first Bragg condition). Alternatively, it may be that the gaps between the first and second walls of each of the said plurality of (first) bodies are different (e.g. where the first and second walls of each of the plurality of (first) bodies are parallel to each other, the gaps between the first and second walls of each of the plurality of (first) bodies may be sized to provide different Bragg conditions). It may be that the open aperture of a first acoustic attenuator (or a plurality of discrete open apertures of the first acoustic attenuator, e.g. discrete open apertures which are spaced from each other along the length of the (first) body of the first acoustic attenuator, typically aligned with each other along the length of the (first) body of the first acoustic attenuator) of the said plurality of acoustic attenuators faces (and is typically in fluid communication with) the open aperture of a second acoustic attenuator (or a plurality of discrete open apertures of the second acoustic attenuator, e.g. discrete open apertures which are spaced from each other along the length of the (first) body of the second acoustic attenuator, typically aligned with each other along the length of the (first) body of the second acoustic attenuator) of the said plurality of acoustic attenuators, and a gap is (or gaps are) provided between the said open apertures (and typically between the bodies of the first and second attenuators). Typically the gap is sized such that resonance of fluid within the cavity of the (first) body of the first attenuator can stimulate resonance of fluid within the cavity of the (first) body of the second attenuator (and typically vice versa), at least when the resonance occurs at a frequency within the said resonant frequency bands of both the first and second attenuators. Typically the first and second acoustic attenuators are oriented differently from each other. It may be that the bodies of the first and second attenuators are provided with at least partially overlapping resonant frequency bands, the said overlapping portions of the resonant frequency bands comprising one or more frequencies of acoustic waves emitted by the said transformer apparatus. Typically the first and second attenuators are provided next to each other (rather than, for example, one of the acoustic attenuators being provided inside the other). Typically the second acoustic attenuator is provided downstream of the first acoustic attenuator (e.g. with respect to acoustic waves emitted by the transformer apparatus). Typically the first and second acoustic attenuators are provided outside each other. Typically the first and second attenuators are provided opposite each other. It may be that one of the first and second walls of the first acoustic attenuator is spaced from one of the first and second walls of the second attenuator such that incident acoustic waves (e.g. emitted from the transformer apparatus) scattered by the said walls (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated. For example, it may be that the first wall of the first acoustic attenuator is spaced from the second wall of the second acoustic attenuator, and that the first wall of the first acoustic attenuator and the second wall of the second acoustic attenuator scatter incident acoustic waves (e.g. emitted from the transformer apparatus) such that said scattered acoustic waves (typically destructively) interfere with each other such that said incident acoustic waves are thereby attenuated. It may be that one of the first and second walls of the first acoustic attenuator is parallel to one of the first and second walls of the second acoustic attenuator, wherein the said one of the first and second walls of the first acoustic attenuator is spaced from the said one of the first and second walls of the second attenuator such that incident acoustic waves (e.g. emitted from the transformer apparatus) having a frequency and angle of incidence on the said walls satisfying a Bragg condition defined by the spacing between them are scattered by the said walls, the said scattered acoustic waves (typically destructively) interfering with each other such that said incident acoustic waves are thereby attenuated. For example, it may be that the first wall of the first acoustic attenuator is parallel to the second wall of the second acoustic attenuator, the first wall of the first acoustic attenuator being spaced from the second wall of the second acoustic attenuator, the first wall of the first acoustic attenuator and the second wall of the second acoustic attenuator scattering incident acoustic waves (e.g. emitted from the transformer apparatus) having a frequency and angle of incidence on the said walls satisfying the Bragg condition defined by the spacing between them (typically destructively) such that they interfere with each other and the said incident acoustic waves are thereby attenuated. Preferably the shortest distance between the (first) bodies of the first and second attenuators whose open apertures face each other is equal to the shortest distance between the first and second walls of the said (first) body of the first attenuator and/or equal to the shortest distance between the first and second walls of the said (first) body of the second attenuator. More generally, in some cases, the spacing between the first and second walls of the first acoustic attenuator, the spacing between the first and second walls of the second acoustic attenuator and a spacing between the first and second acoustic attenuators are the same. It may be that two or more (e.g. three or more) of the first and second walls of the first attenuator and the first and second walls of the second attenuator are arranged (e.g. periodically) so as to attenuate acoustic waves over a (e.g. resonant) frequency band comprising one or more frequencies of acoustic wave emitted by the transformer apparatus (typically by (e.g. multiply) scattering incident acoustic waves emitted by the transformer apparatus, the said scattered waves (typically destructively) interfering with each other such that the said incident acoustic waves are thereby attenuated). This improves the sonic crystal attenuation effect provided by the substantially parallel first and second walls of the (first) body of the first attenuator and/or by the substantially parallel first and second walls of the (first) body of the second attenuator. Preferably the first and second walls of the first attenuator are parallel to each other. Preferably the first and second walls of the second attenuator are parallel to each other. Preferably the first and second walls of the first attenuator are parallel to the first and second walls of the second attenuator. It may be that the gap between the first and second walls of the first acoustic attenuator, the gap between the first and second walls of the second acoustic attenuator and a gap between the first and second acoustic attenuators are equal. This improves the sonic crystal attenuation effect provided by the parallel first and second walls of the first attenuator and/or by the parallel first and second walls of the second attenuator. The smaller the gap, the better the resonance coupling effect between the acoustic attenuators. However, a gap should be maintained between the first and second attenuators (and indeed between the said open apertures) to allow incident acoustic waves (e.g. from the transformer apparatus) to enter and exit the cavities of the (first) bodies of the first and second attenuators by way of their open apertures. Preferably, the shortest distance between the (first) bodies of the first and second attenuators whose open apertures face each other is less than ten times the mean spacing between the first and second walls of the said (first) body of the first attenuator and less than ten times the mean spacing between the first and second walls of the said (first) body of the second attenuator. Preferably, the shortest distance between the (first) bodies of the first and second attenuators whose open apertures face each other is less than five times the mean spacing between the first and second walls of the said (first) body of the first attenuator and less than five times the mean spacing between the first and second walls of the said (first) body of the second attenuator. Preferably, the shortest distance between the (first) bodies of the first and second attenuators whose open apertures face each other is less than two times the mean spacing between the first and second walls of the said (first) body of the first attenuator and less than two times the mean spacing between the first and second walls of the said (first) body of the second attenuator. By the open apertures of the first and second acoustic attenuators facing each other, we mean that there is at least some overlap (preferably a complete overlap) between the open apertures of the first and second attenuators in a direction parallel to the line of shortest distance between the first and second attenuators. A line of shortest distance between a centre of the (first) body comprising a or the said open aperture of the first acoustic attenuator and a centre of the (first) body comprising the open aperture of the second acoustic attenuator facing the said open aperture of the first acoustic attenuator typically passes through both said open apertures. Thus, fluid resonating in the cavity of the (first) body comprising a or the said open aperture of the first acoustic attenuator can stimulate resonance of fluid provided in the cavity of the (first) body comprising a or the said open aperture of the second acoustic attenuator (at least when the resonance occurs at a frequency within the said resonant frequency bands of the first and second attenuators). The open apertures of other acoustic attenuators (or bodies of acoustic attenuators) facing each other should be interpreted accordingly. It may be that a plurality of pairs of first and second acoustic attenuators are provided. It may be that each of one or more open apertures of the first acoustic attenuator of each pair faces a corresponding open aperture of the second acoustic attenuator of that pair. Typically the attenuators within each pair are provided next to each other (rather than, for example, one of the attenuators being provided inside the other). Typically the attenuators within each pair are provided outside each other. Typically the attenuators within each pair are provided opposite each other. It may be that the (first) bodies within each pair are identical to each other, but oriented at 180° to each other. It may be that a plurality of the said pairs of first and second acoustic attenuators are arranged together (e.g. periodically) to form an acoustic barrier. The acoustic barrier may comprise a single layer of the said pairs of first and second acoustic attenuators. It may be that a plurality of the pairs of first and second acoustic attenuators are arranged together to form an enclosure. The enclosure may comprise a single layer of the said pairs of first and second acoustic attenuators. The enclosure formed by the said plurality of pairs of first and second acoustic attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. It may be that the said plurality of acoustic attenuators comprises a pair (or two or more pairs) of said acoustic attenuators, wherein the (typically elongate) body of each of the said attenuators of the said (or each said) pair comprises a first face and a second face, the first face comprising the said open aperture of that body, and wherein the bodies of the said attenuators of the said (or each said) pair are arranged such that their second faces are adjacent to each other and that fluid can flow into or out of the cavities of the said attenuators of the said pair through their respective open apertures (typically without the other attenuator of that pair causing an obstruction thereto). By the second faces of the attenuators of the (or each) said pair being adjacent to each other we mean that the second face of a first attenuator of the pair is provided closer to second face of the second attenuator of the pair than to the first face of the second attenuator of the pair (and vice versa). This arrangement helps to increase the number of attenuators per unit volume. Where resonance coupling is provided between adjacent attenuators, this also helps to optimise the resonance coupling effect between attenuators per unit volume, which increases the level of attenuation provided by the attenuators. This also helps to broaden the frequency range of attenuation. Typically the first and second faces of the each of the said attenuators are separated by a gap. Typically the first and second faces of the said bodies of the attenuators of the said pair are planar faces. Typically the first and second faces of the said bodies of the attenuators of the said pair are substantially parallel to each other. Typically the second faces of the attenuators of the pair abut each other. Typically the second faces of the attenuators of the pair are mechanically coupled to each other. Typically the attenuators within each pair are provided next to each other (rather than, for example, one of the attenuators being provided inside the other). Typically the attenuators within each pair are provided outside each other. Typically the attenuators within each pair are provided opposite each other. It may be that the (first) bodies within each pair are identical to each other, but oriented at 180° to each other. It will be understood that the open apertures of one or both of the attenuators of the (or each) said pair may be elongate open apertures. Where provided, the elongate open apertures may extend along at least a portion of (e.g. at least 30% of, at least 40% of, at least 50% of, at least 90% of) the, or along the entire, length of the (first) body (typically parallel to the longitudinal axis) comprising that elongate open aperture. It may be that the open apertures of one or both of the attenuators of the (or each) said pair are open apertures of respective groups of discrete open apertures (each group comprising two or more open apertures) provided on the (first) body of each said attenuator and arranged such that a line extending in a direction parallel to a longitudinal axis of the said (first) body extends across each of the apertures within the group. Typically the apertures within each group of apertures are aligned with each other along the length of the said (first) body. The said plurality of discrete open apertures within each group (where provided) may comprise a combined aperture length extending along, for example, at least 30% of, at least 40% of, at least 50% of or at least 90% of the length of the (first) body (parallel to the longitudinal axis of the (first) body). Typically the said plurality of discrete open apertures within each group (where provided) together form an elongate aperture area. It may be that the attenuators of the (or each) said pair have at least partially overlapping resonant frequency bands. It may be that the attenuators of the (or each) pair have different resonant frequency bands. It may be that the attenuators of the (or each) pair have resonant frequency bands which do not overlap. It may be that the bodies of the attenuators of the (or each) pair have the same shapes. It may be that the bodies of the attenuators of the (or each) pair are identical to each other (albeit they may be oriented differently from each other, for example at 180° to each other). It may be that the bodies of the attenuators of the (or each) pair have the same shapes but different sizes. It may be that the bodies of the attenuators of the (or each) pair have different shapes. It may be that the second face of the body of a first attenuator of the (or each) pair (typically completely) overlaps the second face of the body of the a second attenuator of that pair, and it may be that the second face of the body of the first attenuator extends beyond the second face of the body of the second attenuator. It may be that a plurality of said pairs are arranged (e.g. periodically) together in a row. It may be that the first attenuator of a first pair is provided adjacent to the second attenuator of a second pair within the said row. It may be that the first attenuator of the first pair is provided adjacent to the first attenuator of a second pair within the said row. It may be that the attenuators of the first pair each have a first resonant frequency band (and typically the bodies of the attenuators of the first pair have the same shape and typically the same size as each other). It may be that the attenuators of the second pair each have a second resonant frequency band different from the first resonant frequency bands (and typically the bodies of the attenuators of the second pair have the same shape and typically the same size as each other). It may be that the bodies of the attenuators of the second pair have different sizes and/or shapes from the attenuators of the first pair. It may be that first and second rows of said pairs of attenuators are provided. It may be that the first and second rows are first and second rows of a plurality of rows of, the said rows being arranged (e.g. periodically) so as to attenuate acoustic waves (e.g. emitted by the transformer apparatus) over a further (e.g. resonant) frequency band. It may be that a said attenuator of each pair in the second row is provided opposite (and may abut and may be mechanically coupled to and may face and may be downstream of, with respect to acoustic waves emitted by the transformer apparatus) an attenuator of a said pair of the first row. It may be that the attenuators of the first row are provided with the same or (more typically) different resonant frequency bands from those of the respective attenuators of the second row which they are provided opposite. It may be that the open apertures of each said pair of the first row face away from (e.g. at 90° to) the pair of the second row provided opposite the said pair of the first row. It may be that the open apertures of each said pair of the second row face away from (e.g. at 90° to) the pair of the first row provided opposite the said pair of the second row. It may be that the first and second rows are separated by a gap. It may be that the first faces of the bodies of the attenuators of the first row are not flush with the first faces of the bodies of the attenuators of the second row they are provided opposite. For example, the first faces of the bodies of the attenuators of the second row may be set back from the first faces of the bodies of the attenuators of the first row which they are provided opposite. Alternatively it may be that the first faces of the bodies of the attenuators of the first row are flush with the first faces of the bodies of the attenuators of the second row they are provided opposite. It may be that the second faces of the bodies of the attenuators of one or more or each said pair of bodies of one of the first and second rows abut each other, and the second faces of the attenuators of one or more or each said pair of bodies of the attenuators of the other of the first and second rows are separated by a gap. For example, it may be that the first and second faces of the bodies of one or both of the attenuators of each said pair of the second row are separated by a smaller gap than the first and second faces of the bodies of one or both of the attenuators of the said pair of the first row which they are provided opposite, and the second faces of the bodies of one or more or each said pair of attenuators of the second row are separated by a gap so that the first faces of the said bodies of the said pair of attenuators of the second row are flush with the first faces of the said bodies of the said pair of attenuators of the first row which they are provided opposite. It may be that the opposing first and second walls (where provided) of the bodies of the attenuators of one or more or each of the said pairs of the first row are separated by a first gap and that the opposing first and second walls (where provided) of the bodies of the attenuators of one or more or each of the said pairs of the second row which are provided opposite the said one or more or each of the said pairs of the first row are separated by a second gap different from the first gap. Accordingly it may be that the first and second walls of the said pairs of the first row are configured to scatter incident acoustic waves of a first frequency (or within a first frequency band) such that the said incident acoustic waves interfere with each other and are thereby attenuated. It may be that the first and second walls of the said pairs of the second row provided opposite the said pairs of the first row are configured to scatter incident acoustic waves of a second frequency (or within a second frequency band) different from the first frequency (or from the first frequency band) such that the said incident acoustic waves interfere with each other and are thereby attenuated. It may be that the frequencies at (or frequency bands across) which the first and second walls of the first and second rows attenuate acoustic waves by this mechanism partially overlap, or alternatively it may be that they do not overlap. It may be that the said plurality of acoustic attenuators comprises a first resonant coupling acoustic attenuator having a said open aperture in fluid communication with, and facing, a said open aperture of a first acoustic attenuator of the said pair, the (first) bodies of the said first resonant coupling acoustic attenuator and the said first acoustic attenuator of the said pair at least partly defining at least partially overlapping resonant frequency bands and a gap being provided between the open aperture of the said first resonant coupling acoustic attenuator and the open aperture of the said first acoustic attenuator of the said pair (and typically between the first resonant coupling acoustic attenuator and the said the said first acoustic attenuator), the gap being sized such that resonance of fluid within the cavity of the said resonant coupling acoustic attenuator can stimulate resonance of fluid within the cavity of the said first acoustic attenuator of the said pair (at least when the resonance occurs at a frequency within the said resonant frequency bands of the said cavities). It may be that the first resonant coupling attenuator is an attenuator of a second said pair of attenuators. It may be that the said plurality of acoustic attenuators further comprises a second resonant coupling acoustic attenuator having a said open aperture in fluid communication with, and facing, the said open aperture of a second acoustic attenuator of the said pair, the (first) bodies of the said second resonant coupling acoustic attenuator and the said second acoustic attenuator of the said pair at least partly defining at least partially overlapping resonant frequency bands and a gap being provided between the open aperture of the said second resonant coupling acoustic attenuator and the said open aperture of the said second acoustic attenuator of the said pair (and typically between the second resonant coupling acoustic attenuator and the said the said first acoustic attenuator), the gap being sized such that resonance of fluid within the cavity of the said second resonant coupling acoustic attenuator can stimulate resonance of fluid within the cavity of the said second acoustic attenuator of the said pair (at least when the resonance occurs at a frequency within the said resonant frequency bands of the said cavities). It may be that the second resonant coupling attenuator is an attenuator of a third said pair of attenuators. Typically the resonant frequency bands of the attenuators of the said pair of attenuators and typically the resonant frequency bands of the attenuators of the said first and second resonant coupling attenuators comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. Typically the said overlapping portions of the resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. It may be that one or more (typically a plurality) groups of attenuators, each said group comprising a said pair of attenuators and said first and second resonant coupling attenuators, are arranged together (e.g. periodically) to form an acoustic barrier. The acoustic barrier may comprise a single layer comprising one or more (typically a plurality) of the said groups. It may be that one or more (typically a plurality) of the said groups are arranged together to form an enclosure. The enclosure may comprise a single layer comprising one or more (typically a plurality) of the said groups. The enclosure may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. It may be that the (first) body of one or more or each of the said one or more acoustic attenuators comprises first and second open apertures in fluid communication with the cavity defined by the said body, the said first and second open apertures being offset from each other around the longitudinal axis of the said body (e.g. offset around the perimeter of the said body in a direction having a component perpendicular to the longitudinal axis of the said body). By providing first and second open apertures in fluid communication with the cavity defined by the said body, the said first and second open apertures being offset from each other around the longitudinal axis of the said body, two masses of fluid will resonate within the cavity (as a result of fluid being able to flow into and out of the cavity through the first and second apertures) when acoustic waves emitted by the transformer apparatus having a frequency within the resonant frequency band of the said body are incident on the attenuator, thereby significantly increasing the acoustic attenuation provided by the attenuator as compared to an attenuator having a cavity of the same volume with only one of the first and second apertures. In addition, the first open aperture can resonantly couple the said cavity of the said body to the cavity of a first adjacent ("nearest neighbour") said attenuator and the second open aperture can resonantly couple the said cavity of the said body to the cavity of a second adjacent ("nearest neighbour") said attenuator (e.g. different from the first attenuator). This helps to improve the resonance coupling effect between said attenuators per unit volume (the said cavity of the said body being resonantly coupled to the cavities of two adjacent said attenuators), which increases the level of attenuation provided. This also helps to broaden the frequency range of attenuation provided. It will be understood that one or both of the first and second open apertures may be elongate open apertures. Where provided, the elongate open apertures may extend along at least a portion of (e.g. at least 30% of, at least 40% of, at least 50% of, at least 90% of) the, or along the entire, length of the (first) body (typically parallel to the longitudinal axis). It may be that the one or both of the first and second open apertures are open apertures of respective groups of discrete open apertures (each group comprising two or more open apertures) provided on the said (first) body and arranged such that a line extending in a direction parallel to a longitudinal axis of the said (first) body extends across each of the apertures within the group. Typically the apertures within each group of apertures are aligned with each other along the length of the said (first) body. The said plurality of discrete open apertures within each group (where provided) may comprise a combined aperture length extending along, for example, at least 30% of, at least 40% of, at least 50% of or at least 90% of the length of the (first) body (parallel to the longitudinal axis of the (first) body). Typically the said plurality of discrete open apertures within each group (where provided) together form an elongate aperture area. It may be that the first and second open apertures are in fluid communication with each other (e.g. through the cavity of the said (first) body). It may be that the first and second open apertures are provided (e.g. directly) opposite each other. It may be that the first and second open apertures of the (first) body face (and are typically in fluid communication with) each other. It may be that the (first) body comprises first and second (typically planar) faces which are opposite each other, and it may be that the first face comprises the first open aperture and the second face comprises the second open aperture. It may be that the first and second open apertures are provided directly opposite each other (typically such there is at least some overlap (preferably a complete overlap) between the first and second open apertures in a direction parallel to the line of shortest distance between the first and second faces). The symmetry provided by having the first and second open apertures directly opposite each other helps to optimise the resonance (and thus acoustic attenuation) performance of the said (first) body of the attenuator. It may be that the first said open aperture is in fluid communication with, and faces, a said open aperture of a first resonant coupling acoustic attenuator of the said plurality of acoustic attenuators. It may be that the second said open aperture is in fluid communication with, and faces, a said open aperture of a second resonant coupling acoustic attenuator of the said plurality of acoustic attenuators. It may be that the (first) body of the attenuator comprising the said first and second open apertures defines a resonant frequency band which at least partially overlaps with resonant frequency bands defined by the (first) bodies of the said first and/or second resonant coupling acoustic attenuators. It may be that gaps are provided between the first and second said open apertures and the said open apertures of the first and second resonant coupling attenuators (and typically between the attenuator and the first and second resonant coupling attenuators). It may be that the gaps are sized such that resonance of fluid in the cavity defined by the (first) body of the said attenuator comprising the said first and second open apertures can stimulate resonance of fluid within the cavities of the said first and second resonant coupling attenuators (at least when the resonance occurs at a frequency within the said resonant frequency bands of the said attenuators). By providing the (first) body with first and second open apertures which resonantly couple the first cavity to the cavities of adjacent attenuators, the resonance coupling effect between attenuators per unit volume is increased, which increases the level of attenuation provided by the attenuators. This also helps to broaden the frequency range of attenuation. Typically the said overlapping portions of the resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. Typically the said attenuator comprising the said first and second open apertures is provided next to the said first and second resonant coupling attenuators (rather than any of the attenuators being inside another). Typically the said attenuator comprising the said first and second open apertures is provided opposite the said first and second resonant coupling attenuators. Typically the said attenuator comprising the said first and second open apertures and the said first and second resonant coupling attenuators are provided outside each other. It may be that one or more (typically a plurality) groups of attenuators, each said group comprising a said attenuator and said first and second resonant coupling attenuators, are arranged together (e.g. periodically, e.g. in a row) to form an acoustic barrier. The acoustic barrier may comprise a single layer comprising one or more (typically a plurality) of the said groups. It may be that first and second said rows of attenuators are provided. It may be that the attenuators within each row are arranged periodically. It may be that the first and second rows are first and second rows of a plurality of rows, the said rows being arranged (e.g. periodically) so as to attenuate acoustic waves (e.g. emitted by the transformer apparatus) over a further (e.g. resonant) frequency band. It may be that one or more or each of the said attenuators in the second row is provided opposite (and may abut and may be mechanically coupled to and may face and may be downstream of, with respect to acoustic waves emitted by the transformer apparatus) a respective attenuator of the first row. It may be that the attenuators of the first row are provided with the same or (more typically) different resonant frequency bands from those of the respective attenuators of the second row which they are provided opposite. It may be that the first and second open apertures of said attenuator of the first row face away from (e.g. at 90° to) the attenuator of the second row provided opposite the said attenuator of the first row. It may be that the first and second open apertures of each said attenuator of the second row face away from (e.g. at 90° to) the attenuator of the first row provided opposite the said attenuator of the second row. It may be that the first and second rows are separated by a gap. It may be that the first faces of the bodies of the attenuators of the first row are not flush with the first faces of the bodies of the attenuators of the second row they are provided opposite. For example, the first faces of the bodies of the attenuators of the second row may be set back from the first faces of the bodies of the first row which they are provided opposite. Alternatively it may be that the first faces of the bodies of the attenuators of the first row are flush with the first faces of the bodies of the attenuators of the second row they are provided opposite. It may be that one or more (typically a plurality) of the said groups are arranged together to form an enclosure. The enclosure may comprise a single layer comprising one or more (typically a plurality) of the said groups. The enclosure may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. It may be that the apparatus comprises an acoustic attenuator (or more than one acoustic attenuator each) having: a first body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a first resonant frequency band across which the first body attenuates acoustic waves; and a second body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a second resonant frequency band across which the second body attenuates acoustic waves, wherein the open apertures of the first and second bodies face each other (and are typically in fluid communication with each other) and the first and second resonant frequency bands at least partially overlap, and wherein the first and second resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the transformer apparatus. Typically the first and second resonant frequency bands comprise one or more of the same frequencies of acoustic waves emitted by the transformer apparatus. By the open apertures of the first and second bodies facing each other, we mean that there is at least some overlap (preferably a complete overlap) between the open apertures of the first and second bodies in a direction parallel to the line of shortest distance between the first and second attenuators. A line of shortest distance between a centre of the first body and a centre of the second body typically passes through both said open apertures. Thus, fluid resonating in the cavity of the first body can stimulate resonance of fluid provided in the cavity of the second body. Typically the first body is oriented differently from the second body. Typically the first and second bodies are provided next to each other (rather than, for example, one of the bodies being provided inside the other). Typically the second body is provided downstream of the first body (e.g. with respect to acoustic waves emitted by the transformer apparatus). Typically the first and second bodies are provided outside each other. Typically the first and second bodies are provided opposite each other. It may be that the first and second bodies are identical to each other, but oriented at 180° to each other. Typically a gap is provided between the said apertures of the first and second bodies (and typically between the first and second bodies). Typically the gap is sized such that resonance of the fluid provided within the cavity of the first body can stimulate resonance of the fluid provided within the cavity of the second body (and typically vice versa), at least when the resonance occurs at a frequency within the said resonant frequency bands of the said first and second bodies. Again, the smaller the gap, the better the resonance coupling effect between the bodies. However, a gap should be maintained between the first and second bodies (and indeed the apertures) to allow incident acoustic waves (e.g. emitted from the transformer apparatus) to enter and exit the cavities of the bodies by way of their open apertures. Typically the first and second bodies have cross sections perpendicular to their longitudinal axes. Preferably, the gap between the first and second bodies is less than ten times the longest dimension of the said cross sections of the first and/or second bodies. Preferably, the gap between the first and second bodies is less than five times the longest dimension of the said cross sections of the first and/or second bodies. Preferably, the gap between the first and second bodies is less than two times the longest dimension of the said cross sections of the first and/or second bodies. Typically the first and second resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. Typically the said overlapping portions of the first and second resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. It may be that the acoustic attenuator comprises a third body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a third resonant frequency band across which the third body attenuates acoustic waves, wherein each of the first and third bodies comprise a first face and a second face, the first face comprising the said open aperture of that body, and wherein the first and third bodies are arranged such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the first and third bodies through their respective open apertures (typically without the other body causing an obstruction thereto). Typically the third resonant frequency band comprises one or more frequencies of acoustic waves emitted by the transformer apparatus. This arrangement helps to optimise the resonance coupling effect between attenuators per unit volume, which increases the level of attenuation provided by the attenuators. This also helps to broaden the frequency range of attenuation. Typically the third body is discrete from the first and second bodies. Typically the first and second faces of each of the said first and third bodies are separated by a gap. Typically the first and second faces of the said first and third bodies are planar faces. Typically the first and second faces of the said first and third bodies are substantially parallel to each other. Typically the second faces of the said first and third bodies abut each other. Typically the second faces of the attenuators of the first and third bodies are mechanically coupled to each other. Typically the first and third bodies are provided next to each other (rather than, for example, one of the bodies being provided inside the other). Typically the first and third bodies are provided outside each other. Typically the first and third bodies are provided opposite each other. It may be that the first and third bodies are identical to each other, but oriented at 180° to each other. It will be understood that the open apertures of one or both of the first and third bodies may be elongate open apertures. Where provided, the elongate open apertures of each said body may extend along at least a portion of (e.g. at least 30% of, at least 40% of, at least 50% of, at least 90% of) the, or along the entire, length of the said body (typically parallel to the longitudinal axis). It may be that the open apertures of the first and third bodies are open apertures of respective groups of discrete open apertures (each group comprising two or more open apertures) provided on the said body and arranged such that a line extending in a direction parallel to a longitudinal axis of the said body extends across each of the apertures within the group. Typically the apertures within each group of apertures are aligned with each other along the length of the said body. The said plurality of discrete open apertures within each group (where provided) may comprise a combined aperture length extending along, for example, at least 30% of, at least 40% of, at least 50% of or at least 90% of the length of the said body (parallel to the longitudinal axis of the (first) body). Typically the said plurality of discrete open apertures within each group (where provided) together form an elongate aperture area. It may be that the first and third bodies have at least partially overlapping resonant frequency bands. It may be that the first and third bodies have different resonant frequency bands. It may be that the first and third bodies have resonant frequency bands which do not overlap. It may be that the first and third bodies have the same shapes. It may be that the first and third bodies are identical to each other (albeit they may be oriented differently from each other, for example at 180° to each other). It may be that the first and third bodies have the same shapes but different sizes. It may be that the first and third bodies have different shapes. It may be that the second face of the first body (typically completely) overlaps the second face of the third body, and it may be that the second face of the first body extends beyond the second face of the third body. It may be that the first and third bodies are first and third bodies of a first pair of first and third bodies. It may be that one or more further pairs of said first and third bodies are arranged (e.g. periodically) together with the said first pair of first and third bodies in a row. It may be that the first body of the first pair is provided adjacent to the third body of a second pair within the said row. It may be that the first body of the first pair is provided adjacent to the first body of a second pair within the said row. It may be that the second body is a first or third body of the second pair. It may be that the transformer apparatus emits acoustic waves having one or more frequencies within the resonant frequency bands of the first and/or third bodies. It may be that the acoustic attenuator is provided in an acoustic wave propagation path of the acoustic waves emitted by the transformer apparatus. It may be that the row extends perpendicularly to an acoustic wave propagation path of acoustic waves emitted by the transformer apparatus. It may be that the first and third bodies of the first pair each have a first resonant frequency band (and typically the same shape and typically the same size as each other). It may be that the first and third bodies of the second pair each have a second resonant frequency band different from the first resonant frequency bands (and typically the same shape and typically the same size as each other). It may be that the first and third bodies of the second pair have different sizes and/or shapes from the first and second bodies of the first pair. It may be that the attenuator comprises first and second rows of said pairs of first and third bodies. It may be that the second row is provided downstream of the first row with respect to acoustic waves emitted by the transformer apparatus. It may be that the pairs of first and third bodies within each of the first and second rows are arranged periodically. It may be that the first and second rows are first and second rows of a plurality of rows of said pairs of first and third bodies, the said rows being arranged (e.g. periodically) so as to attenuate acoustic waves (e.g. emitted by the transformer apparatus) over a further (e.g. resonant) frequency band. It may be that a said body of each pair in the second row is provided opposite (and may abut and may be mechanically coupled to and may face and may be downstream of, with respect to acoustic waves emitted by the transformer apparatus) a body of a said pair of the first row. It may be that the bodies of the first row are provided with the same or (more typically) different resonant frequency bands from those of the respective bodies of the second row which they are provided opposite. It may be that the open apertures of each said pair of the first row face away from (e.g. at 90° to) the pair of the second row provided opposite the said pair of the first row. It may be that the open apertures of each said pair of the second row face away from (e.g. at 90° to) the pair of the first row provided opposite the said pair of the second row. It may be that the first and second rows are separated by a gap. It may be that the first faces of the bodies of the first row are not flush with the first faces of the bodies of the second row they are provided opposite. For example, the first faces of the bodies of the second row may be set back from the first faces of the bodies of the first row which they are provided opposite. Alternatively it may be that the first faces of the bodies of the first row are flush with the first faces of the bodies of the second row they are provided opposite. It may be that the second faces of the bodies of one or more or each said pair of bodies of the first row abut each other, while the second faces of one or more or each said pair of bodies of the second row are separated by a gap. For example, it may be that the first and second faces of the bodies of one or both bodies of each said pair of the second row are separated by a smaller gap than the first and second faces of one or both of the bodies of the said pair of the first row which they are provided opposite, and the second faces of one or more or each said pair of bodies of the second row are separated by a gap so that the first faces of the said bodies of the said pair of the second row are flush with the first faces of the said bodies of the said pair of the first row which they are provided opposite. By providing the first faces of the bodies of the first row flush with the first faces of the second row, the first and second rows are more easily incorporated into an acoustic barrier or enclosure. It may be that the acoustic attenuator comprises a fourth body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a fourth resonant frequency band across which the fourth body attenuates acoustic waves, wherein the said open aperture of the fourth body is in fluid communication with, and faces, the said open aperture of the third body, the third and fourth resonant frequency bands at least partially overlapping, and a gap being provided between the said open apertures of the third and fourth bodies (and typically between the third and fourth bodies), the gap being sized such that resonance of fluid within the cavity of the third body can stimulate resonance of fluid within the cavity of the fourth body (at least when the resonance occurs at a frequency within the said resonant frequency bands of the said third and fourth bodies). It may be that the fourth body is a first or third body of a third pair of first and third bodies. Typically the fourth resonant frequency band comprises one or more frequencies of acoustic waves emitted by the transformer apparatus. Typically the fourth body is discrete from the first, second and third bodies. Typically the third and fourth bodies are provided next to each other (rather than, for example, one of the bodies being provided inside the other). Typically the third and fourth bodies are provided outside each other. Typically the third and fourth bodies are provided opposite each other. It may be that the third and fourth bodies are identical to each other, but oriented at 180° to each other. Typically the said overlapping portions of the third and fourth resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. It may be that the acoustic attenuator comprises a fifth body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a fifth resonant frequency band across which the fifth body attenuates acoustic waves. It may be that the fifth body is the other of the first and third bodies of the said second pair of first and third bodies. Typically the fifth resonant frequency band comprises one or more frequencies of acoustic waves emitted by the transformer apparatus. It may be that the second and fifth bodies each comprise a first face and a second face, the first face comprising the said open aperture of that body, and wherein the second and fifth bodies are arranged such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the second and fifth bodies through their respective open apertures (typically without the other body causing an obstruction thereto). It may be that the acoustic attenuator comprises a sixth body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a sixth resonant frequency band across which the sixth body attenuates acoustic waves. It may be that the sixth body is the other of the first and third bodies of the said third pair of first and third bodies. Typically the sixth resonant frequency band comprises one or more frequencies of acoustic waves emitted by the transformer apparatus. It may be that the fourth and sixth bodies each comprise a first face and a second face, the first face comprising the said open aperture of that body, and wherein the fourth and sixth bodies are arranged such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the fourth and sixth bodies through their respective open apertures (typically without the other body causing an obstruction thereto). It may be that a plurality of the said attenuators are arranged together (e.g. periodically) to form an acoustic barrier. The acoustic barrier may comprise a single layer of the said attenuators. It may be that a plurality of the said acoustic attenuators are arranged together to form an enclosure. The enclosure may comprise a single layer of the said attenuators. The enclosure formed by the said attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. It may be that the said open aperture of the first body is the first of first and second open apertures of the first body which are in fluid communication with the cavity of the first body, the said first and second open apertures of the first body being offset from each other around the longitudinal axis of the first body (e.g. offset around the perimeter of the first body in a direction having a component perpendicular to the longitudinal axis of the said first body). By providing first and second open apertures in fluid communication with the cavity defined by the first body, the said first and second open apertures being offset from each other around the longitudinal axis of the first body, two masses of fluid will resonate within the cavity (as a result of fluid being able to flow into and out of the cavity through the first and second apertures) when acoustic waves emitted by the transformer apparatus having a frequency within the resonant frequency band of the first body are incident on the first body, thereby significantly increasing the acoustic attenuation provided by the first body as compared to (first) body having a cavity of the same volume with only one of the first and second apertures. In addition, the first open aperture can resonantly couple the said cavity of the said body to the cavity of a first adjacent ("nearest neighbour") said body and the second open aperture can resonantly couple the said cavity of the said body to the cavity of a second adjacent ("nearest neighbour") said body (e.g. different from the first adjacent body). This helps to improve the resonance coupling effect between said bodies per unit volume (the said cavity of the said body being resonantly coupled to the cavities of two adjacent said bodies), which increases the level of attenuation provided. This also helps to broaden the frequency range of attenuation provided. It will be understood that one or both of the first and second open apertures may be elongate open apertures. Where provided, the elongate open apertures may extend along at least a portion of (e.g. at least 30% of, at least 40% of, at least 50% of, at least 90% of) the, or along the entire, length of the (first) body (typically parallel to the longitudinal axis). It may be that the one or both of the first and second open apertures are open apertures of respective groups of discrete open apertures (each group comprising two or more open apertures) provided on the said (first) body and arranged such that a line extending in a direction parallel to a longitudinal axis of the said (first) body extends across each of the apertures within the group. Typically the apertures within each group of apertures are aligned with each other along the length of the said (first) body. The said plurality of discrete open apertures within each group (where provided) may comprise a combined aperture length extending along, for example, at least 30% of, at least 40% of, at least 50% of or at least 90% of the length of the (first) body (parallel to the longitudinal axis of the (first) body). Typically the said plurality of discrete open apertures within each group (where provided) together form an elongate aperture area. It may be that the first and second open apertures are in fluid communication with each other (e.g. through the cavity of the said (first) body). It may be that the first and second open apertures are provided (e.g. directly) opposite each other. It may be that the first and second open apertures of the (first) body face each other (and are typically in fluid communication with each other, e.g. through the (first) body). It may be that the (first) body comprises first and second (typically planar) faces which are opposite each other, and it may be that the first face comprises the first open aperture and the second face comprises the second open aperture. It may be that the first and second open apertures of the said first body are provided directly opposite each other (typically such that there is at least some overlap (preferably a complete overlap) between the first and second open apertures in a direction parallel to the line of shortest distance between the first and second faces). The symmetry provided by having the first and second open apertures directly opposite each other helps to optimise the resonance (and thus acoustic attenuation) performance of the said (first) body of the attenuator. It may be that the acoustic attenuator comprises a third body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a third resonant frequency band across which the third body attenuates acoustic waves, wherein the said open aperture of the third body is in fluid communication with, and faces, the second open aperture of the first body, wherein the first and third resonant frequency bands at least partially overlap, and a gap is provided between the second open aperture of the first body and the said open aperture of the third body (and typically between the first and third bodies), the gap being sized such that resonance of fluid within the cavity of the first body can stimulate resonance of fluid within the cavity of the third body (at least when the resonance occurs at a frequency within the said resonant frequency bands of the said first and third bodies). Typically the resonant frequency bands of the first, second and third bodies (where provided) comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. Typically the said overlapping portions of the resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the said transformer apparatus. By providing the first body with first and second open apertures which resonantly couple the first cavity to the cavities of adjacent attenuators, the resonance coupling effect between attenuators per unit volume is increased, which increases the level of attenuation provided by the attenuators. This also helps to broaden the frequency range of attenuation. Typically the first body is provided next to the said third body (rather than one of the bodies being inside the other). Typically the first body is provided opposite the said third body. Typically the first and third bodies are provided outside each other. It may be that the first and third bodies are identical to each other. It may be that the open apertures of one or both of the second and third bodies are each first of first and second open apertures of that body which are in fluid communication with the cavity of that body, the said first and second open apertures of that body being offset from each other around the longitudinal axis of that body (e.g. offset around the perimeter of that body in a direction having a component perpendicular to the longitudinal axis of that body). It may be that a plurality of the said attenuators are arranged together (e.g. periodically, e.g. in a row) to form an acoustic barrier. The acoustic barrier may comprise a single layer of the said attenuators. It may be that the acoustic attenuator comprises first and second rows of first, second and third (and typically further said) bodies. It may be that the bodies within each row are arranged periodically. It may be that the first and second rows are first and second rows of a plurality of rows, the said rows being arranged (e.g. periodically) so as to attenuate acoustic waves (e.g. emitted by the transformer apparatus) over a further (e.g. resonant) frequency band. It may be that one or more or each of the said bodies in the second row is provided opposite (and may abut and may be mechanically coupled to and may face and may be downstream of, with respect to acoustic waves emitted by the transformer apparatus) a respective body of the first row. It may be that the bodies of the first row are provided with the same or (more typically) different resonant frequency bands from those of the respective bodies of the second row which they are provided opposite. It may be that the first and second open apertures of said body of the first row face away from (e.g. at 90° to) the body of the second row provided opposite the said body of the first row. It may be that the first and second open apertures of each said body of the second row face away from (e.g. at 90° to) the body of the first row provided opposite the said body of the second row. It may be that the first and second rows are separated by a gap. It may be that the first faces of the bodies of the first row are not flush with the first faces of the bodies of the second row they are provided opposite. For example, the first faces of the bodies of the second row may be set back from the first faces of the bodies of the first row which they are provided opposite. Alternatively it may be that the first faces of the bodies of the first row are flush with the first faces of the bodies of the second row they are provided opposite. It may be that a plurality of the said acoustic attenuators are arranged together to form an enclosure. The enclosure may comprise a single layer of the said attenuators. The enclosure formed by the said attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. In other embodiments it may be that the cavities of one or more of the acoustic attenuators are spiral shaped. Typically a spiral shaped cavity defines a (spiral shaped) acoustic wave propagation path in fluid communication with the open aperture. Typically the spiral shaped cavity is configured such that acoustic waves received by the open aperture enter the cavity through the aperture and propagate along the acoustic wave propagation path, before being reflected back along the acoustic wave propagation path to the open aperture. Typically the length of the (spiral shaped) acoustic propagation path is a quarter of the wavelength of at least some of the acoustic waves emitted by the transformer apparatus (in use). Accordingly, by the time it takes an acoustic wave of that wavelength to propagate along the spiral path and back to the open aperture, the acoustic wave received by the open aperture is out of phase with the reflected wave by half a wavelength. Accordingly, the reflected acoustic waves and the incident acoustic waves destructively interfere with each other, thereby attenuating acoustic waves emitted by the transformer apparatus. The acoustic attenuators preferably attenuate acoustic waves having frequencies in the range 20 Hz to 1000 Hz, and more preferably in the range 20 Hz to 500 Hz. Typically, a plurality of the said acoustic attenuators is provided such that together they attenuate acoustic waves having frequencies matching multiples of the frequency of the mains supply, for example acoustic waves having frequencies of 100 Hz, 200 Hz and 300 Hz in the UK (where the mains frequency is 50Hz), or for example 120 Hz, 240 Hz and 360 Hz in continental Europe or the United States of America (where the mains frequency is 60 Hz). It may be that one or more (or each) of the said acoustic attenuators comprises a second body provided within the cavity of the (first) body, the second body defining a second cavity and having an open aperture in fluid communication with the second cavity, the second body being configured such that the acoustic attenuator attenuates at least a portion of the acoustic (sound) waves emitted by the transformer apparatus. The portion of the said acoustic waves attenuated by the second body is typically different from the portion of the acoustic (sound) waves attenuated by the acoustic attenuator by way of the configuration of the cavity of the (first) body. For example, it may be that the second body is configured to attenuate acoustic waves of acoustic frequencies different from the acoustic frequencies attenuated by the (first) body/cavity, albeit they may have frequency bands over which they attenuate acoustic waves which partially overlap. The second body may have any of the features of the (first) body discussed above. It may be that one or more (or each) of the said plurality of acoustic attenuators comprising a second body comprises a third body provided within the second cavity of the second body, the third body defining a third cavity and having an open aperture in fluid communication with the third cavity, the third body being configured such that the acoustic attenuator attenuates at least a portion of the acoustic (sound) waves emitted by the transformer apparatus. For example, it may be that the third body is configured to attenuate acoustic waves of acoustic frequencies different from the acoustic frequencies attenuated by the (first) body/cavity and the second body/cavity, albeit they may have frequency ranges which partially overlap. The third body may have any of the features of the (first) body mentioned above. Typically the said plurality of acoustic attenuators are provided around at least part of the transformer apparatus which emits acoustic waves. Typically, the transformer of the transformer apparatus comprises a transformer core and at least one transformer winding. It may be that one or more or each of the acoustic attenuators are provided between the transformer core and the at least one transformer winding (e.g. to attenuate core noise), but more typically the acoustic attenuators are provided around (e.g. the acoustic attenuators form an enclosure comprising) the transformer core and the at least one transformer winding (e.g. to thereby attenuate both core noise and load noise generated in the winding). It may be that the acoustic attenuators are free standing. It may be that the acoustic attenuators are not mechanically coupled to the transformer or to each other. More typically a plurality of the acoustic attenuators are mechanically coupled to each other (e.g. by way of a frame extending between them). It may be that a plurality of the acoustic attenuators are mechanically coupled to the transformer. It may be that the transformer comprises one or more (or two or more or all) of the said acoustic attenuators. It may be that the said one or more acoustic attenuators are integrated into the transformer. By providing the acoustic attenuators as part of the transformer, the transformer is easy to access for maintenance, repair or servicing. Typically the said acoustic attenuators are arranged to allow access to internal components of the transformer. It may be that the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing. It may be that one or more (typically two or more or each) of the acoustic attenuators are coupled to, or form part of, the transformer housing. It may be that one or more of the said acoustic attenuators are mounted to an external surface of the transformer housing. This makes retro-fitting of the acoustic attenuators to existing transformer housing easier. It may be that the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein the housing comprises an internal surface (typically facing towards the said transformer core) and an external surface opposite the internal surface (and typically facing away from the said transformer core), the transformer apparatus further comprising one or more (typically two or more, or three or more) strengtheners (e.g. strengthening elements) provided (e.g. mounted or integrally formed) on the external surface of the housing to thereby strengthen the housing. It may be that each of one or more of the said strengtheners comprises a respective one of the said acoustic attenuators of the said plurality of acoustic attenuators. It may be that each of one or more of the strengtheners comprise a planar surface which is mounted to (e.g. a planar surface of) the transformer housing. It may be that the transformer housing comprises a planar surface which forms an internal surface of the (first) bodies of one or more of the said plurality of acoustic attenuators. It may be that one or more of the said acoustic attenuators are provided by respective modified strengtheners. It may be that one or more of the strengtheners are hollow. It may be that the one or more strengtheners each comprise a (first) body which is tubular. It may be that the strengtheners each comprise a (first) body (which is typically elongate) having a quadrilateral (preferably rectangular or square) shape when viewed in cross section perpendicular to a longitudinal axis of the (first) body. It may be that at least a portion of the transformer housing vibrates (in use), thereby emitting at least a portion of the said acoustic waves emitted by the transformer apparatus and attenuated by the said acoustic attenuators provided by the strengtheners. It may be that the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein one or more (typically two or more of the said plurality) of the acoustic attenuators are provided within the transformer housing (e.g. one or more of the acoustic attenuators are provided between the combination of the transformer core and the transformer winding(s) and an internal surface of the transformer housing, e.g. mounted to an internal surface of the transformer housing). It may be that the transformer is an oil immersed transformer, and a volume between the transformer housing and the transformer core and winding(s) comprises electrically insulating/coolant oil. It may be that one or more (or two or more or each) of the said acoustic attenuators is submerged in the said oil (e.g. if the acoustic attenuators are provided in the transformer housing). It may be that the transformer housing comprises a transformer tank. It may be that the transformer housing comprises an internal surface and an external surface opposite the internal surface. Typically the internal surface defines a volume containing the transformer core and the one or more transformer windings. It may be that one or more (typically two or more of the said plurality) of the acoustic attenuators are mounted to the internal surface of the transformer housing. It may be that one or more (typically two or more or each of the said plurality) of the acoustic attenuators are mounted to the external surface of the transformer housing. It may be that one or more (typically two or more of the said plurality) of the acoustic attenuators are mounted to the external surface of the transformer housing and that one or more (typically two or more of the said plurality) of the acoustic attenuators are mounted to the internal surface of the transformer housing. It may be that the transformer apparatus comprises a transformer cooling system configured to dissipate heat generated by the transformer (in use), and at least a portion of the acoustic waves emitted by the transformer apparatus is emitted by the transformer cooling system. Typically the bodies of one or more of the acoustic attenuators are configured to attenuate at least a portion of the acoustic waves emitted by the transformer cooling system (in use), e.g. using any of the acoustic attenuation mechanisms described herein. Typically the transformer apparatus further comprises a transformer cooling system configured to dissipate heat generated by the transformer (in use), wherein the transformer cooling system comprises one or more (or two or more or all of the said plurality) of the acoustic attenuators. For example, the transformer cooling system may comprise a heat sink comprising one or more of the plurality of acoustic attenuators. It may be that the transformer cooling system is configured to regulate the temperature of the transformer. It may be that the acoustic attenuators are integrated within existing transformer cooling systems. Alternatively, existing transformer cooling systems may be replaced with a transformer cooling system comprising the said acoustic attenuators. It may be that the acoustic attenuators are arranged such that a plurality (or each) of the acoustic attenuators is in contact with one or more adjacent acoustic attenuators. It may be that the transformer cooling system comprises a gap (an air gap) provided between adjacent acoustic attenuators of the said plurality of acoustic attenuators. It will be understood that there is no requirement for acoustic barriers of this type to be connected together in a traditional solid, continuous acoustic barrier arrangement. Indeed, it may be that in some circumstances, better acoustic attenuation performance can be achieved when gaps are provided between the acoustic attenuators. In addition, the (air) gap is typically provided as part of an air flow path along which air heated by the transformer can flow away from the transformer. Accordingly, the acoustic attenuators can attenuate acoustic waves emitted by the transformer with little or no interference on the cooling of the transformer. Typically a plurality of (air) gaps is provided, each of the (air) gaps being provided between adjacent acoustic attenuators, and in some cases between the acoustic attenuators of each pair of adjacent acoustic attenuators. It may be that the acoustic attenuators are arranged together (e.g. periodically) to form an acoustic barrier for attenuating acoustic waves emitted by the transformer apparatus (in use). Typically the acoustic barrier is provided in the path of incoming and outgoing air flow to and from the transformer. Typically the acoustic barrier causes little or no reduction on the rate of air flow to and from transformer. Typically a plurality of the said acoustic attenuators are arranged next to each other. Typically, a plurality of the said acoustic attenuators are arranged together (e.g. periodically) in a single row. It may be that a plurality of the plurality of acoustic attenuators are arranged together (e.g. periodically) to form an acoustic barrier. It may be that a plurality of the said plurality of acoustic attenuators are arranged together to form an enclosure comprising at least part of the transformer apparatus which emits acoustic waves (typically including the transformer). The enclosure may comprise a single layer of the said acoustic attenuators. The enclosure formed by the said plurality of acoustic attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. The acoustic attenuators may be arranged in one or more two-dimensional arrays within an acoustic barrier. An acoustic barrier comprising acoustic attenuators arranged in one or more two-dimensional arrays may function as a (finite) sonic crystal which attenuates incident acoustic waves (e.g. emitted from the transformer apparatus) satisfying a Bragg condition defined by the spacing between subsequent rows (or spacings between planes defined by the said rows). It may be that the acoustic attenuators of the plurality of acoustic attenuators are arranged periodically. For example, the said acoustic attenuators may be arranged to form at least one row, the distance between adjacent acoustic attenuators in the row being periodic (e.g. the spacing between adjacent (first) bodies being identical or varying periodically). The acoustic attenuators of the plurality of acoustic attenuators may be arranged in a plurality of (typically substantially parallel) rows, the distance between adjacent rows being periodic. Subsequent adjacent rows of acoustic attenuators are typically spaced from each other such that incident acoustic waves (e.g. emitted from the transformer apparatus) having a particular frequency and angle of incidence are (typically multiply) scattered by the subsequent rows, the said scattered waves interfering with each other such that the said incident waves are thereby attenuated. Preferably, the (e.g. planes defined by the) rows are parallel to each other, and incident acoustic waves (e.g. emitted from the transformer apparatus) having a frequency and angle of incidence which satisfies a Bragg condition defined by the spacing between the rows (e.g. the spacing between adjacent rows, or the planes defined by the rows, being identical or varying periodically) are (typically multiply) scattered by subsequent rows, the said scattered waves (typically destructively) interfering with each other such that the said incident waves are thereby attenuated. The plurality of rows of acoustic attenuators may form an acoustic barrier. The plurality of rows of acoustic attenuators may form an enclosure comprising the transformer. The enclosure formed by the plurality of rows of acoustic attenuators may be a two sided enclosure, more preferably a three sided enclosure, a four sided enclosure, a five sided enclosure or a six sided enclosure. It may be that one of the sides of the enclosure comprises a roof. It may be that one of the sides of the enclosure comprises a floor. It may be that the enclosure at least partially encloses the transformer apparatus. An air gap may be provided between adjacent acoustic attenuators, the said air gap comprising an acoustically isolated air intake or exhaust port. It may be that a plurality of air gaps are provided between adjacent acoustic attenuators, the said plurality of air gaps comprising an acoustically isolated air intake port and an acoustically isolated air exhaust port. Typically, the air intake port or the air exhaust port comprises a resilient (e.g. rubber) seal providing acoustic isolation. It may be that the air intake port or the air exhaust port comprises a port between adjacent acoustic attenuators. Alternatively, it may be that the air intake port or the air exhaust port comprises a port through an acoustic attenuator. An air gap may be provided between adjacent acoustic attenuators, the said air gap comprising an acoustically sealed wiring port. Typically transformer wiring for interfacing and/or powering the transformer is provided which extends through the acoustically sealed wiring port. The acoustically sealed wiring port may be provided between adjacent acoustic attenuators. Alternatively, the acoustically sealed wiring port may be provided through an acoustic attenuator. Typically the transformer cooling system further comprises one or more air blowers (e.g. fans) configured to blow air heated by the transformer (in use) away from the transformer through the air gap(s) between the said adjacent acoustic attenuators. It may be that the transformer comprises an oil immersed transformer comprising a transformer core and at least one transformer winding immersed in oil, and wherein the transformer cooling system comprises a heat exchanger configured to cool the said oil. The heat exchanger may comprise an oil flow path extending from (e.g. a or the housing of) the transformer to the heat exchanger. The oil flow path typically comprises a loop extending from the transformer to the heat exchanger and back to the transformer from the heat exchanger. It may be that oil flows between the transformer and the heat exchanger passively under convection. In other embodiments, the heat exchanger comprises an oil pump configured to (actively) pump oil between the transformer and the heat exchanger. Typically the heat exchanger comprises one or more (or two or more or all of the said plurality) of the said acoustic attenuators. It may be that the heat exchanger comprises a heat sink comprising one or more (or two or more or all of the said plurality) of the acoustic attenuators. It may be that the acoustic attenuators are made of a material (e.g. a metallic element, alloy or compound) suitable for radiating heat. The heat sink may comprise one or more heat sink fins comprising one or more of the said acoustic attenuators. It may be that the heat exchanger comprises an oil to water heat exchanger, the heat exchanger comprising a first conduit configured to carry oil from the transformer through the heat exchanger and a second conduit adjacent the said first conduit and configured to carry a flow of cooling water such that heat from the oil flowing in the said first conduit transfers to the cooling water flowing in the second conduit, wherein an acoustic attenuator of the said plurality of acoustic attenuators comprises the first and second conduits. It may be that the cooling water flows passively (e.g. under gravity) along the said second conduit. Alternatively, it may be that the cooling system comprises a water pump configured to pump cooling water through the second conduit (either in the same direction as oil flowing in the first conduit or, more preferably, in an opposite direction to oil flowing in the first conduit). One or more (or each) of the acoustic attenuators may comprise a plurality of first conduits configured to carry oil from the transformer through the heat exchanger and a plurality of second conduits, configured to carry a flow of cooling water, the second conduits being interleaved between the first conduits such that heat from the oil flowing in a first conduit is transferred to cooling water flowing through an adjacent second conduit. Typically the first and second conduits are separated by respective heat conducting plates configured to transfer heat from oil flowing in a first conduit to water flowing in a second conduit adjacent to the first conduit. It may be that the (first) bodies of one or more of the said acoustic attenuators comprise one or more (e.g. two or more) first conduits carrying (heated) transformer oil and one or more (e.g. two or more) second conduits carrying cooling water, each of the second conduits being adjacent to one or more of the first conduits such that heat from oil flowing through a first conduit is transferred to cooling water flowing through an adjacent second conduit. Typically at least one of the first conduits and at least one of the second conduits are provided within, and extend along, at least a portion of (e.g. at least 50% of, preferably at least 75%, typically 100%) of the length of the (first) body of one of the said acoustic attenuators. As set out above, the (first) body of one or more (or each) of the acoustic attenuators comprises an inner surface defining the cavity and an outer surface opposite the inner surface. It may be that the first and second conduits are provided between the outer surface and the inner surface. It may be that the first and second conduits are elongate. It may be that the first and/or second conduits have longitudinal axes extending perpendicularly to the shortest distance between the inner and outer surfaces of the (first) body. It may be that the first and/or second conduits have longitudinal axes parallel to the longitudinal axis of the (first) body. Typically the plurality of acoustic attenuators are together arranged to attenuate acoustic waves of one or more frequencies different from the frequencies attenuated by the individual acoustic attenuators. Typically the plurality of acoustic attenuators are together arranged to attenuate acoustic waves emitted by the transformer apparatus (in use). Typically the plurality of acoustic attenuators are together arranged to attenuate acoustic waves having a frequency greater than the frequencies attenuated by the individual acoustic attenuators (although there may be a partial overlap between them). Typically the plurality of acoustic attenuators are together arranged to attenuate acoustic waves emitted by the transformer apparatus (in use) and not attenuated by (typically greater than the frequencies covered by) the individual acoustic attenuators. A second aspect of the invention provides apparatus comprising: transformer apparatus which emits acoustic (typically sound) waves (in use), the transformer apparatus comprising a transformer and a transformer cooling system configured to dissipate heat generated by the transformer (in use), wherein the transformer cooling system comprises one or more acoustic attenuators configured to attenuate at least a portion of the acoustic waves emitted by the transformer apparatus (in use). Each of the said acoustic attenuators typically comprises a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates at least a portion of the acoustic waves emitted by the transformer apparatus (in use). Typically the transformer cooling system comprises a heat sink comprising one or more of the one or more acoustic attenuators. Typically the transformer cooling system comprises a heat exchanger comprising one or more of the one or more acoustic attenuators. It may be that the heat exchanger comprises an oil to water heat exchanger, the heat exchanger comprising a first conduit configured to carry oil from the transformer through the heat exchanger and a second conduit adjacent the said first conduit and configured to carry a flow of cooling water such that heat from the oil flowing in the said first conduit transfers to the cooling water flowing in the second conduit, wherein an acoustic attenuator of the said one or more acoustic attenuators comprises the first and second conduits. It may be that the cooling water flows passively (e.g. under gravity) along the said second conduit. Alternatively, it may be that the transformer cooling system comprises a water pump configured to pump cooling water through the second conduit (either in the same direction as oil flowing in the first conduit or, more preferably, in an opposite direction to oil flowing in the first conduit). It may be that oil flows between the transformer and the heat exchanger passively under convection. In other embodiments, the heat exchanger comprises an oil pump configured to (actively) pump oil between the transformer and the heat exchanger. One or more of the said acoustic attenuators may comprise a plurality of first conduits configured to carry oil from the transformer through the heat exchanger and a plurality of second conduits, configured to carry a flow of cooling water, the second conduits being interleaved between the first conduits such that heat from the oil flowing in a first conduit is transferred to cooling water flowing through an adjacent second conduit. Typically the first and second conduits are separated by respective heat conducting plates configured to transfer heat from the oil flowing in a first conduit to water flowing in a second conduit adjacent to the first conduit. It may be that the (first) bodies of one or more of the said acoustic attenuators comprise one or more (e.g. two or more) first conduits carrying transformer oil and one or more (e.g. two or more) second conduits carrying cooling water, each of the second conduits being adjacent to one or more of the first conduits such that heat from oil flowing through a first conduit is transferred to cooling water flowing through an adjacent second conduit. Typically at least one of the first conduits and at least one of the second conduits are provided within, and extend along, the (first) body of one of the said acoustic attenuators. As set out above, the (first) body of one or more or each of the acoustic attenuators comprises an inner surface defining the cavity and an outer surface opposite the inner surface. It may be that the first and second conduits are provided between the outer surface and the inner surface. It may be that the first and second conduits are elongate. It may be that the first and/or second conduits have longitudinal axes extending perpendicularly to the shortest distance between the inner and outer surfaces of the (first) body (or parallel to a longitudinal axis of the (first) body). A third aspect of the invention provides a transformer comprising: a transformer core; one or more transformer windings; a transformer housing containing the transformer core and the transformer windings, the transformer housing comprising one or more acoustic attenuators, each of the said one or more acoustic attenuators comprising: a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates one or more frequencies of acoustic waves emitted by the transformer (in use). Typically the said one or more acoustic attenuators are integrated into the transformer housing. It may be that the transformer is an oil immersed transformer. It may be that the transformer housing comprises a transformer tank. It may be that the transformer housing comprises insulating/cooling oil. It may be that one or more (or each) of the acoustic attenuators is immersed in the said oil (e.g. the one or more (or each) of the acoustic attenuators may be provided inside the transformer housing). It may be that the transformer core and the one or more transformer windings are immersed in the insulating/cooling oil within the housing. It may be that the transformer housing comprises an internal surface and an external surface opposite the internal surface. Typically the internal surface defines a volume containing the transformer core and the one or more transformer windings. It may be that the one or more acoustic attenuators are mounted to the internal surface of the transformer housing. It may be that the one or more acoustic attenuators are mounted to the external surface of the transformer housing. It may be that one or more of the acoustic attenuators are mounted to the internal surface of the housing and that one or more of the acoustic attenuators are mounted to the external surface of the housing. A fourth aspect of the invention provides a method of attenuating acoustic waves emitted by a transformer apparatus comprising a transformer, the method comprising: the transformer apparatus generating acoustic (typically sound) waves (in use); and attenuating at least a portion of the said acoustic waves by providing one or more acoustic attenuators in an acoustic wave propagation path of the said acoustic waves, each of the said one or more acoustic attenuators comprising a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that at least a portion of the said acoustic waves emitted by the transformer apparatus are attenuated by the acoustic attenuator(s). Typically the method comprises the transformer of the transformer apparatus generating acoustic waves, and attenuating at least a portion of the acoustic waves emitted by the transformer by providing one or more acoustic attenuators in an acoustic wave propagation path of the said acoustic waves, each of the said one or more acoustic attenuators comprising a (first) body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that at least a portion of the said acoustic waves emitted by the transformer are attenuated by the acoustic attenuator(s). It may be that the method comprises receiving acoustic waves emitted by the transformer in the open apertures of (first) bodies of a plurality of acoustic attenuators, the said open apertures being in fluid communication with cavities defined by the (first) bodies and being configured such that the acoustic attenuators attenuate at least a portion of the acoustic waves emitted by the transformer apparatus. Typically the transformer apparatus comprises a transformer cooling system configured to dissipate heat generated by the transformer. It may be that the at least a portion of the acoustic waves emitted by the transformer apparatus are emitted by the transformer cooling system. Typically the cavities of one or more of the acoustic attenuators are configured to attenuate at least a portion of the acoustic waves emitted by the transformer cooling system. It may be that the transformer apparatus comprises a transformer cooling system configured to dissipate heat generated by the transformer, the method further comprising dissipating heat generated by the transformer apparatus using the transformer cooling system, the transformer cooling system comprising one or more said acoustic attenuators. The method may further comprise dissipating heat generated by the transformer by: the transformer heating air adjacent to the transformer to provide heated air; and flowing the heated air away from the transformer through an air gap between a pair of adjacent acoustic attenuators. It may be that the heated air flows away from the transformer by convection. Alternatively, the method may comprise forcing heated air away from the transformer through the said air gap (e.g. using an air blower such as a fan). The method may further comprise: the transformer heating oil adjacent to the transformer to provide heated oil; and flowing the heated oil away from the transformer to a heat exchanger comprising one or more said acoustic attenuators. It may be that the heat exchanger comprises a heat sink comprising the acoustic attenuator. It may be that the heat exchanger comprises an oil to water heat exchanger comprising a said acoustic attenuator. It may be that the method further comprises: the transformer heating oil adjacent to the transformer to provide heated oil; flowing the heated oil away from the transformer to a first conduit of a or the heat exchanger, the said first conduit being provided in the acoustic attenuator; and transferring heat from the heated oil flowing along the first conduit to cooling water flowing along a second conduit adjacent to the first conduit, the second conduit being provided in the said acoustic attenuator. The method may comprise flowing oil back to the transformer after heat has been transferred from the heated oil flowing along the first conduit to cooling water flowing along the second conduit. It may be that the method further comprises: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within the cavity defined by the body of the said acoustic attenuator; and first and second walls of the acoustic attenuator scattering acoustic waves emitted by the transformer apparatus, the said scattered acoustic waves interfering with each other such that the said incident acoustic waves are thereby attenuated. Typically the method comprises the first and second walls scattering acoustic waves emitted by the transformer apparatus, the said acoustic waves having a frequency and an angle of incidence upon the first and second walls which satisfy a Bragg condition defined by a gap provided between the first and second walls, the said (first) body comprising at least one of the first and second walls. Typically the said first and second walls are parallel to each other. It may be that the method further comprises: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a first cavity defined by a first body of one of the said acoustic attenuators comprising a first open aperture in fluid communication with the first cavity; and acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a second cavity defined by a second body of the said acoustic attenuator comprising a second open aperture in fluid communication with the second cavity, wherein the first and second open apertures face each other such that resonance of the fluid provided within the first cavity caused by the said acoustic waves stimulates resonance of the fluid provided within the second cavity. It may be that the method further comprises: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a first cavity defined by a first body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the first cavity; and acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a second cavity defined by a second body of the said acoustic attenuator comprising an open aperture in fluid communication with the second cavity, wherein the open apertures of the first and second bodies face each other (and are typically in fluid communication with each other, the first and second resonant frequency bands typically at least partially overlapping, wherein the overlapping portion of first and second resonant frequency bands typically comprises one or more frequencies of acoustic waves emitted by the transformer apparatus, a gap typically being provided between the open apertures of the first and second bodies (and typically between the first and second bodies), the gap being sized) such that resonance of the fluid provided within the first cavity caused by the said acoustic waves stimulates resonance of the fluid provided within the second cavity (and typically vice versa). It may be that the method further comprises acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a third cavity defined by a third body of the said acoustic attenuators comprising an open aperture in fluid communication with the third cavity, wherein each of the first and third bodies comprise a first face and a second face, the first face comprising the said open aperture of that body, the method further comprising arranging the first and third bodies such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the first and third bodies through their respective open apertures (typically without the other body causing an obstruction thereto). It may be that the method further comprises acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a fourth cavity defined by a fourth body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the fourth cavity, wherein the open apertures of the third and fourth bodies face each other (and are typically in fluid communication with each other, the third and fourth resonant frequency bands typically at least partially overlapping, wherein the overlapping portion of third and fourth resonant frequency bands typically comprises one or more frequencies of acoustic waves emitted by the transformer apparatus), wherein a gap is provided between the open apertures of the third and fourth bodies (and typically between the third and fourth bodies), the gap being sized such that resonance of fluid within the third body caused by the said acoustic waves stimulates resonance of fluid within the fourth body (and typically vice versa). It may be that the said open aperture of a said body of a said attenuator (e.g. the first body described above) is the first of first and second open apertures of the said body which are in fluid communication with the cavity of the said body, the said first and second open apertures of the said body being offset from each other around the longitudinal axis of the said body (e.g. offset around the perimeter of the first body in a direction having a component perpendicular to the longitudinal axis of the said first body). It may be that the method further comprises incident acoustic waves emitted by the transformer apparatus stimulating resonance of fluid within the cavity of the (first) body through the first and second apertures to thereby attenuate the said incident acoustic waves. It may be that the method further comprises providing the first and second open apertures of the said first body directly opposite each other (typically such that there is at least some overlap (preferably a complete overlap) between the first and second open apertures in a direction parallel to the line of shortest distance between the first and second faces comprising the first and second apertures). It may be that the method further comprises acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a third cavity defined by a third body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the third cavity, wherein the said open aperture of the third body is in fluid communication with, and faces, the second open aperture of the first body, wherein the first and third resonant frequency bands at least partially overlap, and a gap is provided between the second open aperture of the first body and the said open aperture of the third body (and typically between the first and third bodies), the gap being sized such that resonance of fluid within the first body caused by the said acoustic waves stimulates resonance of fluid within the third body (and typically vice versa). It may be that the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein the housing comprises an internal surface (typically facing towards the said transformer core) and an external surface opposite the internal surface (typically facing away from the said transformer core), the method further comprising providing one or more (typically two or more, or three or more) strengtheners (e.g. strengthening elements) (e.g. mounted or integrally formed) on the external surface of the housing to thereby strengthen the housing. It may be that each of one or more of the said strengtheners comprises a respective one of the said acoustic attenuators of the said plurality of acoustic attenuators. It may be that the method further comprises mounting a planar surface of each of one or more of the strengtheners to (e.g. a planar surface of) the external surface of the transformer housing. It may be that the method further comprises a planar surface of the transformer housing forming an internal surface of the (first) body of each of one or more of the said one or more acoustic attenuators. It may be that the method further comprises modifying one or more said strengtheners to form a respective acoustic attenuator of the said one or more acoustic attenuators, for example by cutting out one or more open apertures in the said strengtheners. In this case, the strengtheners may originally (prior to modification) comprise a (first) body defining a (hollow) cavity therein such that by cutting out one or more open apertures in the (first) body in fluid communication with the cavity, respective acoustic attenuators of the said one or more acoustic attenuators can be formed. A fifth aspect of the invention provides a method of attenuating acoustic waves emitted by a transformer apparatus comprising a transformer and a transformer cooling system comprising an acoustic attenuator, the method comprising: the transformer generating heat; the transformer apparatus generating acoustic (typically sound) waves; dissipating heat generated by the transformer using the transformer cooling system; and attenuating acoustic waves emitted by the transformer apparatus using the said acoustic attenuator. The acoustic attenuators may have any of the essential or preferred features of the acoustic attenuators described in US2014/0166391 which is incorporated here by reference. The acoustic attenuators may be together arranged in any of the arrangements described in US2014/0166391 which is incorporated here by reference. The preferred and optional features discussed above are preferred and optional features of each aspect of the invention to which they are applicable. Description of the Drawings An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 shows an acoustic attenuator; Figure 2 shows an alternative acoustic attenuator; Figure 3 is a perspective view of an alternative acoustic attenuator; Figure 4 is a cross section through the acoustic attenuator of Figure 3; Figure 5 is a cross section through an alternative acoustic attenuator; Figure 6 is a cross section through a further alternative acoustic attenuator; Figure 7 is a cross section through a yet further alternative acoustic attenuator; Figure 8 is a cross section through a yet further alternative acoustic attenuator; Figure 9 is a cross section through a yet further alternative acoustic attenuator; Figure 10 is a cross section through a yet further alternative acoustic attenuator; Figure 11 is a cross section through a yet further alternative acoustic attenuator; Figure 12 shows another alternative acoustic attenuator; Figure 13 is a side view of a transformer surrounded by a layer of acoustic attenuators; Figure 14 is a plan view of the transformer and acoustic attenuators of Figure 13; Figure 15 is a side view of a transformer surrounded by two layers of acoustic attenuators; Figure 16 is a plan view of the transformer and acoustic attenuators of Figure 15; Figure 17 is a side view of a transformer with integrated acoustic attenuators; Figure 18 is a plan view of the transformer with integrated acoustic attenuators of Figure 17; Figure 19 is a side view of a transformer with internal acoustic attenuators; Figure 20 is a plan view of the transformer with internal acoustic attenuators of Figure 19; Figure 21 is a side view of a system comprising an oil-immersed transformer, a heat sink and acoustic attenuators; Figure 22 is a plan view of the system of Figure 21 ; Figure 23 is a side view of an oil-immersed transformer in communication with acoustic elements operating as a heat sink; Figure 24 is a plan view of the oil-immersed transformer in communication with acoustic elements operating as a heat sink of Figure 23; Figure 25 is a cross section of an acoustic element operable as a heat sink; Figure 26 is a plan view of an enclosure formed by acoustic attenuators as shown in Figure 3 and a transformer; Figure 27 is a plan view of an enclosure formed by acoustic attenuators as shown in Figure 9 and a transformer; Figure 28 is a plan view of an enclosure comprising acoustic attenuators as shown in Figure 10 and a transformer; Figure 29 is a plan view of an enclosure comprising acoustic attenuators as shown in Figure 11 and a transformer; Figure 30 is a sectional view of six elongate acoustic attenuators taken perpendicular to their longitudinal axes, the acoustic attenuators being arranged in back-to-back pairs with their open apertures facing outwards; Figure 31 is a sectional view of six alternative elongate acoustic attenuators taken perpendicular to their longitudinal axes, the acoustic attenuators being arranged in back-to-back pairs with their open apertures facing outwards, the attenuators within each pair being of different sizes; Figures 32 and 33 are sectional view of eight elongate acoustic attenuators taken perpendicular to their longitudinal axes, the acoustic attenuators being arranged in back-to-back pairs with their open apertures facing outwards, the attenuators of two pairs being of different sizes from attenuators of the other two pairs; Figure 34 shows two opposing rows of pairs of attenuators, the said rows being spaced apart from each other; Figure 35 shows a similar arrangement to Figure 34, but with the attenuators of the opposing rows abutting each other and being mechanically coupled to each other; Figure 36 shows a similar arrangement to Figure 35, but with the first faces of the attenuators of the second row being set back from the first faces of the attenuators of the first row; Figure 37 shows a similar arrangement to Figure 35, but with gaps being provided between the second faces of the attenuators of each pair of the second row such that the first faces of the attenuators of each pair of the second row are flush with the first faces of the attenuators of each pair of the first row; Figure 38 is a sectional view of three elongate acoustic attenuators, each of which comprises opposing pairs of elongate open apertures; and Figure 39 is a plan view of a transformer housing comprising a plurality of strengtheners around its perimeter, the strengtheners comprising acoustic attenuators. Detailed Description of an Example Embodiment With reference to Figure 1 , an acoustic attenuator 1 comprises a hollow, elongate, tubular body 2 of length L and diameter D with a continuous open aperture 3 in said body 2 running parallel to a longitudinal axis of the body 2, the aperture 3 having a width W. The body 2 has a first end 4 and a second end 5 opposite the first end, where both the first end 4 and the second end 5 are open. The body 2 comprises an outer surface and an inner surface opposite the outer surface, the inner surface defining an internal cavity 6. The open aperture 3 and the first and second ends 4 and 5 fluidly connect the cavity 6 with the outer surface of the body 2 and its surroundings. The aperture 3 has a length equal to the length L of the body 2. In alternative embodiments, the length of the aperture may be less than the length L of the body 2 (for example, the length of the aperture may be at least 50%, at least 75% or at least 90% of the length L). In other embodiments, the aperture 3 may comprise a plurality of discrete apertures along the length L of the body 2 (which are typically separated by solid portions of the body 2). It may be in some embodiments that one or both of the first end 4 and the second end 5 are covered or partially covered. It may be that the internal cavity 6 is only substantially in fluid communication with the external surface of the body 2 and its surroundings through the open aperture 3. The (solid) body 2 is provided in a fluidic host medium which, unless otherwise stated, will be assumed to be air. However, it will be understood that the fluidic host medium may be any suitable liquid, gas or mixture of liquid and gas. It will also be understood that the fluidic host medium fills the cavity 6 and surrounds the body 2. When an acoustic wave of frequency / is incident on the aperture 3, it causes displacement of air (or other fluidic host medium) from the aperture 3 into the cavity 6. Such a displacement of air causes the air pressure inside the cavity 6 to rise. The increased air pressure inside the cavity 6 exerts a force on the air in the aperture 3, subsequently causing air to be pushed back out of the cavity 6 through the aperture 3 and into the surroundings. Since the air in the aperture 3 has momentum, it continues to travel beyond its initial position inside the aperture 3. This causes the pressure in the cavity 6 to drop, leading to air being subsequently drawn back into the cavity 6 through the aperture 3. These air pressure oscillations usually decay with time. However, when the frequency / of the incident acoustic wave lies within a resonant frequency band of the attenuator 1 (which resonant frequency band is at least partly defined by the aperture 3 and the cavity 6 - see below), the acoustic wave stimulates resonance of the air (or other fluidic host medium) in the cavity 6. A significant proportion of the energy carried by the acoustic wave is transferred to the air in this process, leading to attenuation of the acoustic wave. The particular resonant frequency band of the air in the cavity 6 is dependent on the geometry of the body 2 of the acoustic attenuator 1. More specifically, the resonant frequency band is dependent on the length L of the body 2, the diameter D of the cavity and the width of the aperture W, as well as the thickness of the tubular body 2. The resonant frequency band also depends on the volume of said cavity 6. Although not shown, it may be that the body 2 further comprises a neck extending from the edge(s) of the open aperture 3 into and/or away from the cavity 6. In this case, the said resonant frequency band is also dependent on the length of the neck. The acoustic attenuator 1 may, therefore, be tuned to attenuate acoustic waves with particular frequencies for a given purpose. Acoustic waves incident on the acoustic attenuator 1 and stimulating resonance of the air in the cavity 6 may enter the cavity 6 through the aperture 3 (or through the open ends 4, 5). The resonant properties of the attenuator 1 are substantially material-independent. The body 2 may therefore be formed from any suitable structural or decorative materials such as metals, glasses, pyrex or other plastics. The resonant frequency band is substantially independent of said material used to form the body 2. Acoustic attenuators may comprise more complex designs than illustrated in Figure 1. Figure 2, for example, illustrates an acoustic attenuator 7 comprising an outer body 8 and an inner body 9, both individually of the same type of acoustic attenuator as acoustic attenuator 1. The outer body 8 and the inner body 9 are concentric. The open apertures of the outer body 8 and the inner body 9 overlap such that acoustic waves can easily enter and leave the cavity of the inner body through the open apertures of the outer body 8 and the inner body 9. The cavity of the inner body 9 has a smaller diameter D than that of the outer body 8 and, therefore, the air within the inner body 9 resonates at acoustic frequencies which are different from those at which the air contained within the internal cavity defined by the outer body 8 resonates. Although the resonant frequency bands associated with the outer body 8 and with the inner body 9 are different, they may overlap. The presence of the inner body 9 within the internal cavity of the outer body 8 results in the acoustic attenuator 7 being able to attenuate acoustic waves over a wider frequency range than if only outer body 8 or only the inner body 9 were present. Additional concentric bodies may be included to further widen or otherwise tune the range of frequencies of acoustic waves attenuated. For example, the acoustic attenuator may comprise three, four, five or more concentric bodies. With reference to Figure 3, an alternative monolithic acoustic attenuator 10 comprises a hollow, elongate body 12 of length L. The body 12 comprises four walls 12A, 12B, 12C and 12D. Walls 12A and 12C are parallel to each other. The parallel walls 12A and 12C are separated by a gap, the shortest distance between the parallel walls 12A, 12C being indicated as D in Figure 3. Walls 12B and 12D extend between walls 12A and 12C, converging towards each other from wall 12C to wall 12A which has a shorter width than wall 12C. The four walls 12A, 12B, 12C and 12D are, therefore, arranged to form a trapezoidal cross section (i.e. in cross section perpendicular to a longitudinal axis of the body 12 parallel to its length L) as illustrated in Figure 4. An aperture 13 of width W is provided at an intermediate portion of wall 12A in fluid communication with an internal cavity 14 defined by the body 12. The body 12 is a solid (e.g. steel or plastic) body and (as above) is typically provided in a fluidic host medium which, for the purposes of the discussion below will be assumed to be air. However, it will be understood that the fluidic host medium may comprise any other suitable gas or liquid or a mixture of a gas and a liquid. The aperture 13 has a length equal to the length L of the body 12. In alternative embodiments, the length of the aperture may be less than the length L of the body 12 (for example, the length of the aperture may be at least 50%, at least 75% or at least 90% of the length L). In other embodiments, a plurality of discrete apertures may be provided along the length L of the body 12 (in which case the combined aperture length parallel to the longitudinal axis of the body 12 of the plurality of discrete apertures is typically at least 50%, at least 75% or at least 90% of the length L). The acoustic attenuator 10 attenuates incident acoustic waves by at least two physical mechanisms. The first mechanism is the transfer of energy from incident acoustic waves (having frequencies within the resonant frequency band of the attenuator 10) to the air within the cavity 14 by stimulation of resonance of the air within the cavity 14 (as described above with respect to the acoustic attenuator shown in Figure 1). The particular resonant frequency band of the air in the cavity 14 is again dependent on the geometry of the acoustic attenuator 10 and, in particular, the volume of the cavity 14 (and thus the length L of the body 12 and the distance D). The resonant frequency band also depends on the width W of the aperture 13 and the thickness of the attenuator body 12. Although not shown, it may be that the body 12 further comprises a neck extending from the edge(s) of the open aperture 13 into and/or away from the cavity 14. In this case, the said resonant frequency band is also dependent on the length of the neck. The geometry of the acoustic attenuator 10 may, therefore, be tuned to attenuate acoustic waves over a first frequency band. The second mechanism by which the acoustic attenuator 10 attenuates incident acoustic waves is a one dimensional sonic crystal effect provided by the parallel walls 12A, 12C and the gap provided between them. The propagation of mechanical (acoustic) waves in a medium is usually described by a dispersion relation that relates the (angular) frequency F, and wave vector, k, of the propagating acoustic wave. The dispersion relation for waves travelling in a homogeneous medium is: 2 F = c k, where c is the velocity of sound in the host medium (typically air). Another relation which is useful to define is the Bragg condition: ηλ = 2D sin(a), where n is an integer or a half-integer, λ is the wavelength of the acoustic waves incident on the walls 12A, 12C and D is the shortest distance between parallel walls 12A and 12C, and where the frequency F and the wavelength λ are related to the velocity of the first and second waves c by: c = F The walls 12A, 12C provide density variations ("interfaces") to acoustic waves propagating in the (air) host medium. When incident acoustic waves encounter the interfaces, they transfer part of their energy into secondary, multiply scattered waves which then interfere with each other. As the walls 12A, 12C are parallel, the acoustic waves are strongly dispersed from one wall 12A to the other 12C, and end up filling all available space between the walls 12A, 12C and propagating in every possible direction. When the Bragg condition is satisfied, interference occurs between the scattered waves, leading to the formation of acoustic "band gaps" that prevent acoustic waves with certain frequencies travelling through the body 12. This is due to the modification of the dispersion relation. The scattered waves interfere constructively or destructively depending on the wave frequency and the sonic crystal geometry. A band gap appears when the scattered waves interfere destructively in a given direction, causing the superposition of waves at that frequency to decrease exponentially when traversing the body 12. These properties are strictly true for the frequencies that fall within the complete band gap. For other frequencies, destructive interferences are balanced by constructive ones and waves are transmitted at least partially. Band gaps will occur at multiples (and sub-harmonics) of the fundamental affected frequency. Thus, when acoustic waves of frequency F propagating in a direction having a component parallel to the distance D between the walls 12A, 12C are incident on the walls 12A, 12C at an angle of incidence a, the frequency F and the angle of incidence a satisfying a Bragg condition defined by the gap D between the walls 12A, 12C, the incident acoustic waves are multiply scattered by walls 12A, 12C. The scattered waves (at least partly, or completely) destructively interfere with each other, significantly reducing the transmission of waves having frequency F through the acoustic attenuator 10. Although the first and second walls 12A, 12C are required to be parallel to each other for the Bragg condition to be satisfied (and it is preferable for the first and second walls to be parallel), significant attenuation effects are still achieved even when the first and second walls are not quite parallel to each other. Indeed, acoustic wave attenuation effects have been observed by this effect when the normal to the first wall 12A and the normal to the second wall 12C intersect at angles of up to 20°. It will be understood that as a consequence of the Bragg condition, acoustic waves having a wavelength λ equal to the distance D (or acoustic waves having a wavelength which is a factor of D) are most strongly attenuated. As the one dimensional sonic crystal effect is finite (there being only two scattering surfaces 12A, 12C in this embodiment), it will be understood that acoustic waves across acoustic frequency bands centred on frequencies meeting the Bragg condition (as opposed to only acoustic waves having frequencies which precisely meet the Bragg condition) are attenuated. However, the inventors have discovered that this effect can be used to usefully (multiply) scatter (and thereby attenuate, from the point of view of an observer on an opposite side of the attenuator from an acoustic wave source emitting) acoustic waves having frequencies and angles of incidence on the walls 12A, 12C satisfying the Bragg condition. The acoustic attenuator 10 therefore individually achieves both local resonance- based attenuation of incident acoustic waves having frequencies / and Bragg scattering of incident acoustic waves having frequencies F and angles of incidence a satisfying the Bragg condition. In some cases the frequencies / and F are the same (such that the two mechanisms combine to attenuate acoustic waves having frequencies / and F more strongly), but more typically the frequencies attenuated by the two mechanisms are different. Nevertheless, there may be at least partial overlap between the two frequency bands. The one dimensional sonic crystal mechanism is largely independent of the cross- sectional shape of the attenuator body 12, provided that the shape comprises two substantially parallel walls separated by a distance D. For example, in an alternative embodiment illustrated in Figure 5, an attenuator body 15 has four walls 15A, 15B, 15C and 15D arranged in a substantially square cross section (again the cross section is taken perpendicular to the longitudinal axis of the body 15). The walls 15A and 15C are separated by a distance D and are parallel to each other, as are the walls 15B and 15D. An aperture of width W is provided in the wall 15A. The mechanisms of attenuation are the same as in the embodiment illustrated in Figures 3 and 4, with the one dimensional sonic crystal effect arising due to multiple scattering of incident acoustic waves by the walls 15A and 15C meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). It will be understood that a further one dimensional sonic crystal effect (in a perpendicular propagation plane to that provided by walls 15A, 15C) is provided due to the multiple scattering of incident acoustic waves by the walls 15B and 15D meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident waves). In an another alternative embodiment illustrated in Figure 6, an attenuator body 16 comprises four walls 16A, 16B, 16C and 16D arranged in a substantially rectangular cross section (again the cross section is taken perpendicular to the longitudinal axis of the body 16). The walls 16A and 16C are separated by a distance D and are parallel to each other. The walls 16B and 16D are also parallel to each other. An aperture of width W is provided in the wall 16A. The mechanisms of attenuation are the same as in the embodiment illustrated in Figures 3 and 4, with the one dimensional sonic crystal effect arising due to multiple scattering of incident acoustic waves by the walls 16A and 16C meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). It will be understood that a further one dimensional sonic crystal effect (in a perpendicular propagation plane to that provided by walls 16A, 16C) is provided due to the multiple scattering of incident acoustic waves by the walls 16B and 16D meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). In an another alternative embodiment illustrated in Figure 7, an attenuator body 17 comprises four walls 17A, 17B, 17C and 17D arranged in a substantially parallelogrammatical cross section (again the cross section is taken perpendicular to the longitudinal axis of the body 17). The walls 17A and 17C are separated by a distance D and are parallel to each other. The walls 17B and 17D are also parallel to each other. An aperture of width W is provided in the wall 17A. The mechanisms of attenuation are the same as in the embodiment illustrated in Figures 3 and 4, with the one dimensional sonic crystal effect arising due to multiple scattering of incident acoustic waves by the walls 17A and 17C meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). It will be understood that a further one dimensional sonic crystal effect (in a different propagation plane to that provided by walls 17A, 17C) is provided due to the multiple scattering of incident acoustic waves by the walls 17B and 17D meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). In an another alternative embodiment illustrated in Figure 8, an attenuator body 18 has six walls 18A, 18B, 18C, 18D, 18E and 18F arranged in a substantially hexagonal cross section (again the cross section is taken perpendicular to the longitudinal axis of the body 18). The walls 18A and 18C are separated by a distance D and are parallel to each other. An aperture of width W is provided in the wall 18A. The mechanisms of attenuation are the same as in the embodiment illustrated in Figures 3 and 4, with the one dimensional sonic crystal effect arising due to multiple scattering of incident acoustic waves by the walls 18A and 18C meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation). It will be understood that a further one dimensional sonic crystal effect (in a different propagation plane to that provided by walls 18A, 18C) is provided due to the multiple scattering of incident acoustic waves by the walls 18B and 18E meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). A yet further one dimensional sonic crystal effect (in different propagation planes to those provided by walls 18A, 18C and walls 18B, 18E) is provided due to the multiple scattering of incident acoustic waves by the walls 18D and 18F meeting the Bragg condition defined by the gap between them (and consequent (typically destructive) interference and attenuation of the said incident acoustic waves). In other alternative embodiments, the parallel walls providing the one dimensional sonic crystal effect may be provided as parts of (two) separate bodies provided next to each other. For example, as illustrated in cross section in Figure 9, an acoustic attenuator 19 comprises a first attenuator body 20 and a second attenuator body 21. Both first and second attenuator bodies 20 and 21 have identical triangular cross sections (again the cross sections are taken perpendicular to the longitudinal axes of the bodies 20, 21). Attenuator body 20 comprises three walls 20A, 20B and 20C, none of which are parallel to each other. Attenuator body 21 comprises three walls 21A, 21B and 21C, none of which are parallel to each other. The first and second attenuator bodies 20 and 21 are identical but oriented 180° from each other such that wall 20A is parallel to wall 21A. The first and second attenuator bodies 20, 21 are also provided next to each other in a row arrangement, sufficiently close to each other that a portion of wall 20A is opposite a portion of wall 21A, the shortest distance between said portions of walls 20A and 21A being indicated at D in Figure 9. Open apertures of width W are provided in both walls 20A and 21 A. Each attenuator body 20 and 21 functions separately to attenuate acoustic waves by the local resonance effect described above (typically with substantially the same resonance frequency bands). In addition, the two attenuator bodies 20 and 21 together function synergistically to attenuate acoustic waves due to the one dimensional sonic crystal effect discussed above. More specifically, acoustic waves having a frequency and angle of incidence on the parallel and opposing portions of walls 20A, 21 A satisfying the Bragg condition defined by the distance D between them are multiply scattered by the said parallel and opposing portions of the walls 20A and 21A such that the said incident acoustic waves consequently (typically destructively) interfere with each other and are thereby attenuated. Another alternative acoustic attenuator 22 is illustrated in cross section in Figure 10, the alternative acoustic attenuator 22 comprising a first attenuator body 23 and a second attenuator body 24, each individually having a trapezoidal cross-section (again the cross-sections are taken perpendicular to longitudinal axes of bodies 23, 24) as described above with respect to the acoustic attenuator 10 illustrated in Figures 3 and 4. Indeed, attenuator bodies 23, 24 are identical to each other, but oriented at 180° to each other. Attenuator body 23 comprises four walls 23A, 23B, 23C and 23D, walls 23A and 23C being parallel and separated by a distance D1. Attenuator body 24 comprises four walls 24A, 24B, 24C and 24D, walls 24A and 24C being parallel and separated by a distance D2. Open apertures of width W are provided at intermediate portions of the walls 23A and 24A. The two attenuator bodies 23 and 24 are positioned such that walls 23A and 24A are parallel and adjacent to each other, the apertures in walls 23A and 24A also being in fluid communication with each other, facing each other, and with a direct line of sight between them. The two attenuator bodies 23 and 24 (and thus the open apertures of the attenuator bodies 23, 24) are separated by a distance D3. Each attenuator body 23 and 24 functions separately to attenuate acoustic waves due to the local resonance effect described above. The resonance frequency bands of the bodies 23, 24 are typically the same, or at least there is some overlap between them. The local resonance effect in each body, 23 or 24, is strengthened by the presence of the other. Resonance of air (or other fluidic host medium) in the cavity of body 23 stimulates resonance of air (or other fluidic host medium) in the cavity of body 24, and resonance of air (or other fluidic host medium) in the cavity of body 24 stimulates resonance of air (or other fluidic host medium) in the cavity of body 23. This strong resonance coupling between bodies 23, 24 leads to a stronger acoustic wave attenuation at least in the overlapping portions of the resonant frequency bands of the bodies 23, 24. The two bodies 23 and 24 also attenuate acoustic waves due to a number of different one dimensional sonic crystal effects. More specifically, acoustic waves with frequencies and angles of incidence satisfying Bragg conditions defined by the spacings (D1 , D2, D1 +D3, D2+D3, D1 +D2+D3 - see below) between any pairs of parallel walls taken from the group 23A, 23C, 24A and 24C are multiply scattered, and thus attenuated by the attenuator 22 by the one dimensional sonic crystal effect described above. Scattered waves from different pairings of parallel walls lead to attenuation of acoustic waves of different frequencies determined by Bragg conditions defined by the spacings between them. For example, walls 23A and 23C are separated by a distance D1 and walls 24A and 24C are separated by a distance D2. Walls 23A and 24A are separated by a distance D3, and walls 23C and 24C are separated by a distance D1 +D2+D3. Walls 23A and 24C are separated by a distance D2+D3. Walls 24A and 23C are separated by a distance D1+D3. Each different spacing defines a different Bragg condition. The attenuator 22, therefore, provides several possible frequency bands for acoustic attenuation. It will be understood that typically D1 and D2 are substantially equal, and so the Bragg conditions defined by the spacings between walls 23A, 23C and between walls 24A, 24C are typically the same or similar. In an advantageous embodiment, D1 , D2 and D3 are equal. In this case, the Bragg conditions defined by the spacings D1 , D2 and D3 are the same. This provides an enhanced sonic crystal attenuation effect for acoustic waves satisfying these Bragg conditions. An alternative acoustic attenuator 25 is illustrated in cross section in Figure 1 1 , comprising a first attenuator body 26 and a second attenuator body 27, each having a substantially circular (i.e. circular but for the presence of apertures - see below) cross-section (again the cross sections are taken perpendicular to the longitudinal axes of bodies 26, 27). The attenuator bodies 26 and 27 have the same substantially circular cross-sectional circumferences indicated by diameter D4 on Figure 1 1. Apertures of width W are provided in both bodies 26 and 27. The attenuator bodies 26, 27 are identical, but oriented at 180° to each other such that their open apertures face each other and are in fluid communication with a direct line of sight between them. The two attenuator bodies 26 and 27 are positioned adjacent to each other, the shortest distance between the two bodies being indicated at D5 in Figure 11. The attenuator bodies 26 and 27 function separately to attenuate acoustic waves due to the local resonance effect described above. Moreover, the two attenuator bodies 26 and 27 together function synergistically. More specifically, the acoustic wave attenuation due to local resonance of the air in either body 26 or 27 is strengthened by the presence of the other. Resonance of the air (or other fluidic host medium) in the cavity of body 26 stimulates resonance of the air (or other fluidic host medium) in the cavity of body 27, and resonance of the air (or other fluidic host medium) in the cavity of body 27 stimulates resonance of the air (or other fluidic host medium) in the cavity of body 26. This leads to a stronger coupling between bodies 26, 27, providing stronger acoustic wave attenuation at frequencies within the resonant frequency bands of the bodies 26, 27. An alternative acoustic attenuator 30 is illustrated in Figure 12 and comprises an elongate body 31 having a spiral shape in cross section. The acoustic attenuator 30 has a length L and a diameter D and comprises an aperture 32 having a length substantially equal to the length L of the elongate body 31. In alternative embodiments, the length of the aperture may be less than the length L of the elongate body 31 (for example, the length of the aperture may be at least 50%, at least 75% or at least 90% of the length L). In other embodiments, the aperture 32 may comprise a plurality of discrete apertures along the length L of the elongate body 31. The acoustic attenuator 30 has a first end 33 and second end 34 which are open. It may be in other embodiments that one or both of the first end 33 and the second end 34 are covered or partially covered. The spiral cross section of the acoustic attenuator 30 defines an internal cavity 35 which is in fluid communication with its surroundings through the aperture 32, the first end 33, and the second end 34. The internal cavity 35 is geometrically configured to attenuate incident acoustic waves over a particular frequency band. More specifically, the spiral shaped cavity 35 defines a propagation path between the aperture 32 and the centre of the elongate body 31 , the propagation path having a path length. Incident acoustic waves with a wavelength substantially equal to four times the path length between the aperture 32 and the centre of the elongate body 31 are attenuated. This is because, by the time such incident waves travel along the spiral path and are reflected back to the aperture 32, incident acoustic waves are 180° out of phase. The reflected and incident acoustic waves interfere destructively with one another to thereby attenuate the incident acoustic waves. The individual acoustic attenuators as illustrated in Figures 1-12 may be used specifically to attenuate unwanted acoustic waves emitted by a transformer, typically as part of an acoustic barrier. Figure 13 illustrates schematically one particular embodiment of the present invention. A transformer 36 comprises a transformer kernel 37 (comprising a transformer core, windings and other associated electronics) housed within a transformer housing 38. The transformer 36 may be a dry or an oil- immersed transformer. The transformer 36 emits various types of acoustic noise, including core noise (caused by magnetostriction of the transformer core), load noise (caused by vibrations of the windings due to the interaction between the current in said windings and the leakage magnetic flux generated by the current in said windings) and housing noise caused by various vibrations of the transformer housing 38. Core noise and load noise emanate principally from the transformer kernel 37 while housing noise emanates principally from the transformer housing 38. Acoustic noise may also be emitted by electrical circuitry and components associated with the transformer, such as inverters. Acoustic attenuators of the type shown in Figures 1- 12 can also be used to attenuate such noise, again typically as part of a barrier. In the acoustic barrier configuration illustrated in Figure 13, the transformer 36 is surrounded on four sides by acoustic attenuators 39 arranged in rows a single layer deep. Adjacent acoustic attenuators abut each other so as to form a solid panel surrounding the transformer 4 on all four sides. The acoustic attenuators 39 are individually of the same type as the acoustic attenuators of Figure 1 and may be constructed from any of various materials including metals, glasses, pyrex or other plastics. However, it will be understood that the acoustic attenuators 39 may alternatively take any of the forms previously described (e.g. with reference to any of Figures 1-12). The acoustic attenuators 39 are provided outside the transformer housing 38 in propagation paths of acoustic waves emitted by the transformer 36 such that acoustic waves emitted by the transformer 36 are incident on the acoustic attenuators 39. The acoustic attenuators 39 are spaced apart from the housing 38. The acoustic properties of the acoustic attenuators 39 are tuned to attenuate particular frequency bands of acoustic waves emitted by the transformer 36 by modification of the volumes and aperture widths of each individual acoustic attenuator. Although schematically shown to be the same size, typically three different sizes of acoustic attenuator 39 are provided as part of the enclosure. For example, where the mains electrical supply frequency is 50Hz (e.g. in the UK), a first group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 100Hz, a second group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 200Hz, and a third group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 300Hz. This helps to attenuate load noise generated by a transformer operating at 50Hz. For a mains electrical supply frequency of 60Hz, it may be that the first group is tuned to attenuate acoustic waves having a frequency of 120Hz, a second group is tuned to attenuate acoustic waves having a frequency of 240Hz and a third group is tuned to attenuate acoustic waves having a frequency of 360Hz. Although the individual acoustic attenuators 39 are shown in Figure 14 as abutting each other, they may alternatively be spaced apart from one another. In either case, they are typically mechanically coupled to each other, e.g. by fixed attachment to a common frame extending between them (or a direct coupling between adjacent acoustic attenuators 39). Acoustic noise attenuation may be maximised when the acoustic attenuators are connected directly to one another to form a solid panel. However, by providing gaps between the individual acoustic attenuators 39, air or light or fluids (such as rain water) may flow freely through gaps between the acoustic attenuators 39, which can help cooling of the transformer 36 (if required, see below). Larger gaps may be provided between the acoustic attenuators such that access to the transformer 36 is not hindered (for example, for maintenance work or the inspection or instalment of new parts). Alternatively, the acoustic barriers may be provided close together (e.g. so that adjacent acoustic attenuators abut each other) so as to form a rigid security fence around the transformer 36, thus preventing unwanted access to the transformer 36 or protecting members of the public from the high electrical voltages present. In an alternative embodiment illustrated schematically in Figures 15 and 16, a four sided enclosure formed by a two-layer two dimensional array of acoustic attenuators 39 surrounds the transformer 36. More specifically, one each of the four sides of the enclosure, the transformer 36 is surrounded by a plurality of acoustic attenuators 39 arranged into two subsequent rows (or layers) comprising a first layer of acoustic attenuators 40 and a second layer of acoustic attenuators 41. Again, the first layer and second layer of acoustic attenuators 40 and 41 comprise acoustic attenuators of the type shown in Figure 1 , but it will be understood that they may comprise any of the types of acoustic attenuator shown in Figures 1-12. Although schematically shown to be the same size, typically three different sizes of acoustic attenuator 39 are provided as part of the enclosure. For example, where the mains electrical supply frequency is 50Hz (e.g. in the UK), a first group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 100Hz, a second group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 200Hz, and a third group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 300Hz. This helps to attenuate load noise generated by a transformer operating at 50Hz. For a mains electrical supply frequency of 60Hz, it may be that the first group is tuned to attenuate acoustic waves having a frequency of 120Hz, a second group is tuned to attenuate acoustic waves having a frequency of 240Hz and a third group is tuned to attenuate acoustic waves having a frequency of 360Hz. The acoustic attenuators 39 are oriented with their individual apertures directed towards the transformer housing 38. However, the acoustic attenuators may be oriented in alternative directions. For example, the apertures of each of the acoustic attenuators may face away from the centre of the transformer housing 38, or the individual acoustic attenuators may be arranged in otherwise ordered or random orientations. Optimal acoustic attenuation occurs when the acoustic attenuators are oriented with the apertures directed towards the transformer housing 38. Similarly, the acoustic attenuators may be arranged with their longitudinal axes generally parallel to or perpendicular to the horizontal, or they may be arranged at any angle in between. The acoustic attenuators may be connected to one another (e.g. by fixed attachment to a common frame extending between them or clamped to adjacent attenuators) or they may be spaced apart from one another by gaps. The acoustic attenuators in the first and second layers 40 and 41 are arranged in a two-dimensional periodic lattice. Such an arrangement allows a further mechanism of acoustic attenuation to attenuate incident acoustic waves. More specifically, the attenuators of the adjacent rows 40, 41 are arranged to form a finite sonic crystal. That is, acoustic waves emitted by the transformer 36 and having a frequency and angle of incidence on the first and second rows 40, 41 which satisfy the Bragg condition defined by the spacing between subsequent rows 40, 41 (e.g. the spacing between the centres of adjacent acoustic attenuators 39 of the first and second rows 40, 41) are attenuated by a similar sonic crystal effect to that described above with respect to the acoustic attenuator 10 of Figure 3. When the Bragg reflection condition defined by the gap between subsequent rows 40, 41 is not satisfied, incident acoustic waves are not multiply scattered by the first and second rows, leading to little or no reflection of such incident acoustic waves from the sonic crystal formed by the subsequent layers and so such acoustic waves are not significantly attenuated. The spacing between adjacent rows 40, 41 may be tuned to maximise attenuation of particular wavelengths of acoustic waves emitted by the transformer 36. In general, acoustic waves attenuated by this sonic crystal effect will be of a higher frequency than those acoustic waves attenuated by stimulation of local resonance of air (or other fluidic host medium) within the cavities of the individual acoustic attenuators. The acoustic barriers/enclosure illustrated in Figure 16, therefore, is able to attenuate both low and high frequency acoustic waves emitted by the transformer 36. In alternative embodiments, it may be that the periodic array of acoustic attenuators surrounding the transformer 36 are three, four, five, or more, layers deep. Subsequent layers of acoustic attenuators improve the attenuation of the acoustic noise. In a further embodiment illustrated in Figures 17 and 18, the transformer 36 is again surrounded on four sides by acoustic attenuators 39 arranged in rows a single layer deep. Adjacent acoustic attenuators abut each other so as to form a solid panel surrounding the transformer 4 on all four sides. In this case, the acoustic attenuators 39 are mounted to four sides of the transformer housing 38 to form a four sided enclosure comprising the transformer 36. Again the acoustic attenuators 39 illustrated in Figure 18 are those described in Figure 1 , but it will be understood that any of the acoustic attenuators described in Figures 1 to 12 could be used. Although the open apertures of the acoustic attenuators are shown facing away from the transformer housing 38, it will be understood that the open apertures may alternatively face the transformer 36 (or some may face the transformer 36, and some may face away from the transformer 36). Although schematically shown to be the same size, typically three different sizes of acoustic attenuator 39 are provided as part of the enclosure. For example, where the mains electrical supply frequency is 50Hz (e.g. in the UK), a first group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 100Hz, a second group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 200Hz, and a third group of acoustic attenuators 39 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 300Hz. This helps to attenuate load noise generated by a transformer operating at 50Hz. For a mains electrical supply frequency of 60Hz, it may be that the first group is tuned to attenuate acoustic waves having a frequency of 120Hz, a second group is tuned to attenuate acoustic waves having a frequency of 240Hz and a third group is tuned to attenuate acoustic waves having a frequency of 360Hz. In another alternative embodiment, it may be that the acoustic attenuators form at least part (or all) of the transformer housing 38 themselves (for example for a dry transformer, particularly if gaps are provided between adjacent attenuators 39). In another alternative embodiment as illustrated in Figures 19 and 20, the acoustic attenuators 39 are provided inside the transformer housing 38, the acoustic attenuators 39 being arranged to form a four sided enclosure 43 surrounding the transformer kernel 37 (and provided between the transformer kernel 37 and the transformer housing 38). Such a configuration may be suitable for both oil-immersed transformers (in which case it may be that the fluidic host medium comprises oil) and dry transformers (in which case it may be that the fluidic host medium comprises air). For an oil-immersed transformer, the acoustic attenuators 39 are typically required to be larger than those used in a dry transformer, to account of the change in the speed of sound in oil as compared to the speed of sound in air (given that the attenuators will be immersed in oil). By providing the acoustic attenuators 39 inside the transformer housing 38, said acoustic attenuators 39 are able to attenuate core noise emanating from the transformer kernel 17. The properties (e.g. cavity volumes, aperture widths) of the acoustic attenuators 39 are tuned to attenuate acoustic waves with frequencies substantially similar to those frequencies of acoustic waves known to be emitted by the transformer kernel (for example 100 Hz, 200 Hz and 300 Hz for a 50 Hz transformer, or 120 Hz, 240 Hz and 360 Hz for a 60 Hz transformer - see above). Such acoustic attenuators 39 can also be tuned to attenuate acoustic waves with frequencies substantially similar to the resonant frequencies of the transformer housing 38, such that the acoustic attenuators 39 inhibit resonance of said housing 38. A transformer 36 comprising acoustic attenuators 39 within the transformer housing 38 may also be combined with acoustic barriers external to the transformer housing 38 such that the attenuation of acoustic waves in enhanced, while each set of acoustic attenuators attenuates acoustic waves over different frequency ranges. Again the acoustic attenuators 39 illustrated in Figure 20 are those described in Figure 1 , but it will be understood that any of the acoustic attenuators described in Figures 1 to 12 could be used. Figures 21 and 22 illustrate an oil-immersed transformer 52 comprising a cooling system 53 for dissipating heat generated by the transformer 52 in use. The oil- immersed transformer 52 comprises the same transformer kernel 37 as the transformer 36 (and so the same numeral is used to label said kernel 37) provided in a transformer housing 38. The transformer kernel 37 is immersed in oil 54 which acts as both an electrical insulator within the transformer 52 and as a coolant. Oil heated by the transformer kernel 37 is pumped by a pump 55 into a cooling tank 59. Similarly, cool water 56 is pumped through a system of pipes 57 through the cooling tank 59 by a pump 58. Heat is transferred from the oil 54 to the water 56 through a heat-conductive wall of the pipes 57 such that heated water leaves the cooling system and cooled oil returns into the transformer housing 38. The heat-conductive wall of the pipes 57 is the only barrier between the oil 54 and water 56 such that heat may flow easily from the oil 54 to the water 56. The cooling system 53 therefore acts as a heat sink for the transformer 52. As shown in Figures 21 and 22, an external acoustic barrier of the type shown in Figure 13 is integrated into this system, but gaps are provided between the individual acoustic attenuators 39. Accordingly, pipes 60 can extend through the enclosure formed by the acoustic attenuators 39 so as to permit oil 54 to flow between the transformer housing 38 and the cooling tank 59 through a gap between acoustic attenuators 39. The acoustic attenuators 39 thus attenuate the acoustic noise generated by the transformer 52 without hindering cooling of the transformer kernel 37. Alternatively, an acoustic barrier may be provided which encloses both the transformer housing 38 and the cooling system 53 such that no pipes are required to pass between the individual acoustic attenuators 39. In an alternative embodiment illustrated in Figures 23 and 24, a plurality of acoustic attenuators 63 is provided which form an integral part of the cooling system of the oil immersed transformer 52. More specifically, as shown most clearly in Figure 24, the acoustic attenuators 63 are arranged in four rows, each of which forms an acoustic barrier provided on a different side of the transformer housing 38. Pipes 61 are provided through which heated oil 54 from the transformer 36 is pumped between the housing 38 and the acoustic attenuators 63. Additional chambers 64 are provided which run through the bodies of acoustic attenuators 63 (see Figure 25) to accept oil 54 from the pipes 61 and to provide cooled oil 54 which is sent back to the housing 38 through pipes 61. Cooling water flows in chambers 65 adjacent to the chambers 64 carrying the heated oil. An (e.g. heat conducting, e.g. metallic) interface 66 between the chambers 64, 65 carrying cooling water 56 and the oil 54 to be cooled allows heat to be transferred from the oil to the water, resulting in effective cooling of the transformer kernel 37. The water heated by this process (not shown) can be pumped away to a heat exchanger and looped back into the chambers 65 carrying the cooling water. As illustrated schematically in Figure 25, the first and second chambers 64 and 65 are sealed by the interface 66 such that fluid may not flow between the two chambers 64 and 65. The second chamber 65 is connected to an external pump (not shown in Figures 23, 24) such that cooling water is pumped into the second chamber 65 from a cool water source and out of the cooling system. Heat is transferred from the heated oil in the first chamber 64 to the cool water in the second chamber 65 through the heat conductive interface 66 such that the cool water acts as a heat sink. Cooled oil is permitted to flow back into the transformer housing 38 through pipes 61. The fluids in the first and second chambers 64 and 65 circulate in opposite directions to maximise the efficiency of heat transfer. The acoustic elements 39 thus act as a heat sink for the transformer 52 because heat transferred from the heated oil and the heated water is transferred to the walls of the acoustic attenuator 39 which dissipate heat to the surroundings. In alternative embodiments, the first and second chambers 64 and 65 may comprise first and second pipes, or first and second arrays of pipes, the pipes in direct contact with one another and spatially configured so as to maximise the interfacial area between the first and second chambers 64 and 65, and therefore to maximise the efficiency of heat transfer from the oil to the water. The acoustic elements 63 perform a dual function, attenuating acoustic noise generated by the transformer and acting as a heat sink for the transformer. The generally large surface areas of the acoustic elements 63 provide more effective cooling of the transformer. The integration of the cooling system into the body of the acoustic elements 63 does not have a detrimental effect on the acoustic attenuation capabilities of the acoustic attenuators because the acoustic attenuation is principally dependent on the shape and periodic arrangement of the individual acoustic attenuators and is not substantially dependent on the material used to make said acoustic attenuators. Again the acoustic attenuators 39 illustrated in Figure 24 are based on those described in Figure 1 , but it will be understood that any of the acoustic attenuators described in Figures 1 to 12 could be adapted to provide chambers 64, 65 and interface 66. With reference to Figure 26, an acoustic barrier 70 comprises a plurality of acoustic attenuators 10 as illustrated in Figures 3 and 4. The acoustic attenuators 1 are arranged periodically in each of four rows of acoustic attenuators 10 which together provide a four sided enclosure comprising the transformer 36. The acoustic attenuators 10 form an acoustic barrier which is one layer thick. The acoustic attenuators 10 are arranged periodically (in this case, the spacing between each pair of adjacent attenuators in each of the four rows of the enclosure is identical). A gap is provided between each pair of adjacent acoustic attenuators 10 in the barrier. The acoustic attenuators 10 are also arranged such that the apertures in the bodies of each of the attenuators 10 have a direct line of sight to the transformer 36. Typically the bodies of the attenuators 10 are fixedly coupled to each other by way of a fixed attachment to a common frame extending between them. Typically the transformer 36 comprises a cooling fan which blows heated air out from the enclosure formed by the acoustic attenuators 10 through the gaps between adjacent acoustic attenuators 10. Cool air is permitted to replace the heated air blown out from the enclosure, said cool air entering the enclosure through the gaps between adjacent attenuators. The acoustic barrier 70 attenuates at least some of the acoustic waves generated by the transformer 36 or by the fan. In particular, the acoustic barrier 70 attenuates those acoustic waves generated by the transformer 36 or by the fan with frequencies which stimulate resonance of the air in the individual cavities of the acoustic attenuators 10 and with frequencies and angles of incidence on the walls 12A, 12C of the attenuators 10 which satisfy the Bragg condition defined by the spacing between them (sonic crystal effect). The frequencies of acoustic waves attenuated by the local resonance and sonic crystal effects provided by each of the acoustic attenuators 10 may be the same or they may be different (albeit there may be some overlap between them). For example, although schematically shown in Figure 26 as being of uniform size, it may be that different sizes of acoustic attenuator 10 are provided as part of the enclosure. For example, where the mains electrical supply frequency is 50Hz (e.g. in the UK), a first group of acoustic attenuators 10 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 100Hz and a second group of acoustic attenuators 10 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 200Hz. The distance between the parallel walls 12A, 12C of some or all of the attenuators 10 may be selected such that the one dimensional sonic crystal attenuation effect provided by the individual acoustic attenuators causes acoustic waves of frequencies at 300Hz to be attenuated. For a mains electrical supply frequency of 60Hz, it may be that the first group is tuned to attenuate acoustic waves having a frequency of 120Hz (by local resonance), a second group is tuned to attenuate acoustic waves having a frequency of 240Hz (by local resonance) and the spacings between parallel walls 12A, 12C of the individual attenuators 10 being selected to attenuate acoustic waves having a frequency of 360Hz (by the one dimensional sonic crystal effect). An alternative acoustic barrier 80 is illustrated in Figure 27. The acoustic barrier 80 comprises a single layer of acoustic attenuators 20 as illustrated in Figure 9, the acoustic attenuators 20 again being arranged periodically in each of four rows which together provide a four sided enclosure comprising the transformer 36. Typically the bodies of the attenuators 20 are fixedly coupled to each other by way of a fixed attachment to a common frame. Each adjacent pair of acoustic attenuators 20 within each of the four rows of the barrier is separated by a gap so that heated air can be blown out of the enclosure (as above, by a fan) through the gaps between adjacent acoustic attenuators 20 and cooled air can enter the enclosure through the said gaps. The acoustic barrier 80 attenuates acoustic waves generated by the transformer 36 which stimulate resonance of the air in the individual cavities of the acoustic attenuators 20 and with frequencies and angles of incidence on walls 20A, 21 A which satisfy the respective Bragg condition defined by the spacing between them (see above). As above, the frequencies of acoustic waves attenuated by the local resonance and sonic crystal effects provided by each of the acoustic attenuators 20 may be the same or they may be different (albeit there may be some overlap between them). An alternative acoustic barrier 90 is illustrated in Figure 28. The acoustic barrier 90 comprises a single layer of acoustic attenuators 22 as illustrated in Figure 10, the acoustic attenuators 22 again being arranged periodically in each of four rows which together provide a four sided enclosure comprising the transformer 36. Typically the bodies of the attenuators 22 are fixedly coupled to each other by way of a fixed attachment to one or more frames extending between them. Each adjacent pair of acoustic attenuators 22 within each of the four rows of the barrier is separated by a gap so that heated air can be blown out of the enclosure (as above, by a fan) through the gaps between adjacent acoustic attenuators 22 and cooled air can enter the enclosure through the said gaps. The acoustic barrier 90 attenuates acoustic waves generated by the transformer 36 which stimulate resonance of the air in the individual cavities of the acoustic attenuators 22 and with frequencies and angles of incidence on any pairs of walls 23A, 23C, 24A, 24C of the attenuators 22 which satisfy the respective Bragg conditions defined by the spacing between the said pairs of walls (see above). As above, the frequencies of acoustic waves attenuated by the local resonance and sonic crystal effects provided by each of the acoustic attenuators 22 may be the same or they may be different (albeit there may be some overlap between them). For example, it may be that (although schematically shown in Figure 28 as being of uniform size), different sizes of acoustic attenuator 22 are provided as part of the enclosure. For example, where the mains electrical supply frequency is 50Hz (e.g. in the UK), a first group of acoustic attenuators 22 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 100Hz and a second group of acoustic attenuators 22 may be tuned to attenuate acoustic waves (by stimulation of resonance of air within the cavity) having frequencies around 200Hz. The distance between the parallel walls 23C, 24A (or between parallel walls 23A, 24C, or indeed between any pair of parallel walls of the group 23A, 23C, 24A, 24C) of some or all of the attenuators 22 may be selected such that the one dimensional sonic crystal attenuation effect provided by the individual acoustic attenuators 22 causes acoustic waves of frequencies at 300Hz to be attenuated. For a mains electrical supply frequency of 60Hz, it may be that the first group is tuned to attenuate acoustic waves having a frequency of 120Hz (by local resonance), a second group is tuned to attenuate acoustic waves having a frequency of 240Hz (by local resonance) and the spacings between parallel walls 23C, 24A of the individual attenuators 22 being selected to attenuate acoustic waves having a frequency of 360Hz (by the one dimensional sonic crystal effect). In some embodiments, it may be that D1 =D2=D3, and that D1 is selected to attenuate acoustic waves of frequency 300Hz (or 360Hz) to thereby provide a stronger one dimensional sonic crystal effect comprising four scattering surfaces (23A, 23C, 24A, 24C) with a periodic gap between them. An alternative acoustic barrier 100 is illustrated in Figure 29. The acoustic barrier 100 comprises a single layer of acoustic attenuators 25 as illustrated in Figure 1 1 , the acoustic attenuators 25 again being arranged periodically in each of four rows which together provide a four sided enclosure comprising the transformer 36. Typically the bodies of the attenuators 25 are fixedly coupled to each other by way of a fixed attachment to one or more frames extending between them. Each adjacent pair of acoustic attenuators 25 within each of the four rows of the barrier is separated by a gap so that heated air can be blown out of the enclosure (as above, by a fan) through the gaps between adjacent acoustic attenuators 25 and cooled air can enter the enclosure through the said gaps. The acoustic barrier 100 attenuates those acoustic waves generated by the transformer 36 with frequencies which stimulate resonance of the air in the individual cavities of the acoustic attenuators 25. Figure 30 shows six acoustic attenuator bodies 16 of the type shown in Figure 6 and described above with reference thereto, the said six acoustic attenuator bodies 16 being arranged into three pairs 1 10, 1 12, 114 which are themselves arranged in a row with gaps being provided between adjacent pairs. The attenuator bodies 16 within each pair are oriented at 180° to each other such that their walls 16C are adjacent to each other and abut each other. It may be that the walls 16C of the attenuator bodies 16 within each pair are mechanically coupled to each other (e.g. they may be fastened or bonded to each other). The walls 16A of the attenuator bodies 16, and the elongate open apertures 13 provided in walls 16A, are provided opposite the walls 16C such that fluid can flow into and out of the cavities 14 of the attenuator bodies 16 through the open apertures 13 without obstruction from the other attenuator bodies 16 of the pair. The open apertures 13 of the attenuator bodies 16 of the second pair 112 face open apertures of attenuator bodies 16 of the first and third pairs 1 10, 1 14 respectively. The cavities 14 and open apertures 13 of each of the attenuator bodies 16 define resonant frequency bands which at least partially overlap (and which in fact are identical in the embodiment shown). In addition, the gaps provided between the open apertures which face each other are sized such that fluid resonating in the cavity 14 of an attenuator body 16 of the second pair 1 12 stimulates fluid resonance of fluid in the cavity 14 of the attenuator body 16 of the first or third pair 1 10, 1 14 whose aperture its own aperture faces (and typically vice versa) through the facing apertures (at least when the resonance occurs at a frequency within the overlapping portion of the said resonant frequency bands of the attenuator bodies). The attenuator bodies 16 of the said pairs define resonant frequency bands comprising one or more frequencies of acoustic wave emitted by the transformer 36 and/or components or circuitry (such as inverters) associated with the transformer. Accordingly, the first, second and third pairs 110-1 14 may be provided as part of an acoustic barrier for attenuating acoustic waves emitted by the transformer 36, housing 38 or (e.g. electrical) components or circuitry (such as inverters) associated with the transformer. This arrangement helps to increase the fluid resonance coupling between attenuators per unit volume, which in turn helps to increase the attenuation provided by the attenuators, and increases the overall resonant frequency spectrum of the attenuators (thereby increasing the frequencies across which the attenuators attenuate acoustic waves). It may be that the apertures 13 do not directly face the acoustic waves emitted by the transformer apparatus. For example, the transformer apparatus may emit acoustic waves towards the pairs of attenuator bodies 110, 112, 1 14 from the left or right hand side in the view of Figure 30. It will be understood that the pairs of attenuators 1 10, 112, 1 14 may comprise attenuator bodies of any alternative suitable shape. As shown in Figure 30 it may be that the attenuator bodies of each of the pairs 110, 1 12, 1 14 are identical to each other (albeit within each pair it may be that the bodies are oriented differently from each other). Alternatively the attenuator bodies within each of the pairs 1 10, 1 12, 1 14 may be provided with different sizes and/or shapes from each other. For example, as illustrated in Figure 31 , a plurality of identical pairs of attenuator bodies 1 10', 1 12', 1 14' may be provided, each pair comprising first and second attenuator bodies 15', 15" of the type shown in Figure 5 (similar features will be referred to using the same reference numerals as Figure 5 but also including ' and " respectively therein) and described above with reference thereto which have the same shape (with a square cross section perpendicular to their longitudinal axes) but different sizes from each other. That is, the first attenuator body 15' of each pair is of a smaller size than the second attenuator body 15" of that pair. The first and second attenuator bodies 15', 15" of each pair have adjacent and abutting faces 15'C, 15"C. The face 15"C of the second attenuator 15" completely overlaps and extends beyond the face 15'C of the first attenuator 15'. As the volumes of the cavities defined by the attenuator bodies 15', 15" are different from each other, the attenuator bodies 15', 15" have different resonant frequency bands (which may or may not overlap with each other). As shown in Figure 31 , the second attenuator body 15" of the first pair 1 10' faces the first attenuator body 15' of the second pair 1 12' and the second attenuator body 15" of the second pair faces the first pair 15' of the third pair. Where there is no overlap in the resonant frequency bands of the attenuator bodies 15', 15", it may be that there is no resonant coupling between the attenuator bodies of adjacent pairs. It may be that different pairs of attenuators within the row have different shapes and/or sizes and/or resonant frequency bands from other pairs of that row. For example, as shown in Figure 32, a row may comprise two adjacent inner pairs 120, 122 of attenuator bodies 16 of the type shown in Figure 6 and two outer pairs 124, 126 of attenuator bodies 16' of the type shown in Figure 6 (similar features of body 16' to body 16 will be referred to using the same reference numerals but also including ' therein), each of the outer pairs being adjacent to one of the inner pairs 120, 122 (the attenuator bodies within each pair being arranged as set out above with respect to Figure 30). Within each pair 120, 122, 124, 126, the attenuator bodies have the same size, shape and resonant frequency band. However, the attenuator bodies of the inner pairs 120, 122 are of a smaller size (and have different resonant frequency bands) from the outer pairs 124, 126. The inner pairs 120, 122 are identical to each other, while the outer pairs 124, 126 are identical to each other. As shown in Figure 32, apertures 13 of the attenuator bodies 16 of the inner pairs 120, 122 provided opposite each other face each other. As those attenuator bodies 16 have identical resonant frequency bands, there is a strong resonance coupling between the attenuator bodies 16 of the inner pairs 120, 122. However, as the attenuator bodies 16, 16' do not have even partially overlapping frequency bands, there is little (if any) resonance coupling between the inner pair 120 and outer pair 126 and between inner pair 122 and outer pair 124 despite the fact that apertures 13 of attenuator bodies 16 of the said inner pairs 120, 122 face and are in fluid communication with apertures 13' of attenuator bodies 16' of the adjacent outer pairs 124, 126. In some embodiments, it may be that the four pairs of attenuators 120-126 are a repeating unit of attenuators which are stacked on top of each other in use. In this case, there will be adjacent pairs of attenuators 16' between which there is resonance coupling (between adjacent repeating units). As shown in Figure 33, the pairs 120-126 may be re-arranged such that pairs 120, 124 are the inner pairs and pairs 126 and 122 are the outer pairs. In this case, there will be no resonance coupling between any of the pairs 120-126 within the row of four pairs 120-126. However, it may be that the four pairs of attenuators 120-126 are a repeating unit of attenuators which are stacked on top of each other in use. In this case, there may be adjacent pairs of attenuators 16' between a first pair of repeating units and/or adjacent pairs of attenuators 16 between a second pair of repeating unit which provide resonance coupling. As shown in Figure 34, first and second rows 130, 132 may be provided, the first row 130 comprising three adjacent pairs 134, 136, 138 of attenuator bodies 16' (of the type shown in Figure 6 and described above with reference thereto) and the second row 132 comprising three adjacent pairs 140, 142, 144 of attenuator bodies 15 (of the type shown in Figure 5 and described with reference thereto above). The attenuator bodies within each pair are identical to each other and are arranged as described above with reference to Figure 30. The pairs within each row 130, 132 are identical to each other and are arranged as described above with reference to Figure 30, and therefore resonance coupling occurs between attenuator bodies 16' of the pair 136 of the first row and a respective attenuator body 16' of adjacent pairs 134, 138. Similarly, resonance coupling occurs between attenuator bodies 15 of the pair 142 of the second row 132 and a respective attenuator body of the adjacent pairs 140, 144. The cavities and apertures of the attenuator bodies of the first row 130 at least partly define first resonant frequency bands across which they attenuate incident acoustic waves (by stimulation of resonance of fluid within the cavities by incident acoustic waves) and apertures and cavities of the attenuator bodies of the second row define second resonant frequency bands across which they attenuate incident acoustic waves (by stimulation of resonance of fluid within the cavities by incident acoustic waves) different from the first resonant frequency bands. The attenuator bodies 16' of each pair 134, 136, 138 of the first row 130 are provided opposite respective attenuator bodies 15 of each pair 140, 142, 144 of the second row 132. The first and second rows 130, 132 are spaced from each other by a gap. The distances between the opposing walls 16'B, 16'D of the attenuator bodies 16' of the first row 130 are different from the distances between the opposing walls 15B, 15D of the attenuator bodies 15 of the second row 132, so that they define different frequencies (or frequency bands) across which the opposing walls of those attenuator bodies scatter incident acoustic waves such that the said scattered acoustic waves interfere with each other and are thereby attenuated. The distance between the opposing walls 16'D, 15B of opposing attenuators between the first and second rows 130, 132 may be the same as the distances between opposing walls 16'B, 16'D of the attenuator bodies 16' of the first row 130 or as the distances between the opposing walls 15B, 15D of the attenuator bodies 15 of the second row 132 (so as to improve the attenuation effect provided thereby) or the distance between the opposing walls 16'D, 15B of opposing attenuators between the first and second rows 130, 132 may be different from the distances between opposing walls 16'B, 16'D of the attenuator bodies 16' of the first row 130 and the distances between the opposing walls 15B, 15D of the attenuator bodies 15 of the second row 132 so as to define a further frequency across which incident acoustic waves are scattered such that they interfere with each other and are thereby attenuated. Typically the said distances are selected to attenuate frequencies of acoustic wave emitted by the transformer apparatus. The first and second rows 130, 132 are typically spaced from each other to thereby define a further resonant frequency band across which incident acoustic waves are attenuated. The first and second rows 130, 132 may be first and second rows of a plurality of rows which are spaced (e.g. periodically) from each other to thereby define a further resonant frequency band across which incident acoustic waves are attenuated. Typically the said spacing is selected to attenuate frequencies of acoustic wave emitted by the transformer apparatus. As shown in Figure 35, it may be that instead of a gap being provided between the first and second rows, opposing attenuator bodies of the first and second rows may abut each other. As also shown in Figure 35, it may be that the first faces 15A, 16Ά of the opposing attenuator bodies between rows are flush with each other. Alternatively, as shown in Figure 36 (in which an alternative second row 132' comprising pairs of attenuators 16 of the type shown in Figure 6 are provided instead of the attenuators 15 of the type shown in Figure 5), it may be that first faces 16A, 16Ά of the opposing attenuator bodies are not flush with each other. In the embodiment shown in Figure 36, the first faces 16A of the attenuators 16 of the second row 132' are set back from the from the first faces 16A' of the attenuators 16' of the first row 130. It is typically preferable for the first faces of the opposing attenuators of the first and second rows to be flush with each other so as to provide a more practical arrangement for use in an acoustic barrier, and to enhance the fluid coupling effect which can be achieved with attenuator bodies of adjacent pairs of attenuator bodies within each row (as the distance between opposing apertures within the row will be decreased), where possible. Figure 37 shows adjacent first and second rows 130", 132", the first row comprising adjacent pairs 134", 136'" of attenuator bodies 15" of the type shown in Figure 5 and described above with reference thereto arranged as described above with reference to Figure 30, the second row also comprising pairs 140", 142" of attenuator bodies 15"' of the type shown in Figure 5 and described above with reference thereto (similar features will be referred to using the same reference numerals as Figure 5 but also including " and "' respectively therein), but where the adjacent faces 15"'C of adjacent attenuator bodies 15"' are spaced apart from each other within the second row 132" such that the first faces 15"'A of the attenuator bodies of the second row are flush with the first faces 15"'A of the attenuator bodies of the first row 130" which they oppose. As above, this provides a practical arrangement for using the first and second rows 130", 132" in an acoustic barrier, and to enhance the fluid resonance coupling effect which can be achieved between adjacent attenuator bodies 15"' of adjacent pairs within the second row 132" (as the distance between opposing apertures within the row will be decreased). Figure 38 shows first (top), second (middle) and third (bottom) attenuator bodies 15^IV which are similar to the attenuator bodies 15 shown in Figure 5, but wherein in each case a second elongate open aperture 13 is provided in the wall 15^I C, the said second open aperture 13^v being in fluid communication with the cavity 14^IV of the attenuator body. Similar features will be referred to using the same reference numerals as Figure 5 but also including ^lv therein. The first, second and third attenuator bodies 15^IV are arranged in a row with gaps provided between adjacent attenuator bodies 15^IV. The two open apertures 13^IV, 13^v of each attenuator body 15^IV are offset from each other around the longitudinal axis of the attenuator body 15^IV and are provided directly opposite each other. By providing the open apertures 13^IV, 13^v, two masses of fluid will resonate within the cavity (as a result of fluid being able to flow into and out of the cavity through the two apertures 13^IV, 13^v) when acoustic waves emitted by the transformer apparatus having a frequency within the resonant frequency band of the attenuator body 15 are incident on the apertures 13 , 13 , thereby significantly increasing the acoustic attenuation provided by the attenuator body 15^IV. The symmetry provided by having the open apertures 13^I , 13^v directly opposite each other helps to optimise the resonance (and thus acoustic attenuation) performance of the attenuator body 15^IV. In addition, each of the open apertures 13^IV, 13^v can resonantly couple the cavity 14^IV defined by each said attenuator body 15^I to the cavities of first and second adjacent ("nearest neighbour") attenuator bodies. This helps to improve the resonance coupling effect between attenuator bodies per unit volume (the said cavity of the said body being resonantly coupled to the cavities of two adjacent attenuator bodies), which increases the level of attenuation provided. This also helps to broaden the frequency range of attenuation provided. In the illustrated embodiment, the first, second and third attenuator bodies are identical to each other and are therefore provided with identical resonant frequency bands. The open apertures 13^IV, 13^V of the second attenuator body 15^IV face open apertures of the first and third attenuator bodies 15^IV respectively. The gaps provided between the open apertures which face each other are sized such that fluid resonating within the cavity 14^IV of the second attenuator body 15^IV stimulates resonance of fluid within the first and third attenuator bodies 15^IV (and typically vice versa) through the facing apertures (at least when the resonance occurs at a frequency within the said resonant frequency bands of the attenuator bodies). By providing the attenuator bodies 15^IV with first and second apertures which each permit fluid resonance coupling with neighbouring attenuators, the fluid resonance coupling between attenuators per unit volume can be increased, which in turn helps to increase the attenuation provided by the attenuator arrangement, and increases the overall resonant frequency spectrum of the attenuator arrangement (thereby increasing the frequencies across which the attenuator arrangement attenuates acoustic waves). It may be that the apertures 13^IV, 13^v do not directly face the acoustic waves emitted by the transformer apparatus. Figure 39 shows a transformer housing 150 which houses a transformer core 152 (and typically one or more transformer windings, not shown in Figure 39). The transformer housing 150 has an internal surface 154 which faces the transformer core and an external surface 156 which is opposite the internal surface 154 and faces away from the transformer core 152. A plurality of strengthening elements 158 is provided on the external surface of the housing to thereby strengthen the structure of the housing. The strengthening elements 158 comprise attenuator bodies 15^I of the type shown in Figure 38 and described with reference thereto. The strengthening elements are arranged in rows of eight around each of four walls of the housing such that the apertures 13^IV, 13^v of the attenuator bodies 15^IV (other than the apertures at the end of each row) face respective apertures 13^IV, 13^V of adjacent attenuator bodies 15^IV, the apertures which face each other being spaced such that resonance of fluid within each of the said attenuator bodies 15^IV stimulates resonance of fluid in the said adjacent attenuator bodies through the facing apertures, at least when the resonance is at a frequency within the resonant frequency bands of adjacent attenuator bodies 15^IV. The first, second and third attenuator bodies 15^IV (and typically opposing first and second walls thereof) define (e.g. resonant) frequency bands comprising one or more frequencies of acoustic wave emitted by the transformer 36, housing 38 and/or components or circuitry (such as inverters) associated with the transformer. Typically incident acoustic waves emitted by the transformer apparatus are scattered by the opposing first and second walls 15^IVB, 15^IVD of the said attenuator bodies, the scattered waves (typically destructively) interfering with each other to thereby attenuate the said incident acoustic waves. Resonant coupling also typically occurs between adjacent ones of the attenuator bodies 15^IV having open apertures 13^IV, 13^v which face each other. Accordingly, the first, second and third attenuator bodies 15^IV may be provided as part of an acoustic barrier for attenuating acoustic waves emitted by the transformer 36, housing 38 or (e.g. electrical) components or circuitry (such as inverters) associated with the transformer. It may be that the (planar) walls 15^IVB (or 15^IVD) of the attenuator bodies 15^IV are mounted to the external surface 156 of the housing 150. Alternatively, it may be that the external surface 156 of the housing 150 forms the (planar) walls 15^IVB (or 15^I D) of the attenuator bodies 15^IV. It may be that the strengthening elements 158 (and thus the attenuator bodies 15^IV) are integrally formed with the transformer housing. The square cross sectional shape of the attenuator bodies (taken perpendicular to their longitudinal axes) is structurally advantageous particularly when the strengthening elements (and thus the attenuator bodies) are formed separately from the transformer housing because the planar walls 15^IVB (or 15^I D) of the attenuator bodies 15^IV mounted to the external surface of the transformer housing provide a wide load path through which load can be transferred from the transformer housing to the strengthening elements. It will be understood that the cavities and apertures of the attenuator bodies 15^IV are configured to attenuate acoustic waves emitted by the transformer apparatus. In existing arrangements of this type, tubular hollow strengthening elements may be provided on the external surface of the transformer housing which do not perform acoustic attenuation. Accordingly, it may be that the arrangement of Figure 39 is formed by modifying existing strengthening elements provided on an external surface of an existing transformer housing. It may be that the step of modifying the said existing strengthening elements comprises forming (e.g. cutting out) the said open apertures 13^IV, 13^v in the said existing strengthening elements. As shown in Figure 39, the apertures 13^IV, 13^v of the attenuator bodies 15^IV do not directly face the transformer apparatus, but are provided in fluid communication therewith. It will be understood that the attenuator bodies 15^IV could be replaced with any of the pairs of attenuators shown in Figures 30 to 37. Further modifications and variations may be made within the scope of the invention herein disclosed. For example, although the acoustic attenuators forming each of the acoustic barriers in Figures 26-29 are illustrated as being identical to each other, it may be that some of the acoustic attenuators of the plurality of acoustic attenuators forming the barrier are different from each other. In this case, it may be that each of the acoustic attenuators is of the same type (e.g. of the type shown in Figures 1 , 2, or of the type shown in any other Figure), but having different resonant frequency bands or Bragg conditions. Alternatively, it may be that the plurality of acoustic attenuators forming the acoustic barrier comprise different types of acoustic attenuator (e.g. a first one or group of the acoustic attenuators may be of a type shown in one of the Figures 1-9, and a second one or group of the acoustic attenuators may be of a type shown in one of Figures 1-9 different from the first one or group of acoustic attenuators). It will be understood that any relevant selection may be made, dependent on the frequencies of acoustic waves emitted by the transformer apparatus 20 which need to be attenuated, and how much attenuation is required/desired. It will also be understood that there may or may not be gaps provided between adjacent acoustic attenuators in the enclosures of Figures 14, 16, 18, 20, 22, 24, 26- 29. Gaps can be advantageous where for example the transformer also generates heat because heated air can disperse through the gaps, and cool air can enter the enclosure through the gaps. However, in some cases, it may be that there are no gaps between adjacent attenuators. For example, adjacent attenuators may abut each other to form unitary panels. It will also be understood that, although it can be beneficial for the open apertures of the acoustic attenuators to have a direct line of sight to the transformer (as shown in Figures 26-29), it may be in other embodiments that there is no direct line of sight between the open apertures and the transformer (as shown in Figures 18, 24). However there should at least be fluid communication between the transformer and the open apertures. It will also be understood that, although the walls (or adjacent rows) providing the sonic crystal effect described above are said to be parallel in the exemplary embodiments, it may be that the walls providing the sonic crystal effect are not exactly parallel and the one dimensional sonic crystal effect is still observed. For example, it may be that a transversal line extending between the said walls intersects the walls with corresponding angles between the said transversal and the respective walls which differ from each other by 20° or less. Preferably, the corresponding angles differ from each other by 10° or less, more preferably by 5° or less, more preferably by 2.5° or less, more preferably by 1 ° or less, even more preferably the corresponding angles are the same. It will also be understood that, although the acoustic attenuators are illustrated as being oriented vertically in the appended figures, the acoustic attenuators may alternatively be oriented horizontally (or indeed in any suitable orientation). It will also be understood that each of the pairs of attenuators within each row of the embodiments of Figures 32-36 could be replaced with (single) attenuators of the type shown in Figure 38.

Claims

Claims 1. Apparatus comprising: transformer apparatus which emits acoustic waves, the transformer apparatus comprising a transformer; and one or more acoustic attenuators provided in an acoustic wave propagation path of the said acoustic waves emitted by the transformer apparatus, each of the said one or more acoustic attenuators comprising a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates at least a portion of the said acoustic waves emitted by the transformer apparatus.

2. The apparatus according to claim 1 wherein the one or more acoustic attenuators comprise a plurality of acoustic attenuators, each of the said plurality of acoustic attenuators comprising a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates at least a portion of the said acoustic waves emitted by the transformer apparatus.

3. The apparatus according to claim 1 or claim 2 wherein one or more of the said acoustic attenuators comprises: opposing first and second walls, the second wall being substantially parallel to the first wall, the said body of the or each of said acoustic attenuators comprising at least one of the first and second walls, wherein the aperture and the cavity of the said body at least partly define a resonant frequency band across which the said body attenuates incident acoustic waves, and wherein the first and second walls are separated by a gap, wherein the transformer apparatus emits acoustic waves which are scattered by the first and second walls, the said scattered waves interfering with each other such that the said incident acoustic waves are thereby attenuated.

4. The apparatus according to claim 3 wherein the said body of the or each of the said acoustic attenuators comprises the first wall and the first wall comprises the open aperture.

5. The apparatus according to claim 3 or claim 4 wherein the said body of the or each of the said acoustic attenuators comprises the first and second walls.

6. The apparatus according to any one of claims 3 to 5 wherein the said body of the or each of the said acoustic attenuators has a cross section perpendicular to its longitudinal axis which is trapezoidal.

7. The apparatus according to any one of claims 3 to 6 wherein the shortest distance between the first and second walls of one or more or each of the said acoustic attenuators is equal to a wavelength of acoustic waves emitted by the transformer apparatus.

8. The apparatus according to any one of claims 3 to 6 wherein the shortest distance between the first and second walls of one or more or each of the said acoustic attenuators is substantially equal to an integer or a half-integer number of wavelengths of acoustic waves emitted by the transformer apparatus.

9. The apparatus according to claim 3 wherein one or more or each of the said acoustic attenuator(s) comprises a second body comprising the second wall, the said second body being provided next to the first body.

10. The apparatus according to any one of claims 3 to 9 as dependent on claim 2 wherein each of the plurality of the said acoustic attenuators comprises opposing first and second walls, the second wall being substantially parallel to the first wall, the said body of the said acoustic attenuator comprising at least one of the first and second walls, wherein the aperture and the cavity of the said body at least partly define a resonant frequency band across which the body attenuates incident acoustic waves, and wherein the first and second walls are separated by a gap, wherein the transformer apparatus emits acoustic waves which are scattered by the first and second walls, the said scattered waves interfering with each other such that the said incident acoustic waves are thereby attenuated.

1 1. The apparatus according to claim 10 wherein the open aperture of a first acoustic attenuator of the said plurality of acoustic attenuators faces the open aperture of a second acoustic attenuator of the said plurality of acoustic attenuators, and a gap is provided between the said open apertures.

12. The apparatus according to claim 1 1 wherein the bodies of the first and second attenuators are provided with at least partially overlapping resonant frequency bands, the said overlapping portions of the resonant frequency bands comprising one or more frequencies of acoustic waves emitted by the said transformer apparatus.

13. The apparatus according to claim 1 1 or claim 12 wherein one of the first and second walls of the first acoustic attenuator is parallel to one of the first and second walls of the second acoustic attenuator, and wherein the said one of the first and second walls of the first acoustic attenuator is spaced from the said one of the first and second walls of the second acoustic attenuator such that incident acoustic waves scattered by the said walls and having a frequency and angle of incidence on the said walls satisfying the Bragg condition defined by the spacing between them interfere with each other such that said incident acoustic waves are thereby attenuated.

14. The apparatus according to any one of claims 1 1 to 13 wherein the gap between the first and second walls of the first acoustic attenuator, the gap between the first and second walls of the second acoustic attenuator and a gap between the first and second acoustic attenuators are equal.

15. The apparatus according to any one of claims 3 to 14 wherein a transversal line extending between the first and second walls intersects the first and second walls with corresponding angles between the said transversal and the respective first and second walls differing from each other by 20° or less.

16. The apparatus according to claim 2, or to any one of claims 3 to 15 as dependent on claim 2, wherein the said plurality of acoustic attenuators comprises a pair of said acoustic attenuators, wherein the body of each of the said attenuators of the said pair comprises a first face and a second face, the first face comprising the said open aperture of that body, and wherein the bodies of the said attenuators of the said pair are arranged such that their second faces are adjacent to each other and that fluid can flow into or out of the cavities of the said attenuators of the said pair through their respective open apertures.

17. The apparatus according to claim 16 wherein the said plurality of acoustic attenuators comprises a first resonant coupling acoustic attenuator having a said open aperture in fluid communication with, and facing, a said open aperture of a first acoustic attenuator of the said pair, the bodies of the said first resonant coupling acoustic attenuator and the said first acoustic attenuator of the said pair at least partly defining at least partially overlapping resonant frequency bands and a gap being provided between the open aperture of the said first resonant coupling acoustic attenuator and the open aperture of the said first acoustic attenuator of the said pair, the gap being sized such that resonance of fluid within the cavity of the said resonant coupling acoustic attenuator can stimulate resonance of fluid within the cavity of the said first acoustic attenuator of the said pair, and a second resonant coupling acoustic attenuator having a said open aperture in fluid communication with, and facing, the said open aperture of a second acoustic attenuator of the said pair, the bodies of the said second resonant coupling acoustic attenuator and the said second acoustic attenuator of the said pair at least partly defining at least partially overlapping resonant frequency bands and a gap being provided between the open aperture of the said second resonant coupling acoustic attenuator and the said open aperture of the said second acoustic attenuator of the said pair, the gap being sized such that resonance of fluid within the cavity of the said second resonant coupling acoustic attenuator can stimulate resonance of fluid within the cavity of the said second acoustic attenuator of the said pair.

18. The apparatus according to claim 16 or claim 17 comprising first and second rows, each of the said first and second rows comprising one or more said pairs of said acoustic attenuators, the second row being provided downstream from the first row with respect to acoustic waves emitted by the transformer apparatus.

19. The apparatus according to claim 18 wherein the attenuators within one or more or each of the said pairs of the first row are provided opposite respective attenuators of respective pairs of the second row, and wherein the first faces of the said attenuators of the first and second row which face each other are flush with each other.

20. The apparatus according to claim 19 wherein the second faces of the bodies of the attenuators of one or more or each said pair of bodies of one of the first and second rows abut each other, and the second faces of the attenuators of one or more or each said pair of bodies of the attenuators of the other of the first and second rows are separated by a gap.

21. The apparatus according to claim 2, or to any one of claims 3 to 20 as dependent on claim 2, wherein the body of each of one or more of the said plurality of acoustic attenuators comprises first and second open apertures in fluid communication with the cavity defined by the said body, the said first and second open apertures being offset from each other around the longitudinal axis of the said body.

22. The apparatus according to claim 21 wherein the first and second open apertures are provided directly opposite each other.

23. The apparatus according to claim 21 or claim 22 wherein the first said open aperture is in fluid communication with, and faces, a said open aperture of a first resonant coupling acoustic attenuator of the said plurality of acoustic attenuators, and the second said open aperture is in fluid communication with, and faces, a said open aperture of a second resonant coupling acoustic attenuator of the said plurality of acoustic attenuators, wherein the body of the attenuator comprising the said first and second open apertures defines a resonant frequency band which at least partially overlaps with resonant frequency bands defined by the bodies of the said first and second resonant coupling acoustic attenuators, and gaps are provided between the first and second said open apertures and the said open apertures of the first and second resonant coupling attenuators, the gaps being sized such that resonance of fluid in the cavity defined by the body of the said attenuator comprising the said first and second open apertures can stimulate resonance of fluid within the cavities of the said first and second resonant coupling attenuators.

24. The apparatus according to any one preceding claim comprising an acoustic attenuator having: a first body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a first resonant frequency band across which the first body attenuates acoustic waves; and a second body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a second resonant frequency band across which the second body attenuates acoustic waves, wherein the open apertures of the first and second bodies face each other and the first and second resonant frequency bands at least partially overlap, and wherein the first and second resonant frequency bands comprise one or more frequencies of acoustic waves emitted by the transformer apparatus.

25. The apparatus according to claim 24 wherein the acoustic attenuator comprises a third body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a third resonant frequency band across which the third body attenuates acoustic waves, wherein each of the first and third bodies comprise a first face and a second face, the first face comprising the said open aperture of that body, and wherein the first and third bodies are arranged such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the first and third bodies through their respective open apertures.

26. The apparatus according to claim 25 wherein the acoustic attenuator comprises a fourth body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a fourth resonant frequency band across which the fourth body attenuates acoustic waves, wherein the said open aperture of the fourth body is in fluid communication with, and faces, the said open aperture of the third body, the third and fourth resonant frequency bands at least partially overlapping, and a gap being provided between the said open apertures of the third and fourth bodies, the gap being sized such that resonance of fluid within the cavity of the third body can stimulate resonance of fluid within the cavity of the fourth body.

27. The apparatus according to claim 24 wherein the said open aperture of the first body is the first of first and second open apertures of the first body which are in fluid communication with the cavity of the first body, the said first and second open apertures of the first body being offset from each other around the longitudinal axis of the said first body.

28. The apparatus according to claim 27 wherein the first and second open apertures of the said first body are provided directly opposite each other.

29. The apparatus according to claim 28 wherein the acoustic attenuator comprises a third body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the cavity and the at least one open aperture at least partly defining a third resonant frequency band across which the third body attenuates acoustic waves, wherein the said open aperture of the third body is in fluid communication with, and faces, the second open aperture of the first body, wherein the first and third resonant frequency bands at least partially overlap, and a gap is provided between the second open aperture of the first body and the said open aperture of the third body, the gap being sized such that resonance of fluid within the cavity of the first body can stimulate resonance of fluid within the cavity of the third body.

30. The apparatus according to any of claims 3 to 29 as dependent on claim 2 wherein a plurality of the said plurality of acoustic attenuators are mechanically coupled to each other.

31. The apparatus according to any one of claims 3 to 30 as dependent on claim 2 wherein a plurality of the said plurality of acoustic attenuators are arranged to form an enclosure comprising at least part of the transformer apparatus which emits acoustic waves.

32. The apparatus according to any one preceding claim wherein the transformer comprises one or more of the said acoustic attenuators.

33. The apparatus according to any one preceding claim wherein the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein one or more of the said acoustic attenuators are coupled to, or form part of, the transformer housing.

34. The apparatus according to any one preceding claim wherein the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein the housing comprises an internal surface and an external surface opposite the internal surface, the transformer apparatus further comprising one or more strengtheners provided on the external surface of the housing to thereby strengthen the structure of the housing.

35. The apparatus according to claim 34 wherein each of one or more of the said strengtheners comprises a respective one of the said acoustic attenuators.

36. The apparatus according to claim 35 wherein each of one or more of the strengtheners comprise a planar surface which is mounted to the transformer housing.

37. The apparatus according to claim 35 or claim 36 wherein the transformer housing comprises a planar surface which forms an internal surface of the bodies of one or more of the said plurality of acoustic attenuators.

38. The apparatus according to any one of claims 34 to 37 wherein one or more of the said acoustic attenuators are provided by respective modified strengtheners.

39. The apparatus according to any one preceding claim wherein the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein one or more of the said acoustic attenuators are provided within the transformer housing.

40. The apparatus according to any one preceding claim wherein the transformer apparatus further comprising a transformer cooling system configured to dissipate heat generated by the transformer, wherein the transformer cooling system comprises one or more of the said one or more acoustic attenuators.

41. The apparatus according to claim 40 as dependent on claim 2 wherein the transformer cooling system comprises an air gap provided between adjacent acoustic attenuators of the said plurality of acoustic attenuators.

42. The apparatus according to claim 41 wherein the transformer cooling system further comprises one or more air blowers configured to blow air heated by the transformer away from the transformer through the air gap between the said adjacent acoustic attenuators.

43. The apparatus according to any one of claims 40 to 42 wherein the transformer comprises an oil immersed transformer comprising a transformer core and at least one transformer winding immersed in oil, and wherein the transformer cooling system comprises a heat exchanger configured to cool the said oil.

44. The apparatus according to claim 43 wherein the heat exchanger comprises a heat sink comprising one or more of the acoustic attenuators.

45. The apparatus according to claim 43 or claim 44 wherein the heat exchanger comprises an oil to water heat exchanger, the heat exchanger comprising a first conduit configured to carry oil from the transformer through the heat exchanger and a second conduit adjacent the said first conduit and configured to carry a flow of cooling water such that heat from the oil flowing in the said first conduit transfers to the cooling water flowing in the second conduit, wherein an acoustic attenuator of the said plurality of acoustic attenuators comprises the first and second conduits.

46. The apparatus according to claim 45 wherein the bodies of one or more of the said acoustic attenuators comprise one or more first conduits carrying transformer oil and one or more second conduits carrying cooling water, each of the second conduits being adjacent to one or more of the first conduits such that heat from oil flowing through a first conduit is transferred to cooling water flowing through an adjacent second conduit.

47. The apparatus according to claim 46 wherein at least one of the first conduits and at least one of the second conduits are provided within, and extend along, at least a portion of the length of the body of one of the said acoustic attenuators.

48. The apparatus according to any one preceding claim wherein the plurality of acoustic attenuators are together arranged to attenuate acoustic waves of one or more frequencies different from the frequencies attenuated by the individual acoustic attenuators.

49. Apparatus comprising: transformer apparatus which emits acoustic waves, the transformer apparatus comprising a transformer and a transformer cooling system configured to dissipate heat generated by the transformer, wherein the transformer cooling system comprises one or more acoustic attenuators configured to attenuate at least a portion of the acoustic waves emitted by the transformer apparatus.

50. The apparatus according to claim 49 wherein the transformer cooling system comprises a heat sink comprising one or more of the one or more acoustic attenuators.

51. The apparatus according to claim 49 or claim 50 wherein the transformer cooling system comprises a heat exchanger comprising one or more of the one or more acoustic attenuators.

52. The apparatus according to claim 51 wherein the heat exchanger comprises an oil to water heat exchanger, the heat exchanger comprising a first conduit configured to carry oil from the transformer through the heat exchanger and a second conduit adjacent the said first conduit and configured to carry a flow of cooling water such that heat from the oil flowing in the said first conduit transfers to the cooling water flowing in the second conduit, wherein an acoustic attenuator of the said one or more acoustic attenuators comprises the first and second conduits.

53. A transformer comprising: a transformer core; one or more transformer windings; a transformer housing containing the transformer core and the transformer windings, the transformer housing comprising one or more acoustic attenuators, each of the said one or more acoustic attenuators comprising: a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that the acoustic attenuator attenuates one or more frequencies of acoustic waves emitted by the transformer.

54. A method of attenuating acoustic waves emitted by a transformer apparatus comprising a transformer, the method comprising: the transformer apparatus generating acoustic waves; and attenuating at least a portion of the said acoustic waves by providing one or more acoustic attenuators in an acoustic wave propagation path of the said acoustic waves, each of the said one or more acoustic attenuators comprising a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity, the body being configured such that at least a portion of the said acoustic waves emitted by the transformer apparatus are attenuated by the acoustic attenuator(s).

55. The method according to claim 54 wherein the transformer apparatus comprises a transformer cooling system configured to dissipate heat generated by the transformer, the method further comprising dissipating heat generated by the transformer apparatus using the transformer cooling system, the transformer cooling system comprising one or more said acoustic attenuators.

56. The method according to claim 54 or 55 comprising dissipating heat generated by the transformer by: the transformer heating air adjacent to the transformer to provide heated air; and flowing the heated air away from the transformer through an air gap between a pair of adjacent acoustic attenuators.

57. The method according to any one of claims 54 to 56 further comprising: the transformer heating oil adjacent to the transformer to provide heated oil; and flowing the heated oil away from the transformer to a heat exchanger comprising one or more of the said acoustic attenuators.

58. The method according to any one of claims 54 to 57 further comprising: the transformer heating oil adjacent to the transformer to provide heated oil; flowing the heated oil away from the transformer to a first conduit of a or the heat exchanger, the said first conduit being provided in one of the said acoustic attenuators; and transferring heat from the heated oil flowing along the first conduit to cooling water flowing along a second conduit adjacent to the first conduit, the second conduit being provided in the said acoustic attenuator.

59. The method according to any one of claims 54 to 57 comprising: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within the cavity defined by the body of the said acoustic attenuator; and first and second walls of the acoustic attenuator scattering acoustic waves emitted by the transformer apparatus, the said acoustic waves having a frequency and an angle of incidence upon the first and second walls which satisfy the Bragg condition defined by a gap provided between the first and second walls, the said body comprising at least one of the first and second walls.

60. The method according to any one of claims 54 to 59 further comprising: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a first cavity defined by a first body of one of the said acoustic attenuators comprising a first open aperture in fluid communication with the first cavity; and acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a second cavity defined by a second body of the said acoustic attenuator comprising a second open aperture in fluid communication with the second cavity, wherein the first and second open apertures face each other such that resonance of the fluid provided within the first cavity caused by the said acoustic waves stimulates resonance of the fluid provided within the second cavity.

61. The method according to any one of claims 54 to 59 further comprising: acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a first cavity defined by a first body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the first cavity; and acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a second cavity defined by a second body of the said acoustic attenuator comprising an open aperture in fluid communication with the second cavity, wherein the open apertures of the first and second bodies face each other such that resonance of the fluid provided within the first cavity caused by the said acoustic waves stimulates resonance of the fluid provided within the second cavity.

62. The method according to claim 61 further comprising acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a third cavity defined by a third body of the said acoustic attenuators comprising an open aperture in fluid communication with the third cavity, wherein each of the first and third bodies comprise a first face and a second face, the first face comprising the said open aperture of that body, the method further comprising arranging the first and third bodies such that their second faces are adjacent to each other and that fluid can fluid into or out of the cavities defined by the first and third bodies through their respective open apertures.

63. The method according to claim 62 further comprising acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a fourth cavity defined by a fourth body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the fourth cavity, wherein the open apertures of the third and fourth bodies face each other, wherein a gap is provided between the open apertures of the third and fourth bodies, the gap being sized such that resonance of fluid within the third body caused by the said acoustic waves stimulates resonance of fluid within the fourth body.

64. The method according to claim 62 wherein the said open aperture of the first body is the first of first and second open apertures of the first body which are in fluid communication with the cavity of the first body, the said first and second open apertures of the first body being offset from each other around the longitudinal axis of the said first body.

65. The method according to claim 64 further comprising providing the first and second open apertures of the said first body directly opposite each other.

66. The method according to claim 65 further comprising acoustic waves emitted by the transformer apparatus stimulating resonance of a fluid provided within a third cavity defined by a third body of one of the said acoustic attenuators comprising an open aperture in fluid communication with the third cavity, wherein the said open aperture of the third body is in fluid communication with, and faces, the second open aperture of the first body, wherein the first and third resonant frequency bands at least partially overlap, and a gap is provided between the second open aperture of the first body and the said open aperture of the third body, the gap being sized such that resonance of fluid within the first body caused by the said acoustic waves stimulates resonance of fluid within the third body.

67. The method according to any one of claims 54 to 66 wherein the transformer comprises a transformer core and at least one transformer winding provided in a transformer housing, wherein the housing comprises an internal surface and an external surface opposite the internal surface, the method further comprising providing one or more strengtheners on the external surface of the housing to thereby strengthen the structure of the housing.

68. The method according to claim 67 wherein each of one or more of the said strengtheners comprises a respective one of the said acoustic attenuators of the said plurality of acoustic attenuators.

69. The method according to claim 68 further comprising mounting a planar surface of each of one or more of the strengtheners to the external surface of the transformer housing.

70. The method according to claim 68 or claim 69 further comprising a planar surface of the transformer housing forming an internal surface of the body of each of one or more of the said one or more acoustic attenuators.

71. The method according to any one of claims 67 to 70 comprising modifying one or more said strengtheners to form a respective acoustic attenuator of the said one or more acoustic attenuators.

72. A method of attenuating acoustic waves emitted by a transformer apparatus comprising a transformer and a transformer cooling system comprising an acoustic attenuator, the method comprising: the transformer generating heat; the transformer apparatus generating acoustic waves; dissipating heat generated by the transformer using the transformer cooling system; and attenuating acoustic waves emitted by the transformer apparatus using the said acoustic attenuator.