WO2000057585A1 - Loudspeaker with thermally compensated impedance - Google Patents

Abstract

Current passing through an acoustic transducer of a loudspeaker (104) warms the transducer's coil (106). As the coil warms, its resistance increases. Referred to as compression, this resistance increase will lower the power output or volume of the loudspeaker transducer (104). To compensate for the compression, a negative temperature coefficient thermistor device (110) or circuit is coupled in series between an amplifier (102) and the acoustic transducer (104). As the current passing through the thermistor (110) warms it, the thermistor's resistance decreases, thereby offsetting to some degree the increase in resistance of the coil.

Description

LOUDSPEAKER WITH THERMALLY COMPENSATED IMPEDANCE

Field of Invention

The invention relates to acoustic loudspeakers.

Background A loudspeaker transduces an electrical waveform to an equivalent acoustic waveform. Although there are several different types of transducers used in loudspeakers to perform this function, the most common type includes a coil moving in a linear fashion to vibrate a sound radiator. The mechanical force to move the coil is generated by the interaction of a current passing through the coil and a transverse magnetic field disposed radially across a gap formed in magnet assembly, in which the coil moves. As the current of the input signal oscillates, so does the coil. A coil is comprised of one or more wires wound around a substrate. The substrate provides mechanical rigidity.

To avoid problems associated with designing and building a single transducer having a relatively flat frequency response across the entire audio range, multiple transducers are used in most loudspeakers for high fidelity audio systems. Each transducer is optimized to reproduce sound in a particular portion of the audio range. An electronic circuit called a crossover network divides an input signal into several signals, one for each transducer. Each of these signals carries the power of the input signal at the frequencies within the intended frequency range of the transducer to which it is being sent. The volume of sound generated by a loudspeaker (as represented by the acoustic pressure of the sound) is a function of the magnitude of the current of the input electrical signal. A source signal - e.g. the signal from a radio tuner or a compact disc player - has a relatively low current and voltage. Because of the comparatively high current levels necessary to drive a loudspeaker, especially at high volume levels, an amplifier is required for generating the loudspeaker's input signal. Generally, the function of the amplifier is to generate a signal having the same waveform as the source signal, but with greater power in order to supply the current necessary to drive the transducer at desired volume levels. To avoid distorting the waveform of the input signal to the loudspeaker, amplification must be linear. Generally, a comparatively high power amplifier is more expensive than a low power amplifier, especially if low distortion and noise are desired.

Due to the resistance inherent in a coil of a loudspeaker transducer, the coil will naturally tend to heat. The greater the magnitude of the current, the greater the heating. The coil has a relatively low thermal mass; therefore, it will tend to heat quickly and cool quickly. The coil's thermal energy will heat the other parts of the transducer, loudspeaker and surrounding air. The magnets and other portions of the transducer, and the air surrounding the transducer, have greater thermal mass than the coil, and thus tend to warm up and cool down more slowly. Consequently, the temperature of the coil will tend to rise when the loudspeaker is played, especially at higher volumes.

As the temperature of the coil increases, its resistance increases. This increase leads to a drop in the current to the loudspeaker. Corresponding to this decrease in current is a decrease in the force generated by the coil and, consequently, in the pressure of the sound created by the loudspeaker. This decrease in sensitivity of a loudspeaker is generally referred to as compression, and is considered a form of distortion of the speaker. Correcting for compression by turning up the power of an amplifier creates a greater risk of the amplifier introducing distortion in its output signal.

Summary of the Invention An objective of the invention is to compensate for the increase in impedance, particularly resistance, of a coil of an acoustic transducer caused by heating, and thereby alleviate compression.

In accordance with one aspect of a preferred embodiment of the invention, at least one negative coefficient thermistor is placed in series in a circuit formed between an output of an amplifier and an input to a transducer of a loudspeaker. As the temperature of the thermistor rises, it resistance decreases. The thermistor is selected so that its temperature correlates to the temperature of the coil in a predictable manner, at least within a given range of temperatures or period of time. An increase in resistance of the transducer's coil caused by an increase in temperature is thereby offset, at least to some degree, by a decrease in the thermistor's resistance caused by a corresponding increase in temperature due to thermal events, including for example current passing through the thermistor and coil. In accordance with another aspect of a preferred embodiment of the invention, a negative coefficient thermistor, or circuit with a plurality of such thermistors, having a thermal mass substantially the same as a loudspeaker coil is placed in series with the coil. Heating of the thermistor and coil caused by the same event - for example, the current passing through both the coil and the thermistor - will thus result in similar rises in temperature in both the thermistor and coil, assuming that both also have similar resistances.

In accordance with another feature of a preferred embodiment of the invention, a circuit with one or more negative coefficient thermistors are thermally coupled to a magnet assembly of, or other structure functioning as a heat sink for, a loudspeaker transducer. The thermistor circuit is connected in series with a signal driving the speaker. As the transducer's coil conducts current, it generates heat. The heat is transferred to the heat sink. By thermally coupling the thermistor circuit to the magnet assembly, the thermistors remain warmed by the heat sink, the heat sink giving up the heat that it originally absorbed from the thermistor and/or heat received from the coil. This arrangement overcomes a problem of a thermistor cooling off as a result of its impedance dropping and of the coil, which remains hot as a result of its increased impedance, reducing current flow through the series circuit formed by the coil and thermistor. Such cooling will reintroduce the impedance of the thermistor to the coil, thereby compounding the problem of high impedance.

According to yet another feature, to improve radiant heat transfer from a coil of a loudspeaker transducer to a heat sink structure, such as a magnet assembly or a heat sink set on top of a magnet assembly, a thin, nonmagnetic metal tube is inserted into a flux gap, between a center pole and a coupling between a coil of a loudspeaker transducer and heat sink structure, such as the transducer's magnet assembly, a separate heat sink structure on top of a magnet assembly, or a combination of a heat ink structure and magnet assembly, The tube functions to absorb radiant heat given off by the coil and to conduct the heat out of the flux gap. If the coil expands due to heating, it will move even closer to the heat conducting tube, thereby increasing the heat transfer rate. The tube may, for example, be connected at its top end to an aluminum heat sink structure thermally connected to a magnet assembly. The heat sink structure may also include cooling fins to assist in heat dissipation into the air away from the coil. The reciprocal motion of the transducer's diaphragm will tend to generate air currents through the fins to further improve heat transfer. A thermistor may be incorporated into such a heat sink assembly for the purposes described above, thereby allowing the heat of the coil to be more effectively transferred to the thermistor and improving thermal coupling of the coil and thermistor.

These and other features and advantages of the invention, in its preferred embodiments, are described below in connection with the appended drawings.

Brief Description of the Drawings

FIG. 1 is a schematic drawing of a circuit comprising a loudspeaker transducer, an audio amplifier and a thermistor coupled in series between the audio amplifier and loudspeaker transducer.

FIG. 2 is a perspective drawing of a loudspeaker having a single transducer and a thermistor mounted in close proximity to the transducer.

FIG. 3 is a schematic drawing of an alternate embodiment of the circuit of FIG. 1 , with a plurality of discrete thermistor devices coupled in series between the loudspeaker transducer and an audio amplifier.

FIG. 4 is a perspective drawing of the loudspeaker transducer of FIG. 2, mounted in an enclosure but with a thermistor mounted in the enclosure.

FIG. 5 is a schematic drawing of a loudspeaker comprising multiple acoustic transducers, each of which is connected to a crossover network through a thermistor, the loudspeaker being connected to an audio amplifier. FIG. 6 is a perspective drawing of a loudspeaker with a thermistor mounted within a pocket formed in a heat sink assembly attached to the top of a magnet assembly. FIG. 7 is an enlargement of a portion of FIG. 6.

Description of Preferred Embodiments In the following description, like numbers refer to like parts.

FIG. 1 is a functional schematic of a representative example of an audio system 100. The audio system includes an audio amplifier 102 that is electrically coupled to loudspeaker 104. The audio amplifier functions to amplify a low power audio signal to generate a relatively high power audio signal having substantially the same wave form for purposes of driving the loudspeaker at a desired listening volume. The loudspeaker 104 includes an acoustic transducer 106. As will be described in connection with a subsequent figure, the loudspeaker may be comprised of more than one transducer. The transducer is mounted in an enclosure 108. An enclosure may take the form of a plastic, metal or wood cabinet or casing. It may also take the form of a door or trunk of a car, or the hollow wall or ceiling of a building. Furthermore, the amplifier may be integrated with the loudspeaker by, for example, mounting the amplifier within the enclosure. As will be described in connection with FIG. 2, the transducer includes in the preferred embodiment a moving coil, suspended within a fixed magnetic field, that moves an acoustic radiator.

In series with the transducer 106, in a circuit formed by the amplifier and transducer, is thermistor 110. The thermistor possesses a resistance to current flow that varies with its temperature in a predictable manner within at least a predetermined temperature range. It has predetermined or known resistance at or about room temperature (i.e. approximately sixty-eight degrees Fahrenheit) and a known thermal coefficient with which its resistance at any given temperature within a predetermined range can be predicted within acceptable tolerances. The thermal coefficient is negative: as the temperature of the thermistor rises, its resistance decreases. Referring now to FIG. 2, acoustic transducer 200 is a broad band transducer suitable for use in a loudspeaker for high fidelity sound reproduction. It is, however, only a representative example of a transducer that can be used in the circuit of FIG. 1. Acoustic transducer 200 includes a coil 202 that drives a cone-shaped acoustic radiator 204. The radiator 204 is suspended from a basket 206 by resilient foam roll 208 that allows the radiator to translate along a center axis of the coil while remaining centered within the basket. The basket provides structural support for mounting the transducer within a wall of an enclosure or other structure. The coil is, in turn, attached to the radiator so that it is suspended within a magnetic flux gap 210 formed between metal post 212 and magnet assembly 214. Wire leads 216 and 218 are used to electrically connect the coil 202 to a signal source, such as amplifier 102 (FIG.1).

Thermistor 220 is connected in series with lead 218. It could, alternately, be connected to the other lead 216. The thermistor has known impedance, or more particularly resistance, at room temperature, and a known negative thermal coefficient. The thermistor is selected so that an increase in resistance in the coil due to a thermal event or under a given condition will be offset, to some degree, by a drop in the resistance of the thermistor in reaction to the same or corresponding thermal event or under the same or corresponding thermal condition. Generally speaking, the thermal coefficient of the thermistor is selected so that the expected change in the respective temperatures of the thermistor and the coil in at least certain, predefined operating conditions, will result in offsetting, though not identical, changes in the resistances of the thermistor and coil and thereby alleviate compression.

Compression in a transducer's sensitivity or response to an input signal is primarily caused by high power audio signal, although the temperature of the coil's environment - e.g. the other components of the loudspeaker and the air around the coil - can contribute to compression by inhibiting the cooling of the coil. Therefore, in one embodiment, the thermistor is chosen so that, at room temperature, its resistance is substantially equivalent to the resistance of the coil. A thermal event such as a transient high power signal or a sustained signal will result in both the coil and the thermistor initially consuming similar amounts of power when both are at room temperature. Furthermore, the thermistor preferably has the same or nearly the same thermal mass as the coil 202. The thermistor and coil will therefore initially tend to warm at the same rate due the roughly equivalent amounts of power they each consume, and thus the temperature of each will rise to approximately the same levels.

With this arrangement, the combined (i.e. summed) resistance of the coil and thermistor is intended to be held at a relatively constant level - or more precisely within some acceptable range of resistances ~ over a given range of temperatures in response to heating caused by power in the audio signal. The term "relatively constant" means that the combined resistance does not deviate more than some predetermined amount from the impedance of the combination at some predetermined temperature, for example room temperature. The predetermined impedance would normally be the nominal or rated impedance of the transducer (e.g. four or eight Ohms). Furthermore, the predetermined deviation preferably results in compression that is not discernable by a listener, e.g. less than six decibels at any given frequency of interest. Ideally, the combined resistances of the thermistor and the coil remains relatively constant under those conditions normally encountered during operation, or during operation of the transducer at some predetermined, relatively high power level for some predetermined period of time. However, once the impedance of the coil and the thermistor significantly differ from each other, so too will the amount of warming each will experience due to the current. Thus, although compression will tend to be alleviated, the combined resistances will not necessarily always be held at a constant, or relatively constant, level under all conditions.

Referring briefly to FIG.3, should a discrete thermistor device selected for use with a particular transducer, e.g. a thermistor 302, have a thermal mass less than that of the coil, a plurality of such discrete thermistor devices may be arranged into a circuit that is an equivalent of a single, discrete thermistor device of substantially equal thermal mass placed in series between the amplifier and transducer without introducing extra impedance. For example, in the illustrated circuit, each thermistor device 302 has a nominal or measured impedance (i.e. impedance at room temperature measured with minimal current) of 4 ohms.

Two thermistor devices 302 are connected in series to form a first 8 ohm branch. Two additional thermistor devices are connected in series to form a second 8 ohm branch. The first and second branches are connected in parallel with each other between the amplifier 102 and transducer 106. The amplifier sees a 4 ohm impedance across the circuit 304.

Referring back to FIG. 2, the thermistor 220 is also preferably located where thermal influences of the environment on the thermistor approximate the thermal influences on the coil in the coil's environment. The relatively large thermal mass of the transducer's magnet assembly 214 will tend to absorb and reradiate heat given off by the coil. The air within the flux 212 may remain warm for at least some period of time since it may not be easily vented. Furthermore, heat from the transducer's magnet assembly will warm the air around the coil. Thus, in most circumstances, the coil's temperature will be expected to rise once a substantial current is connected to it, and remain at elevated temperatures over a relatively long-term despite any short-term drop in the amplitude of the current. Referring now to FIG. 4, since the transducer will also tend to warm the air within the transducer's enclosure, placing the thermistor anywhere within the enclosure, as schematically represented in FIG. 1 and illustrated in FIG. 4, will enable it to compensate to some degree for the heating of the coil due to ambient heating. In FIG. 4, the thermistor 220 is actually located near the back of the enclosure 400, and is connected to one of the terminals on terminal block 402. Terminal block 402 has two sets of terminals for connecting electrical wires from an amplifier to the loudspeaker: primary terminals 304a and 304b, which are high impedance connections (e.g. 8 ohms); and alternate terminals 306a and 306b, which are low impedance connections (e.g. 4 ohms). Only the rear portions of the terminals can be seen in the figure. The output of an amplifier, such as amplifier 102 (FIG. 1), is connected to either the primary or the alternate terminal set. If connected to the primary terminal set, the amplifier will be connected to the transducer 200 through thermistor 220. In this configuration, the coil and terminal 304a, thermistor 220, lead 308, coil 202, lead 310 and terminal 304b form a circuit through which an audio signal will pass. If connected to the alternate terminal set, the thermistor will be by-passed. The alternate circuit will include only the terminals 306a and 306b and leads 308 and 310 connected in series with coil 202.

Referring back to FIG. 2, the thermistor may alternately be placed in physical contact with the transducer 200, in particular a metal plate that forms part of its magnet assembly 214. Direct contact may be necessary to better approximate the effect on the coil of the heating of the other parts of the transducer and the surrounding air.

In FIG. 5, two transducers 502 and 504 and a crossover network 506 are arranged in enclosure 508 to form a single loudspeaker 500. The transducers may also be located in different enclosures or separate compartments within an enclosure. The crossover need not be in the enclosure. To provide temperature compensation for the coil of transducer 502, thermistor 510 is placed in series between the crossover network 506 and transducer 502. Similarly, thermistor 512 is placed in series between crossover network 506 and transducer 504 to provide thermal compensation for the coil of transducer 504. Because crossover networks are typically designed to function with certain transducer impedance, thermally compensating a transducer will result in better performance of the crossover network. Additionally, each transducer may have coils of differing thermal masses and thermal coefficients (i.e., temperature vs. impedance characteristics). Therefore, placing the thermistor in series between the crossover network and the transducer allows for better matching of the thermal masses and coefficients of the coil and thermistor. It should be noted that the thermistors 510 and 512 are coupled in series between amplifier 102 and the respective transducers 502 and 504. Alternately, a thermistor device or circuit can be placed in series between amplifier 102 and crossover network 506 and still the advantage of some degree thermal compensation of the transducers, but not all of the advantages noted above in connection with FIG. 5. Finally, the addition of impedance between an amplifier and a loudspeaker transducer is generally undesirable. It lowers the efficiency of the system and consumes additional power. Should a thermistor or thermistor circuit be introduced as described above, it will be desirable to reduce the impedance of a coil to offset the additional impedance. This offset can be accomplished through lowering the impedance of the coil.

Standard impedance for loudspeakers is 4 or 8 ohms. Thus, if the coil would normally have an 8 ohm impedance, it will need to be reduced to 4 ohms if a 4 ohm thermistor or thermistor circuit is used. Referring now to FIG's 6 and 7, acoustic transducer 610 includes a frame 612, in the shape of a basket, from which is suspended a cone-shaped diaphragm 614, which is in the shape of a cone. Collar 615 enhances coupling of high frequency movement of the diaphragm to the air. Suspension 616 allows the diaphragm to move linearly in a reciprocating fashion along an axis defined by a center of cylindrical pole 617. The suspension includes two, compressible foam rings 618 and 620 and a foam roll 622. Foam ring 618 is attached to an outer circumference of the diaphragm; foam ring 620 is attached to the frame. The foam rings compress, stretch and bend to accommodate movement of the cone during its excursions, but otherwise function to keep the cone substantially centered within the frame. Magnet assembly 626 includes a bottom steel plate 628, a magnet 630 and two steel top plates 632 and 634. The magnet and top plates have a center hole and form a donut shape through which pole 617 extends. Pole 617 is attached to or, as shown, integrally formed with plate 628. A foam button 636 acts as a bumper to stop downward excursion of the diaphragm and to prevent the end of the voice coil from hitting the back plate 628. A voice coil assembly reciprocates in a conventional, linear fashion within a cylindrically shaped, annular flux gap 642 formed between pole 16 and the inside surfaces of donut shaped magnet assembly 626.

Formed at regularly spaced intervals around the circumference of pole 616 are a plurality of ridges 644. Each ridge is oriented in the general direction of the axis of the pole and movement of the voice coil assembly, but turned at an angle, resulting in each ridge running in an helical fashion partway around the pole. With this helical arrangement of the ridges, when the coil assembly touches a ridge, the coil assembly is typically very close to touching or is touching another, adjacent ridge. Each ridge has a low friction outer surface. Furthermore, it is preferred to be compressible. It is formed, in the preferred embodiment, using Teflon® tape overlaying cotton thread. However, the compressibility and resiliency can be altered by using different core materials, if desired. By keeping the ends of the ridges open and using a relatively porous cotton thread, air trapped within the ridge can act as a dampening mechanism. The cotton thread acts to create resistance to slow the flow of air through the ridge as it is being displaced. Additionally, as the coil is likely to engage a length of each of two adjacent ridges when it hits the post, air is momentarily trapped between the adjacent ridges and can only escape by flowing in a generally axial direction along the pole. Confining the flow of air in this fashion may also tend to dampen lateral movement of the coil toward the pole. The larger the lateral forces, the more the ridge and the surrounding air is compressed and the larger the lateral dampening.

Although magnet assembly 626 acts also as a heat sink, an additional structure 646 that functions as a heat sink is connected to the top of the magnet assembly. It is made of nonmagnetic metal, preferably one that conducts heat well, such as aluminum. The heat sink structure 646 includes a plurality of fins 648 that increase the surface area to improve transfer of heat to the air. The structure also includes a top plate 650, with which the fins are integrally formed in the preferred embodiment, and a bottom plate 652. The bottom part 652 is attached to the magnet assembly. Between the two plates is formed at least one pocket or cavity. A negative coefficient thermistor 654 is placed within the cavity so that it, preferably, has good thermal contact with the top and bottom plates 650 and 652. The thermistor is connected electrically in series between the coil 656 and a signal source, in the manner previously described. Only one of the two electrical wires 658 is shown. This wire extends through on opening 660 in the top plate 650. As the thermistor conducts current and warms, its heat is transferred to heat sink structure 646. As the thermistor cools to below that of the heat sink structure, it will tend to absorb heat from the heat sink structure. Thus, the thermistor will tend to remain near the same temperature of the heat sink structure. To improve transfer of heat from coil 656, a nonmagnetic, thin, metal tube 662 is inserted into flux gap 642. Tube 662 receives heat radiated from coil 656 and conducts it toward heat sink structure 646. The tube includes a lip 664 for contacting the heat sink structure to transfer heat by conduction. Heat from the coil is thereby transferred to the heat sink structure 646, which in turn will heat thermistor 654. A thermal coupling is thereby formed between the coil and the thermistor so that the temperature of the thermistor more closely tracks the temperature of the coil.

The foregoing loudspeakers and audio circuits include preferred embodiments of the invention. The invention is defined by the following claims. Modifications, including substitutions, rearrangements and omissions for or of the various elements of the described loudspeakers and audio circuits, may be made to the described loudspeakers without departing from the scope of the claims. What is claimed is:

Claims

Claims

1. A loudspeaker comprising: an acoustic transducer, the transducer having a coil of a predetermined impedance at room temperature; and means for resisting current flow coupled in series with the coil, the means for resisting having a predetermined impedance at room temperature and a negative thermal coefficient, whereby, as the temperature of the means for resisting rises, its resistance decreases.

2. The loudspeaker of Claim 1 , wherein the coil and the means for resisting have substantially the same thermal mass.

3. The loudspeaker of Claim 1 wherein the coil and the means for resisting have substantially the same resistance at room temperature.

4. The loudspeaker of Claim 1 , wherein the means for resisting includes thermistor means.

5. The loudspeaker of Claim 4, wherein the coil and the thermistor means have substantially the same thermal mass.

6. The loudspeaker of Claim 4, wherein the thermistor means includes a plurality of discrete thermistors.

7. The loudspeaker of Claim 6, wherein the coil has a thermal mass that is substantially the same as the plurality of thermistors' collective thermal mass.

8. The loudspeaker of Claim 6, wherein the coil has the same resistance at room temperature as the plurality of thermistors.

9. An audio system comprising: an audio amplifier; a loudspeaker including an acoustic transducer electrically coupled with the audio amplifier, the transducer having a coil; and means for resisting current flow electrically coupled in series between the audio amplifier and the coil, the means for resisting having a predetermined impedance at room temperature and a negative thermal coefficient, whereby, as the temperature of the means for resisting rises, its resistance decreases.

10. The audio system of Claim 9, wherein the loudspeaker further includes a crossover network electrically coupled in series between the audio amplifier and the means for resisting.

11. The audio system of Claim 10, wherein the loudspeaker includes a second acoustic transducer electrically coupled to the crossover network and a second means for resisting current flow electrically coupled between the crossover network and the second transducer; the second means for resisting current flow having a predetermined impedance at room temperature and a negative thermal coefficient, whereby, as the temperature of the means for resisting rises, its resistance decreases.

12. The audio system of Claim 11 wherein the second transducer includes a second coil, the second coil and the second means for resisting have approximately equal thermal masses and room temperature resistances.

13. The audio system of Claim 11 , wherein the second means for resisting includes thermistor means.

14. The audio system of Claim 13, wherein the second coil and the thermistor means have approximately equal thermal masses and room temperature resistances.

15. The audio system of Claim 13, wherein the thermistor means includes a plurality of discrete thermistors.

16. The audio system of Claim 9, wherein the coil and the means for resisting have approximately equal thermal masses and room temperature resistances.

17. The audio system of Claim 9, wherein the means for resisting includes thermistor means.

18. The audio system of Claim 17, wherein the coil and the thermistor means have approximately equal thermal masses and room temperature resistances.

19. The audio system of Claim 9, wherein the means for resisting includes a plurality of discrete thermistor devices.

20. The audio system of Claim 19, wherein the coil has a thermal mass that is substantially the same as the plurality of thermistors' collective thermal mass, and a room temperature resistance that is substantially equal to the circuit resistance of the plurality of thermistors.

21. A loudspeaker comprising: an acoustic transducer, the transducer having a coil of a predetermined impedance at room temperature; means for resisting current flow coupled in series with the coil, the means for resisting having a predetermined impedance at room temperature and a negative thermal coefficient, whereby, as the temperature of the means for resisting rises, its resistance decreases; the means for resisting current flow mounted in thermal contact with a heat sink that is thermally coupled with the coil.

22. The loudspeaker of Claim 21 , wherein the transducer includes a magnet assembly defining a flux gap in which the coil reciprocates, and a metal tube inserted within the flux gap, between the coil and the magnet assembly, for conducting heat out of the flux gap generated by the coil.

23. The loudspeaker of Claim 22 wherein the heat sink is the magnet assembly.

24. The loudspeaker of Claim 22, wherein the heat sink includes a structure mounted on the magnet assembly, the metal tube in thermal contact with the structure.

25. A loudspeaker comprising an acoustic transducer, the transducer having a coil, a magnet assembly defining a flux gap in which the coil reciprocates, and a metal tube inserted within the flux gap, between the coil and the magnet assembly, for conducting heat generated by the coil out of the flux gap.

26. The loudspeaker of Claim 25 further including a heat sink structure mounted on the magnet assembly, the metal tube in thermal contact with the heat sink structure.