US2908772A - Electroacoustical transducer - Google Patents

Description

Oct. 13, 1959 n. ADLER ELECTROACOUSTICAL TRANSDUCER Filed ll'y 24. 1957 2 Sheets-Sheet 1 l ggg!!! INVENTOR. oer @Qd/ef j p Oct. 13, 1959 R. ADLER 2,908,772

v ELEcTRoAcousTrcAL TRANSDUCER File@ uy 24, 1957 2 Sheets-Sheet zzonzey United StatesA Patent z,9os,772 A ELEcrRoAcoUsncAL rRANsDUcER Robert Adler, Northfield, Ill., assignor to Zenith Radio Corporation, a corporation of Delaware Application May 24, 1957, Serial No. 661,348

6 Claims. (Cl. 179-111) This invention relates generally to electroacoustical transducers and more particularly to a new and improved condenser microphone of simple and economical construction and having substantially greater sensitivity than has heretofore been possible.

As is well known, maximum power is extracted from a generator when the load impedance is a .conjugate of the generator impedance. However, in order to secure a maximum transfer of energy between two connecting networks, if the respective phase angles of the two networks are not adjustable, the absolute value of the impedance of one should be equal to the absolute value of the impedance of the other.

Air, like any other medium capable of carrying waves, can be assigned an acoustical characteristic impedance which is defined as the quotient ofthe pressure by the particle velocity in a sound wave. Similar to the reactance of a coil to alternating current, a microphone diaphragm has a resistance to alternating motion which can be expressed in terms ofa resonant impedance,-

sometimes referred Vto as its mechanical impedance, indicative of the molecular friction or damping loss within the diaphragm. The resonant impedances of the diaphragms of most conventional microphones, whether of condenser, piezoelectric or crystal, magnetic, or magnetostrictive type, are generally several times larger than the acoustical characteristic impedance of air. Consequently,

it isdesirable to provide some means for matching these two impedances to insure a transformation of all o f -the acoustical energy into Ythe desired electrical energy so that maximum sensitivity of the microphone is attained.

It is also generally desirable to provide the microphone structure with a protective enclosure in order to isolate the 'diaphragm and other delicately constructed components from the deleterious effects of dust, moisture, and the like, and in addition, toprovide some degree of protection from physical abuse. Such enclosures normally include a faceplate including a grille which is apertured to permit passage of the sound waves to be intercepted by the diaphragm. The grille, however, creates a partial obstruction to the incoming sound Waves and thus may cause an additional decrease in the sensitivity of the microphone.

Therefore one of the principal objects of this invention is to devise a new and improved microphone which utilizes a unique aperturedjgrille without adversely-affecting the sensitivity of the microphone.

Another object-of the'invention is to device a new and` improved microphone which utilizes an apertured grille inv a novel manner to provide a substantial match between the acoustical characteristic impedance of air and the resonant impedanceof the diaphragm, whereby the sensitivity of the microphone is substantially increased.

A further object of this invention isto devise a new and improved condenser microphone which is relatively simple, economical, troublefree, and

to mass production techniques.

readily adaptable Another object of this-invention is to devisesuch a i new'and improved condenser microphone having a higher a thickness substantially equal to sensitivity than heretofore possible.

In accordance with one aspect of the present invention, vthere has been devised a new and improved microphone comprising a vibratile diaphragm resonant at a predetermined frequency of incident sound vibrations and having a predetermined mechanical impedance at its resonant frequency. Means are provided for substantially matching the mechanical impedance of the diaphragm with Ithe acoustical impedance of air. These means include a facelplate including a multi-apertured grille parallel to the diaphragm and spaced therefrom by a distance substantially equal to and establishing an otherwise substantially closed accus tic cavity therebetween. characteristic dimension which is 21t-1 4 The spacingbetween the peripheries of adjacent aper- Atures 'in the grille is less than 7\/2 and the grille has a percentage aggregate perforate area with respectto its totalsurface area substantially equalto Za -Z-dX 100% z where Za is the acoustical impedance of air, Zd is the mechanical impedance of the diaphragm, k is the wave length of the resonant sound vibrations in air, and m and n are integers.

In' accordance with another aspect of the present in.

vention, there has been devised a new and improved condenser microphone having a predetermined acoustical.

resonant frequency.l The microphone comprises an insulating support member having a substantially at surface and a metallic stationary plate member imbedded in the support member. The stationary plate member has a substantially iiat exposed surface coplanar with the surface of the support member. A frame-shaped spacer member is provided having two oppositely disp posed substantially parallel surfaces and having a thicklness which is at least one order of magnitude smallerv than the wave length in air corresponding to the resonant frequency of the microphone. The spacer is in surface contact with the support member and substantially enycompasses the exposed surface of the stationary plate member; the shortest distance between the` spacer and the stationary plate member is substantially greater than the spacer thickness. There is also provided a vibratile membrane having a substantially uniform thickness which is atv least two orders of magnitude smaller than the wave length corresponding to the resonant frequency of uniform and closed acoustic chamber therebetween.

One of the characteristic dimensions of the chamber is thus equal to the thickness of the spacer and, along with the distributed mass of the membrane, determines the 'j resonant frequency of the microphone. The membrane is stressed by an amount only to insure tautness but not by -an amount suicient to substantially affect the reso.

nant frequency.

The features of the present invention which are be.

lieved to be novel are set forth with particularity in the appended claims. The organization and manner of op4 Patented Oct.l 13, 1959,

The grille has at least one' greater than M2 and 3 i eration of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify likeY elements, and in which:

Figure l is an elevational viewof` a microphone constructed in accordance with the present invention;

lFigure 2 is a cross-sectional view of the microphone as viewed along the line 2-2 of Figure 1;

Figure 3 is a cross-sectionalv view of the microphone as viewed along the line 3 3 of Figure l; and

' Figure 4 is an exploded view of the microphone assembly.

For some applications, it may be desirable to provide a diaphragm resonant frequency silghtly higher than the frequency of intended use of the microphone. The sensitivity of a condenser microphone increases rapidly as the thickness of the air cushion behind the diaphragm decreases, and the resonant frequency of the diaphragm varies directly with the square root of the density of the air. Thus, if the microphone is operated in a locality of substantially reduced barometric pressure, the resonant frequency of the microphone is reduced to a value in the region of the frequency of intended use. As a result, the sensitivity of the microphone does not decrease, but is maintained constant or may even increase somewhat when operated in a locality of lower barometric pressure. In this manner, maximum utility, regardless of geographical location, may be obtained. The invention is described with'pa'rticular reference to an ultrasonic microphone for use in conjunction with a remote control system operating at about 40 kilocycles, and the resonant frequency of the diaphragm is established at 45 kilocycles.

With reference to the drawings, a microphone constructed in accordance with the present invention comprises an insulating support having a substantially flat and smooth surface 11 formed on one side thereof, A metallic and substantially rectangular-shaped plate electrode 12 is embeddedv in support |10 and is held in position by cement or other suitable means. Electrode 12 is provided' with a substantially flat and smooth surface 13 which is coplanar with face 11 of support 10. A metallic and substantially rectangular frame-shaped spacer 14 is in surface contact with face 11 of support 10 and completely encompasses, yet is electrically insulated from, the exposed' surface of electrode 12. For reasons to be more apparent hereinafter, spacer 14 is provided with fiat and smooth oppositely-disposed parallel surfaces, one of which is in contact with support 10. A metallic-coated vibratile plastic membrane 15 is in surface contact with theexposed surface of spacer 14. Membrane 15 is tautly stretched across spacer 14 to insure a parallel relationship thereof with. respect to the exposed face of electrode 12 but is not maintained under sufficient lateral stress to materially affect its resonant frequency. An acoustic cavity is thus formed between membrane 15 and electrode 12.. A metallic gasket 16 and a plastic Kgasket 17, each of uniform thickness, are placed in juxtaposition with the exposed side of' diaphragm 15, and the entire assembly is held' together by means of a spring pressure plate 18 which is mechanically connected to face plate 19 by a pair of screws 20. The maximum pressure exerted by plate 18 on the assembly is determined by tubular spacer members 21 interposed between pressure plate E18 and face plate` 19 and surrounding screws 20. The entire.

assembly is held in proper alignment by dowel pins 22 press-fitted into face plate 19l and cooperating with suitable indexing holes 23y (Figure 4) formed in the various elements.

Face plate 19 includes a rectangular-shaped grille, shown more clearly in Figure 1 as dotted line 24, having substantially smooth oppositely disposed parallel faces and further having a uniform thickness t as shown in Figure 3'. Grille 24 is provided with a plurality of paraland a width w, as indicated in Figure l.

25, each having a length l Slots 25 preferably extend for the full Width of grille 24 and are equally spaced from one another to form a slot pattern lel rectangular-shaped slots 'which is centrally disposed with respect to grille 24.

As shown in Figure 3, grille 24 is parallel to diaphragm 1S and is spaced therefrom by a distance d. Face plate 19 is provided with a rectangular cavity 26 to establish an otherwise closed and uniform acoustic chamber between grille 24 and diaphragm 15.

As shown in Figure 2, metallic-coated vibratile membrane 15 constitutes one electrode of a variable condenser and is electrically returned to ground potential by conductor 27 connected to spacer 14, which, in turn, is in electrical contact with membrane 15. Fixed electrode 12 is electrically connected to a source of unidirectional operating potential, conventionally designated B+, through a resistor R; electrode i12 may be connected to the inputvcircuit of a suitable amplifier through a coupling condenser (not shown).

In operation, sound waves enter the microphone through slots 25 and impinge upon diaphragm electrode '115 to vary the spacing and hence the capacity between electrodes 12 and 15. This change in capacity is reflected as a corresponding change in voltage developed across resistor R, in a manner fully analogous with the electrical operation of condenser microphones of known construction.

'The acoustic system of the illustrated condenser microphone is not, however, quite so simple and will be explained in terms of an example of the actual mechanics involved.

In the design of prior condenser microphones, it has been the general practice to provide a diaphragm electrode spaced from a fixed electrode and laterally stressed by a predetermined amount until the desired resonant frequency of the diaphragm is obtained; in some constructions, the stress imposed may even be of suicient magnitude to approach the elastic limit of the diaphragm. `In a condenser microphone embodying the present invention, diaphragm 15 is tautly stretched across spacer 14 to insure a parallel relationship between diaphragm 15 and fixed plate 12 but is not stressed by an amountl to substantially affect the resonant frequency of the diaphragm.

In the preferred construction, diaphragm 15 is cornposed of a polyester film, such as that manufactured by I. E. du Pont de Nemours and Company and sold under the name of Mylar, provided with `an evaporated aluminum film of a thickness sufficient to give a resistance of approximately l0 ohms per square. It has been found that an aluminum coated Mylar diaphragm having a thickness of' 1/2 mil has a mass per unit area sufiicient to resonate at approximately 45 kc. with a 4 mil air cushion, an air gap sufficiently large to permit the use of rnass production assembly techniques without encountering short circuits and arc-overs.

In order to reduce electrical leakage between diaphragm 15 and electrode 12, spacer 14 is dimensioned so that the shortest distance between spacer 14 and electrode 12, shown as s in Figure 2, is `substantially greater than the depth of the air cushion which is equal to the thickness of spacer 1-4 or 4 mils; in the preferred embodiment the spacing s maybe 32 mils.

' The acoustical characteristic impedance for air at room temperature is approximately equal to 42 dynes/om.2 per cm./second, while 1/2- mil thick aluminum-coated Mylar` Howl ever, experimental measurements indicate that the microphone in open air, with the face plate removed, hasa Q of only 3. Thus, it isevident that there exists, `somewhere in the microphone structure, a friction or damping loss equivalent to 3 times the'characteristic impedance of air, making a total resistance 4 times that which would be expected from air damping alone.

In the microphone of the present invention, face plate 19 and particularly grille 24 are uniquely constructed to provide a substantial impedance match between the acoustical characteristic impedancc of air and the resonant impedance of diaphragm 15 to improvethe sensitivity of the assembly. The characteristic impedance of the grille increases in inverse proportion tothe fractional open area; that is, the characteristic impedance increases as the fractional open area decreases andY decreases as the fractional open'area increases. 'Ihe fractional open area of 'grille 24 is defined as the ratio of the aggregate open area of slots 25 to the total surface area'of the grille.

The resonant impedance of the diaphragm is matched with the characteristic impedance of air by providing a grille of a thickness substantially equal to an odd number of one-quarter wavelengths at the operating frequency and having a fractional open area substantially equal to where Za and Zd are the characteristic impedance of air andthe resonant impedance of the diaphragm,- respectively. Moreover, grille 24 is 'spacedfrom diaphragm 15 by an integral number of one-half` wavelengths at the operating frequency; 'an obviously equivalent construction is one in which the spacing d is made as close to zero as possible without inhibiting vibration of the diaphragm, and this construction is` accordingly within the spirit and scope of the invention. In the example shown, the thickness t of grille 24 is substantially equal to onequarter `wave length at the desired operating frequency of 40 kilocycles, or approximately 0.086 inch. The spacing between grille 24 and diaphragm 15 is one-half wavc` length at 40 kilocycles, or 0.172 inch. Moreover, as diaphragm 1S has a resonant impedance equal to 3 times the acoustical characteristic impedance of air, the characteristic impedance of (grille 24' should be substantially equal to \/3 times the characteristic impedance of air. Therefore, the maximum opening of grille 24 is reduced to approximately and the total aggregate `open area of slots 23 is substantially 58% of the total surface area of grilleV 24.

To avoid undesirable phase errors, it is preferred that slots 25be parallel to and spaced from one another so that the distance between adjacent slots is substantially less than one-half wave-length of sound in air at 40 kc., or 0.172 inch. Slots 25 are preferably rectangular and extend vsubstantially across the entire width of grille 24 as viewed in Figure 1. The slots are also preferably equally spaced from one another and arranged in a pattern which extends substantially the entire length of gnue 24.

- In utilizing the previously described microphone as a pick-up device, for example' in a remote control apparatus such as that shown and described on pages 156-161 of the March 1, 1957, issue of Electronics, it may be desirable to provide a compromise between the maximum sensitivity and the directivity of the microphone. With the microphone oriented in a position shown in Figure 1 of the drawings, the horizontal directivity is either increased or decreased, as desired, by effecting a corresponding increase or decrease in the Width W of the diaphragm.Y By the same token, the vertical directivity is either increased or decreased, as desired, by eecting a corresponding increase ,or'decrease inthe llength Lfof the diaphragm. The idealsituation would of course exist in the apparatus above mentioned when the microphone possesses the characteristics of maximum sensitivity and minimum directivity; that is, it is desirable that the microphone have a maximum of sensitivity in response to incoming sound vibrations ofthe control frequency approaching the diaphragm from `any and` all directions, s0 that theV response, is not at all4 dependant uponV the angle of incidence of the irnpinging sound wave fronts.`

VHowever, an increase in either of the characteristic surface dimensions of the diaphragm elects a corresponding increase in sensitivity butalso .causes the microphone `to bemore directional which may bepundesirable for some applications. In theremote control apparatus mentioned, it is desired to dimension the diaphragm, and consequently the grille of the microphone, so that width Wr is substantially equal to one wavelength at the operating frequency, or approximately 0.35 inch; length L is dirnensioned to be substantially equal to two wavelengths at the operating frequency, or approximately 0.70 inch. As a result, the microphone has a directivity of substantially plus or minus 30 in a vertical direction, and has a directivity of plus or minus in a horizontal direction. It is to be noted, however, an important feature of such a construction is that a change in the width dimension of the diaphragm does` not substantially affect the vertical directivity, and conversely, a change in the length dimension of the diaphragm does not substantially affect the horizontal directivity.

Microphonesconstructed in accordance with the present invention possess ,many desirable characteristics thatl have not heretoforebeen available to the art. Firstly, it is quite evident that surfaces 11 and 13 of support 10 and xed electrode 12, respectively, are very easily rendered coplanar with respect tol one another simply by lapping or otherwise machining the two surfaces simul-v taneously. Secondly, the depth of the air cushion be tween diaphragm. 15-and electrode 12, and consequently the resonant frequency of the diaphragm for a given distributed mass, is determined simply by the thickness of onant frequency. As la result of its unique construction,`

the microphone is extremely simple, rugged and economical and yet providesrimproved sensitivity as compared with prior condenser microphones. M g

While the invention has been shown and described as embodied in a condenser microphone, the impedance matching arrangement of the invention may be employed with equal or even greater advantage in 'piezoelectric and other types of microphones, since the impedance mismatch encountered in ksuch other devices` is generally greater than that encountered in a condensermicrophone. While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in itsbroader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

l. A microphone structure comprising in combination:

comprising a face plate having a multi-apertlured grille parallel to said diaphragm and spaced therefrom by a distance substantially equal to 7 and establishing an otherwise substantially closed acoustic cavity between said grille and said diaphragm, said grille having at least one characteristic dimension which is greater than M2 and a thickness substantially equal to the spacing between the peripheries of adjacent apertures in said grille being less'than M2 and said grille having a percentage aggregate perforate area with respect to its total surface area substantially equal to \/Za/Zd l%, where Za is the acoustical impedance of air, Zd is said mechanical impedance, is the wave length of said resonant sound vibrations in air, and m and n are integers.

2. A microphone structure comprising in combination: a. vibratile diaphragm resonant at a predetermined frequency of incident sound vibrations and having a predetermined mechanical impedance at said resonant frequency; and means for substantially matching said diaphragm impedance with the acoustical impedance of air, comprising a `face plate having a multi-aperturedl grille, parallel to saiddiaphragm and spaced therefrom by a distance substantially equal to M2 and establishing an otherwise substantially closed 4acoustic cavity between said grille and said diaphragm, said grille having at least one characteristic dimension which is greater than M2 and a thickness substantially equal to the spacing between the peripheries of adjacent apertures in said grille being less than M2 and said grille having a percentage aggregate perforate Iarea with respect to its total surface area substantially equal to Za Zd where Za is the acoustical impedance of air, Zd is said mechanical impedancet. is the wave length of said reso- -nant sound vibrations in air, and n is an integer.

3. A microphone structure comprising in combination: a vibratile `diaphragm resonant `at a predetermined frequencyof incident sound vibrations and having a predetermined -mechanical impedance at said resonant frequency; and means for substantially matching said diaphragm impedance with the acoustical impedance of air,

comprising vva vface plate having a multi-apertured grille parallel to said diaphragm andl spaced therefrom by a distance substantially equal to 2 and establishing an otherwise substantially closed acoustic cavity between said grille and said diaphragm, said grille having' at least one characteristic dimension which is greater than M2 and a thickness substantially equalv to M 41, the spacing between the periphcries of adjacent apertures in said grille being less than M2 andi said grille having a percentage aggregate perforate area with respect toits totalI surface area substantially equall to the Za VZX 100% where Za is the acoustical impedance of air, Zd is said mechanical impedance, A is the wave length of said sound vibrations in air and m is an integer.

4. A microphone structure comprising in combination: a vibratile diaphragm resonant at a predetermined superaudible frequency of incident sound vibrations and having a predetermined mechanical impedance 'at said resonant frequency, said diaphragm further having characteristic surface. dimensions substantially greater than M2; and means for substantially matching said diaphragm impedance with the acoustical impedance of air, comprising a face plate :having a grille provided with a plurality of parallel slots, parallel to said diaphragm and spaced therefrom by a distance substantially equal to M2 and establishing an otherwise closed acoustic cavity between `said grille and diaphragm, said grille having characteristic surface dimensions substantially equal to corresponding characteristic surface dimensions of said diaphragm and a thickness substantially equal to the spacing between the peripheries of adjacent slots in said grille being less than M2 and said grille having a percentage yaggregate perforate area with respect to its total larea substantially equal to \/Za/Zd where Za is the acoustical impedance of air, Zd is said mechanical impedance, and x is the wave length of said resonant sound vibrations in air.

5. A condenser microphone having a predetermined acoustical'resonant frequency comprising: an insulating support member having a substantially ilat surface; a metallic stationary plate member embedded in said sup'- port member and having a substantially ilat exposed sur- -face coplanar with said support member surface; a frameshapedV spacer member having two oppositely disposed substantially parallel surfaces and having a thickness which is at least one order of magnitude smaller than the wave length in air corresponding to said resonant frequency, said spacer being in surface contact with said support member and substantially encompassing said exposed plate surface, and the shortest distance between said spacer and plate member being substantially greater than said spacer thickness; anda vibratile membrane having a substantially uniform thickness which is at least two orders of magnitude smaller than said wave length and in. surface contact with the opposite surface of said spacer than saidy support member, said membrane being substantially taut `and parallel to said exposed plate surface to form a substantially runiform and closed acoustic chamber therebetween one characteristic dimension of which is equal to said spacer thickness,y said characteristic dimension along with the distributed mass of said mem-` brane determining said resonant frequency and said membrane stressed -by an amount only to insure tautness and insufficient to substantially affect said resonant frequency.

6.. A condenser microphone having a predetermined acoustical resonant frequency comprising: an insulating. support member having a substantially ilat surface; a metallic stationary plate member embedded in said support member and having a substantially dat exposed surface coplanar with said support member surface; a metallic frame-shaped spacer member having two oppositely disposed substantially parallel surfaces and having a thickness which is at least one order of magnitude smaller than the wave length in air corresponding to said resonant fre-y quency, said spacer being in surface contact with said support member and substantially encompassing said exposed plate surface, and the shortest distance betweenl said spacer and plate member being substantially greater than said spacer thickness; and a metallic coated vibratile membrane having. `a substantially uniform thickness which is at least two orders of magnitude smaller than said wave length. and in surface contact with the opposite surface of said spacer than said support member7 said membraney being substantially taut and parallel to said exposed plate surface to form a substantially uniform and closed acoustic chamber therebetween one characteristic dimen sion of which is equal to said spacer thickness said char acteristic dimension along with the distributed mass of said membrane determining said resonant frequency and said membrane stressed by an amount only to insure tautness and insuicient to substantially affect said resonant frequency.

Moreland M ay 30, 1950 Grosskopp et al. Apr. 2, 1957,

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