US2351400A - Microphone - Google Patents

Description

June 13, 1944. R` W` CARUSLE l 2,351,400

MICROPHONE Filed July 29, 1940 5 -Shee'i.:s-Shee'c 1 7?/ '-g j J7 Y Z3 V Q" 33'@ Z INVENTOR JL w. CMM@ @www ATTORNEY June 13, 1944. R. w. CARLISLE 2,351,400

MICROPHONE Filed July 29, 1940 3 Sheets-Sheet 2 Voli:

INVENTOR 53"" mencin o smaim ATTORbEY June 13, 1944. R. w. CARLISLE MICROPHONE Filed July 29, 1940 5 Sheets-Sheet 5 INVENTOR fw (4x/dbf@ S LILMMLQQ ATTORNEY in Figs. 12a and 13j Patented AJune 13, 194'4 UNITED STATES .PATENT OFFICE Richard W. Carlisle, Elmsford, N.

Sonotone Corporation,

poration of New York Application July 29, 1940, Serial No. 348,108

14 Claims.

This invention relates to microphones and more particularly to microphones of the type required for hearing aids and similar applications in which a small microphone structure of high sensitivity over the important part of the speech frequency range is required.

Among the objects of the invention is a microphone which, notwithstanding its great compactness essential for applications, such as hearing aids, has high sensitivity to speech frequency vibrations propagated over the air and hasits acoustic elements so arranged inside a housing as to suppress the disturbing eects of cord and clothes rubbing vibrations imparted to the microphone and thus reduce microphone output.

The foregoing and other objects of the invention will be best understood from the following description of exemplications thereof, reference being had to the accompanying drawings Wherein Fig. l is a cross-sectional view through a microphone of the invention along line l-l of Fig. 2;

Fig. 2 is a cross-sectional view along line 2 2 of Fig. 1;

Fig. 3 is a of Fig. 2;

Fig. 4 is an elevational view illustrating the interior of the rear shell of the microphone, some of the parts being broken oi;

cross-sectional view along line 3 3 the noise level of the Fig. 5 is a diagrammatic view exemplifying the use of the microphone as a part of a hearing aid; Figs. 6 to 8 illustrate diilerent vibration modes of a circular diaphragm;

Fig. 9 is an enlarged cross-sectional view showing the coupling .of the resonant cavity to the microphone diaphragm of Fig. 1; j

Fig. 10-is a rear view'of the cavity plate of Fig. 9:

Fig. ll is atypical response curve of the microphone;

Fig. 12 is a curve illustrating the diaphragm vibration modes of the microphone unitshown Figs. 12a and 13 are views similar to Figs. 9

and l0 illustrating another arrangement of the microphone`unit of the invention;

' Fig. 14 is a curve illustrating the diaphragm vibration modes of the microphone unit shown in Figs. 14a and 15? and Figs. 14a and 15 are views similar to Figs. 9

and 10 illustrating a further arrangement of the microphone unit of the. invention.

Deafened persons are hearing aid must operate with Y., assigner to to conceal their impairment. To be useful, a hearing aid device must be suitable for comfortable and inconspicuous wear on the body of the user. Only relatively small batteries are practical :for such hearing aids. As a result, the microphone as well as all the other elements of the a high sensitivity. In addition, the microphone and the other elements of a hearing aid ldevice must not only be very small and light in weight to avoid being bur densome to the user, but they must be able to respond fairly uniformly at high eillciency to all important speech frequencies and they should operate satisfactorily under all conditions of use, without requiring special attention and frequent reconditioning.

vutilizing the microphonic eifect of carbon contacts for producing the electric output, present difficulties because they have to be operated in a variety of angular positions when tilted by the body of the user on which they are worn. In addition, since the microphone and the electric cords which`connect it to the other elements of the hearing den under the clothing of the user, the microphone is subjected to vibrations caused by rubbing of the clothes against the microphone casing and the cord, thereby raising in a very disturbing way thenoise level of the microphone.

Many diiiiculties have also been encountered with prior dust entering the microphone and causing damage to the control unit, such as the switch and rheostat. mechanism usually associated withxsuch hearing aid microphones.

According to one phase of the invention, a microphone Othighsensitivity and good overall response and very compact overall dimensions is'provided byarranging a flat microphone unit having two outwardly facing diaphragms in a flat housing so as to form around the periphery -of the microphone unit a peripheral acoustic chamber which is vcoupled to the exterior area very sensitive and seek 55 the housing parts serving as the support furthe aid have to beworn more or less hidf hearing aid microphones because of through thin air spaces extendingphone unit being so supported in the housing that noise vibrations reaching the cord. part of the housing are eil'ectively prevented from being mechanically transmitted to the other part of the housing.

According to other phases of the invention, a microphone formed of a vibro-electric translating device including a nvibratory, diaphragm is combined with acoustic exciting means for causing the diaphragm to vibrate at one yor more higher modes,land particularly dis-symmetrical 1:V

modes, so as to modify the frequency response of the microphone, and specinc arrangements are provided for exciting a microphone diaphragm to vibrate ,at one or more higher modes. and particularly dis-symmetrical modes, so as to modify its output response.

Other features of the invention involve a microphone unit enclosed in a hat casing so as to form around the periphery of the microphone unit an acoustic chamber which is coupled to Y.

ythe diaphragm through a thin airspace extending in the front of the diaphragm having a thickness of the order of s to 64 of an inch, and the combination of such microphone with a switch ing and rheostat-mechanlsm mounted in the thin air space and enclosed-in a protective material that resists passage of dust but is highly pervious to sound so that the air space has substantially the same acoustic properties in the presence or absence of the protective layer. v A hearing aid microphone exemplifying the invention is shown in Figs. l to 4. It comprises a housing formed of a rear shell 20 and a front shell 2l enclosing a circular. substantially ilat interior. The two housing shells 20, 2l are made by molding asynthetic resin material over formed sheets of strong material, such as duralumin, so as to keep the walls of the shells 20, '2| extremely thin and reduce to a minimum its overall ,bulk. without giving `the exterior a metallic characte objectionable in a hearing aid'microphone.

The rearshell 20 has, at four points spaced at an angle of about against its vertical axis,"

indicated by a dash-dotted line, four anchoring posts 24 aligned opposite similar posts 25 extending from'the front shell. The two housing shells 20, 2l are clamped to each other by screws 25' extending throughthe: posts. The posts 25 of the front shell 2|' have short pillar-Alike extensions about 11g Aof an inch above the edge of the shell, so as to provide between the facing edges of the two shells a 'peripheral acoustic vent 26 from the exterior to the interior space of the housing. f

As shown in Figs. 2 to 4, the bottom portion of the rear shell 20 has a forwardly projecting ledge portion 21 which serves as a support for three terminal bushings 28 designed to receive the plug terminals of a plug at the end of a cord 29 which connects the electrical elements of the microphone to the other elements of the hearing aid y y Worn on the body of the user. 'Ihe edge portion vof the front shell facing `the terminal ledge 21 is spaced therefrom by a gap, and the mechancord leading from the microphone, and the microsmall cross-sectional area of the four pillar posts 25, the disturbances of the microphone output by noises transmitted to the microphone housing, through propagation along the cord, are radically reduced.

This is probably due 'to the fact that in -a microphone arranged in the general way described above, rubbing vibrations transmitted by the cord to the rear shell are propagated along the direction ofits main plane in which it is very eicctive in attenuating such vibrations, whereas if such 'vibrations are propagated from the rear shell to the front shell, they are imparted 'theretov in a general direction perpendicular to its main surface in which direction it is more readily set into vibrations.

Inside the housing shells 20, 2l is mounted the microphone unit 30. The elements of the microphone unit are held in their operating positions by'a spider member 3l, formed, for instance, of a metal die-casting, having a circular rim 32 and a central supporting ring 33 spaced from the outer rim by the spider arms. The central ring 33 is slightly offset and has secured thereto a central supporting plate 34 which is aligned in the central plane of the rim 32.

The edges of the two similar circular vibratory diaphragms 36 are held against the opposite edges of the spider rim 32 by ferrules 31, suitably attached tothe rim as by pins or screws, a spring washer 38, peripherally be'nt into wavy shape, keeping the edge of each diaphragm clamped to the rim 32.

The-two diaphragms 3B' are used to drive two vibro-electric energy translating units lheld on the opposite sides of the center plate of the spider 3|, so as to produce by the combined action of both diaphragms 36-.a relatively large electric output in response to sound propagated in the air surrounding themicrophone housing 20, 2l,

I have found that by arranging a ilat microphone unit having two outwardly facing diaphragms in a at housing so as to form around the periphery of the microphone unit a peripheral acoustic chamber, which iscoupled to each daphragm through thin air spaces extending in front of the diaphragm and to the exterior air through narrow acoustic inlet openings or vents distributed substantially uniformly around the periphery, the two* microphone diaphragms will be excited in proper phase relation by sound propagated from the exterior air, and that they will operate with great sensitivityv overhthe prncipal part ofthe speech frequency range important for a hearing aid,if the peripheral acoustic chamber, the thin diaphragm coupling spaces and the exterior inlet vents are designed to resonate in the principal part of the speech frequency y range, and particularly over the range between about 1000 and 2500 cycles.

Theprovision of a peripheral acoustic cham'- ber which is coupled to the exterior through relatively narrow slots and to two vibratorily excited .microphone diaphragms by very narrow air spaces, it is possible to obtain a highly sensitive microphone that occupies a very narrow space.

The narrow air spaces extending in front of the diaphragms may be readily designed so that the rate of acoustic propagation through the air spaces is made equal to the rate of mechanical propagation through the diaphragms, and that the acoustic energy is transmitted from the air spaces to the diaphragms at not only the fundaaccordance with the principles outlined above,.

so as to provide an acoustic cavity around the rim of the microphoneunit which is coupled to the exterior through relatively narrow acoustic inlet vents and to the diaphragms through relatively narrow air spaces, the two diaphragms will be excited in proper phase relation so as to give an output much greater than that obtainable with a microphone having a single diaphragm and requiring a space of about the same order, as heretofore required for single diaphragm microphones.

An exterior inlet vent slot of about to n, of an inch width and distributed substantially uniformly around the periphery is effective in securing efiicient acoustic coupling of the peripheral chamber with the exterior.

Satisfactory propagation of sound from the peripheral chamber to the center of the diaphragms was found to occur if the fiat walls of the microphone housing are spaced 'from the facing diaphragm by about 3/64 to n, of an inch, the spacing depending on the hardness or stiiness of the vibratory diaphragms 36. vHarderdiaphragms operate better with a slightly wider spacing than softer diaphragms.

The peripheral air chamber is in general designed to distribute the acoustic impulses equally to the front and rear diaphragms and to be partially resonant with the acoustic inertance of the exterior inlet vent slots in the higher part of the principal speech frequency range between about 1500 and 2500 cycles.

In order to provide suihcient excitation of the diaphragms 36 -at the lower part of the frequency range, the space between the diaphragme is connected to the peripheral cavity by internal vent holes 39 designed so as to resonate in the low frequency range, for instance, in the region of about 400 cycles.

Additional excitation of the diaphragms in dis-symmetrical modes may be secured by special acoustic resonators, as will be explained hereinafter.

Although thefvarious features of the invention disclosed herein are applicable to Rochelle salt crystal, electromagnetic, condenser microphones, and the like, a specific exemplification of the invention as applied to a microphone of the carbon ball type, that is widelyused in the hearing aid field, will here be described.

Each of' the two diaphragms A36 is of carbon, and cooperates with a pair of carbon electrodes 4|, each having a plurality of conical cavities 43, each containing a quantity of loose granules of microphonic material; such as carbon balls, resting between the conical cavity surface and the facing portion of the diaphragm so as to modulate a D. C. exciting current passing therebetweenin response to sound reaching the diaphragm.

In the form shown, the cavityelectrodes 4| are of semi-circular shape, and each pair of. elec. trodes 4| facing a diaphragm 36 is assembled into a self-sustaining mechanical unit within a ring 45 of metal, for instance, being insulatinglyy spaced from each other and from the lring by radial insulating strips 46 and peripheral insulating strips 41. The two pairs of semi-circular cavity electrodes 4|, each pair joined into a mechanical unit within the clamping ring 45, are clamped to the opposite sides of the `central spider plate 34 by a single screw 43 and nut 43' whichare insulated from the adjacent conducting elements by insulating collars 49, 49', the electrodes being insulated from the spider plate 34 by insulating sheet portions 5|.

Against the periphery of each semi-circular electrode 4| is held clamped an arcuate contact 'strip 52 of metal having a tail portion 53 connected, as by soldering, to the inward tips of two pairs of terminal pins 54, radially held within insulating sleeves seated in the spiderrim 32, thel pair of terminal pins 54 serving as terminals for one pair of oppositely facing microphone cavity electrodes 4|, and the other pair of terminal pins 55 serving similarly as terminals for the other pair of oppositely facing microphone cavity electrodes.

When used as a part of a microphone amplifier hearing aid illustrated diagrammatically in Fig. 5, a microphone arrangement of the type described above will give very important operating advantages.

Such Wearable ,hearing aid usually consists of `a microphone 30 with a control unit 60, a hearleading to the battery 64 to a contact strip-68' leading to the microphone and a rheostat 69 leading to the receiver 6|. When the control unit is in the position shown, it connects the microphone 30 in series with the amplifier wind ing 62 to' the battery for actuating the amplifier microphone 63 with modulated speech frequency input currents, and it connects the rheostat 69 in series with the receiver 6| and the amplifier microphone 63 to the battery for actuating the receiver 6| with -amplied output currents.

To secure maximum sensitivity, such wearable hearing aids are usually equipped with carbon ball microphones having, as shown, a carbon diaphragm and a carbon block with a plurality of cone-shaped cavities, each cavity containing a Quantity of carbon balls which form a heap resting in the space between the lower inclined wall of the cavity and the diaphragm'and form a resistancepath which varies the resistance be tween the carbon block and the diaphragm in accordance with the vibrations imparted to the diaphragm by sound waves propagated in the air for modulating the transmitter current passing between the transmitter electrodes and producing correspondingly modulated sound-frequency currents.

The carbon balls perform two functions. First,

through the contacts with the diaphragm andcavities of a transmitter may assume in operation a variety of diilerent formations, and `if the microphone is tilted from the normal vertical.v

to lthe horizontal position, a condition will be reached in which the contacts are broken. In

addition, it is important to prevent the application of more than about 2l/2 volts to a diaphragml way shown diagrammatically in Fig. 5.

When such a microphone arrangement is tilted from the vertical to the horizontal position, the pressure on the balls facing the lower diaphragm will increase as the pressure on the ballson the upper 'side is decreased, with the result that the total direct current thr'ough the microphone remains substantially constant until the unit Ais nearly horizontal.` for the stable operation of a microphone ampliiler hearing aid because variations of the direct V current flowing through the microphone upset the stability of the microphoneaxnplier.

vangular positions as they are tilted by thevbody of the user and the loose carbon balls inthe quired gap from the facing surfaces of the housing shells 20, 2|. small sponge rubber pads 16 are cemented to the surfaces of the four pairs of posts 24, 25 facing the rim 32 of the microphone unit, the sponge rubber pads being proportioned to have sufficient thickness to assist the iiexible suspension arms 12', 13' in keeping the micro-v phone unit in its properly spaced position, while at the same time having sumcient vibration absorbing` capacityq to effectively suppress propaga-.

tion of noise vibrations from the shell to the niicrophone unit 30 and to reduce `the effect of rubbing noises on the microphone output to a negli gible value.

sulated from the underlying metallic reinforcing plate 22 of the housing 'shell 20 by a thin sheet Y or layer of insulating material 6I extending over and united to substantially the entire inwardly facing surfaces of the reinforcing plate.

The metallic connector strips 12, 13, as well as the other metallic operating elements located on the interior surface of the housing shell 20 described hereinafter are insulatingly held 25 thereon in their proper operating position by 'I'his is extremely important pletely insulated. All elements held in position At the same time, the voltage of the battery i is at all times distributed over two series connected microphone electrode groups, so that no electrode group is subjected to a voltage higher than 21/2 volts even if a 41/2 volt battery is used.

In the arrangement shown in Figs. l to 4, the series parallel interconnections between the four microphone electrode groups of the microphone unit 30 formed by the two diaphragme 36 and the I four cavity electrodes 4i, insulatingly mounted between the diaphragms, are completed by two segmental metallic connector strips 12, 13 which are secured to the internal surface of the rear shell 20, each connector strip 12, 13 having a pair of long inwardly projecting arms 12', 13' which have inwardly foldedA extremely flexible portions,

the ends of which are connected, as by clamping,

y under the heads of the two pairs of cavity-electrode'terminals pins 54,v 55, respectively, so that each of the connector strips 12, 13 connects one pair of oppositely facing cavity electrodes v5| in parallel andv that each of the two diaphragms 36 which make contact with the carbon balls held in the cavity electrodes 5i, complete two series connections between each pair of cavity electrodes 5i facing it, in the way illustrated in'` brations, the flexible suspension arms are made as thin as consistent with the requirement that In the practical 70 they should resist corrosion. construction of such microphone, the suspension `arms are about. .002 to .003 inch thick.

In order to hold the microphone unit 30 in its proper operating position within the housing and maintain its two'qiaphragms 36 spaced by the rethreaded studs 62 and at nuts' 63, the studs .62 'having heads molded in the insulating body of the shell 26 and insulatingly projecting through holes in its reinforcing plate 22 so as to be comby screw studs, such as studs 82, are so arranged and located that their dat nuts 63 face the area of the overlying microphone diaphragm 36, and the thickness of the nuts 63 is substantially equal to the height of the clamping ferrule overlying the edge'of the diaphragm 38 so that the screws and nuts do not increase the overall `thickness of the microphone assembly. A

By following the principles of the design outlined above, it was possible to compress within a unit having an overall diameter of not more than 3% of an inch and an overall thickness of not more than .83 of an inch, a hearing aid microphone that has all the desirable features of the best heretofore available hearing aid microing with two driving diaphragms and was able to give agreater output at a much lower. noise level.

In addition, a way was also foundfor mounting in the cramped acoustic cavity space of the extremely fiat housing of such microphone the elements of thesWitching and rheostat mechanism, required when such microphone is used as a part-of a wearable microphone ampliiier hearing aid, without impairing the acoustic eiect of .the cavities and propagation spaces upon which itshigh sensitivity and goodlfrequency response depends.

As shown in Figs. l to 4, a unitary three-armed control member 60 is made fromA thin high strength elastic non-corrosible metal, such as Z-nickel, and is pivoted `at its hub on a pivot stud 65 held in the rear shell, so that by operating its control arm which extends through the upper .-'acoustic slot 26 and vhas an exterior operating knob 65, its other two arms 65, will slide on a fiat thin contact strip 66 and a flat rheostat 59 held clamped by insulated screw studs 82 and nuts 63 extending from the rear shell 20. Although the distance from the pivot 65 ofthe control member 60 to the contact `strip 66 and the rheo- -stat 69 is very short, its twoslidingcontact arms 65, 66 are. given theresiliency essential ,for obtaining reliable wiping contact engagement by forming them with looped undulationsI aligned in 'I'he metallic connector strips 12, 13 are in-v phones, and that at the same time was operatride firmly on the surface of the rear shell 20 rather than move without support.

A contact plate B1 of thin sheet metal having low friction resistance, such as nickel silver, is disposed between the reinforcing plate 22 of Duralumin and the lever Bil,k because the high friction resistance of aluminum would result in scraping. If a low friction metal is used for the shell reinforcing plate 22, the contact plate 61 may be eliminated.

Asshown in Fig. 3, a thin connector strip extending-from the contact plate 61 of the control-member 60 is connected to the center terminal bushing of the microphone shell 20, the two outer terminal bushings being connected by similar contact strips to the microphone electrode connector strip 54 and one end of the rheostat 69, the other microphone electrode connector strip 55ibeing connected by a similar thin connector strip to the Aswitch contact strip 68.

Great difliculties have been encountered in the past with rheostats built into the microphone of the hearing aids because the cavity of the microphone casing must have acoustic inlets and the dust which invariably reaches the contact surface of Ithe rheo'stat through the acoustic inlets is ground in by the moving contact intothe rheostat surface. As shown in Figs. 1 to 4, in the arrangement of the invention, the rheostat 69 is formed of an extremely thin wire-wound bobbin which is sunk into a recess in the rear shell 20 and Iprojects only slightly more than nl, inch above its surface.

A thin closely-woven sheet 81 of fabric is placed over the elements of the control mechanism Il to the-periphery of the rear shell 20 so as to permit free operation of the control member 60, while at the same time impeding the entrance of dust or lint into the neighborhood of the rheostat.` effective in practical construction.

This effective protection of the rheostat against dust is secured'without any additional space by \using for the protective sheet 81 a fabric'which is air-pervious, but dust impervious. Fine cotton fabric, such asl cotton lawn, was found satisfactory in actual use.

By designing the control mechanism in accordance with the principles described above, it may be combined with a. compact hearing aid microphone of the invention without occupying any space required for giving the acoustic chambers in the microphone housing the characteristics essential for the satisfactory operation of the microphone.

It has been long known that plain diaphragms vibrate=not only with a fundamental mode, but also with higher modes. Morse, "Vibrations'and Sound, 1936, pp. 133-181, deals with such vibrations. Figs. 6 to 8 of the drawings illustrate the different vibration modes of a circular diaphragm, Fig. 6 showing the fundamental mode, Fig. 'I a first dis-symmetrical mode and Fig. 8 a second dis-symmetrical mode.

Similar modes occur in rectangular dia.- phragms. In general, a membrane may be defined as a thin elongated member, such as a sheet Such arrangement was found very oi' rubber, capable of sustaining tension, but not compression, and a plate may be defined asa. thin solid body which can sustain tension as well as compression.

The explanations given by Morse for vibrating plates which are rigidly clamped Vat the edges apply in a general way also to diaphragms which are not clamped at the edges, the frequency ratios of the successive modes of a diaphragm varying somewhat from those of a plate, but the general shape of a diaphragm vibrating with different modes is substantially the same as that of a plate.

As far as I am aware, no attempt was ever made to devise amicrophone yusing a higher dis* symmetrical vibration mode ofa diaphragm as a part of its operating system. I have found that dis-symmetrical modes of a diaphragm may be used with great advantage in microphone vibrating systems and that various arrangements may -be used for driving the diaphragm in such dis-symmetrical modes.

- According to the invention, acoustic means are combined with a rdiaphragm forming part of a microphone so as to drive the diaphragm at one or more dis-symmetrical modes; or such a diaphragm is combined with a plurality of acoustic driving means to simultaneously actuate it in a number of symmetrical modes, the modes being used to operate a microphone transducer element.

Asl acoustic means for driving a diaphragm of a microphone at one or more dis-symmetrical modes, acoustic chambers may be used. According to the inventiomthe acoustic means for driving a diaphragm at a desired mode are formed of acoustic chambers having a volume and small vent apertures to the exterior so proportioned and arranged as to resonate substantially at the same frequency, as the desired mode of a given microphone diaphragm.

According to the invention, such a modalresonance acoustic chamber of the desired characteristics is acoustically coupled to a, moving part of the diaphragm so as to drive it acoustically. 'Ihe acoustic coupling may consist of a. narrow slit $5 formed between the diaphragm and an extension on the edge of the acoustic chamber enclosure. the width and the depth of the coupling slit being so proportioned as to have considerable inertance which keeps the air to a certain extent from'circulating through it except at frequencies below those at which the chamber is to be eifective in exciting the diaphragm at the desired mode.

According to the invention, a number of chambers are coupled to the diaphra m and used at .high frequencies without inter ering with the action of others used at lower frequencies by properly proportio'ning the slit by which the high frequency chambers are coupled to the diaphragm, so that they may be uncoupled from the diaphragm at lowy frequencies.

In practical designs, a narrow slit about 1 to 5 millimeters wide and about 1,/4 to 1/ millimeter deep formed between the diaphragm and an extension of the chamber enclosure is effective for exciting diaphragms to operate at its higher modes and particularly at dis-symmetrical modes.

In designing such coupling slit, consideration should be given to the fact that narrow slits 'offer little acoustic impedance at low frequencies while they offer high impedance at high frequencies.

The vibration modes of a diaphragm may be analyzed and determined by measuring the current passing between the vibrating diaphragm diaphragm and an exploring electrode.

and a single carbon ball interposed between the In case of a circular diaphragm of carbon .020 inch thick and 21% of an inch diameter, the fundamental mode is at about 800 cycles, a dis-symmetrical harmonic occurs near 2700 cycles, and another dis-symmetrical harmonic occurs between 4000 and 5000 cycles.

Figs. 9 and 1-0 illustrate one practical way for acoustically coupling to a circular carbon diaphragm 30 of a microphone, such as shown in Figs. 1 to 4, a modal resonant chamber so as to excite the diaphragm at higher mode and thereby broaden and improve the .microphone response.

To the outer edges of a circular carbon diaphragm 36 oi the microphone is-clamped acircular chamber plate 9| having a large opening 92 exposing the major part of the diaphragm area, and an odset plate portion 93 forming adjacent a segmental region of the diaphragm an acoustic cavity having a volume and small outlet vents 94 proportioned so that the cavity 93 has the desired resonant characteristics. As seen from a comparison with the similar diaphragm of Fig. 8, the cavity 93 extends .over a region of the diaphragm which is bounded by nodes of the dissymmetrical mode shown in Fig. 8, except that instead of extending the resonant cavity 93 over the entire haii of the diaphragm periphery on whichk the dis-symmetrical mode occurs, it is made somewhat shorter in arcuate length, leaving out the end regions in which the velocity of the diaphragm is negligible at the modal -frequency.

The large opening 92 of the chamber plate 9| exposes the major part of the diaphragm 3E to the sound waves reaching the microphone. The vent holes 94 of the acoustic chamber 93 are so proportioned as to cause the chamber s3 to resonate at the frequency of the particular mode at which the microphone, diaphragm is to vibrate. In designing such acoustic resonant chambers 93 for exciting the diaphragm to vibrate at its higher modes, the manner in which the chamber is coupled to the diaphragm is of importance. In the case shown in Figs. 9 and 1Y0, the chamber plate portion 95 which forms the boundary between the chamber acoustic cavity S3 and the large segment of the diaphragm 33 which is exposed through hole 92, is placed sufficiently close to the diaphragm 36 as to form a slit 95 which keeps air from circulating through it except at the frequencies at which the chamber is to resonate. In the specic embodiment of the invention shown, the radial length of the slit 95 is 114 to of an inch and the thickness of the slit is about .010 to .020 inch.

In order to prevent dust from reaching the slit 95 and become jammed therein against thediaphragm, a close-grained screen 96 of silk or similar sound-pervious, but dust-impervious, fabric is secured, as by cementing, to the outwardly bent flange portions of the chamber plate which border the opening 92 through which the diaphragm is exposed. When using such modal excitation chamber for the diaphragm of the microphone unit 30 shown in Figs. 1 to 4, the outer shell 2| has, as shown,

formed therein holes 98 in front of the openings 94 of the modal excitation chamber in order to of the inventlon,;such as shown in Figs. 1 to 4, 75

using a modal resonant cavity 93, such as shown in Figs. 9 and 10. The same microphone, without the resonant chamber S3, but with a flatter front shell 2| spaced fromthe front diaphragm 3G by the same gap as the rear shell 20, will have very little response above 3000 cycles, as indicated by the curve portion B. Th inner vent holes 39 in the rim of the microphone unit 30 raise the response in the low frequency region down to below 400 cycles, as indicated at C. By closing the inner vent holes 39, the low frequency response may' be lowered, as indicated at D, to fit the needs of a deafened individual having good hearing at low frequencies. The resonance effect of the peripheral acoustic chamber of the microphone housing and its exterior inlet vents, and the resonance effect of the narrow diaphragm coupling spaces raise the response in the region between 1000 and 2500cycles, as indicated at E.

Figs. 12, 12a and 13 show another arrangement for exciting a microphone diaphragm to vibrate at a higher as well as its fundamental mode. To the opposite sides of the rim of a spider |0| are clamped two diaphragms 36, as in Figs. 1 to 4. The spider I0| has a transverse partition |02, so as to form between the inwardly facing segmental regions of the two diaphragms an acoustic cavity |03 having a volume and ventopenings |04 through the spider rim proportioned so that the cavity |03 is resonant in the frequency range of the desired higher mode, as in the arrangement of Figs. 9 and 10.

Two pairs of carbon blocks |05, |06 having carbon-ball-holding outwardly-facing cavities are insulatingly held adjacent the diaphragms 36 by spider arms |08 between the two inwardly facing regions of the diaphragms 36, which are excited to vibrate with the fundamental and higher dissymmetrical vibration modes of the diaphragm for producing, in response to sound reaching the microphone unit, an electric output having a correspondingly broadened frequency response.

Figs. 14, 14a and 15 illustrate another microphone arrangement having diaphragms excited at fundamental and higher dis-symmetrical modes. 'Io the rims 32 of a mounting spider is clamped a diaphragm n36 and an overlying chamber plate lli. as in Figs. 1 to 4. The chamber plate has two large openings H2, ||3 through which the diaphragm 38 is exposed, and two offset plate portions forming, in front of the facing portions of the diaphragm, two modal-resonant cavities i4, I5 with vent holes H6, ||1, respectively. The volume and the vent holes of the two cavities H4, ||5 are proportioned so that the cavities are resonant at the frequency of the desired higher mode.

Two carbon block electrodes |05, |06 with ball cavities are suitably supported opposite the other side of the diaphragm 36, so that the carbon balls held in the block cavities are actuated by the fundamental and higher dissymmetrical modes of the diaphragm 36, as in Figs. 1 to 4, 9, 10 and 12, 13.

The principles of the invention explained in connection with specific exempliiications thereof will suggest to those skilled in the art many other applications and modifications of the same. It is accordingly desired that the appended claims be construed broadly, and that they shall not be limited to the specific details shown and described in connection with exempliiications thereof.

I claim:

1. In a microphone having a vibratory mechano-electric energy translating element: a diaphragm one side of which is exposed to an admental and modal' frequencies; said vibratory element being actuated by a portion of the diaphragm which/is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of the diaphragm directly exposed to said exterior space. y

2. In a microphone having a vibratory mechano-electric energy translating element: a diaphragm one side of which is exposed to an' adjoining air space in which sound is propagated;

said diaphragm having a fundamental resonant frequency in the speech frequency range and having a segmental modal region of a higher modal-frequency mode; means including awall portion overlying a portion of the modal region and forming with it a segmental acoustic enclosure having vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged and proportioned so as to resonate at said higher modal frequency and to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; said vibratory element being'actuated by a portion of the diaphragm which is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of thediaphragm directly exposed to said exterior space.

3. In a microphone having a vibratory mechano-electric energy translating element: a fiat diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; said diaphragm having a fundamental resonant frequency in the speech frequency range and having a modal region of a higher modalfrequency mode; means including a wall portion overlying a portion of the modal region and forming with it an acoustic enclosure having Vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged and proportioned so as to resonate at said higher modal frequency and to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; said vibratory element being actuated by a portion of the diaphragmwhich is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of the diaphragm directly exposed to said exterior space.

4. In a microphone having a vibratory mechano-electric energy translating element: a flat diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; means for supporting said diaphragm along its periphery; said diaphragm'having a fundamental resonant frequency in the speech frequency range and having a modal region of a higher modalfrequency mode; means including a wall portion overlying a portion of the modal region and forming with it an acoustic enclosure having vent enclosure and said vent opening portions being arranged and ,proportioned so as to resonate at said higher modal frequency and. to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; said vibratory element being actuated by a portion of the diaphragm which is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of the diaphragm directly exposed to said exterior space.

5. In a microphone having a vibratory mechano-electric energy translating element: a flat, circular diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; means for supporting said diaphragm along its periphery; said diaphragm having a fundamental resonant frequency in the speech frequency range and having a modal region of a higher modal-frequency mode; means including a wall portion overlying a portion of the modal region 'and formingwith it an acoustic enclosure having vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged and proportioned so as to resonate at saidlhigher modal frequency and to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; said vibratory element being actuated by a portion of the diaphragm which is set into vibration through excitation by the sound waves in said enclosure and' by sound waves acting directly on the part of the diaphragm directly exposed to said exterior space.

6. In a microphone having a vibratory mechano-electric energy translating el-ement: a diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; said diaphragm having a fundamental resonant frequency in the speech frequency range and having a segmental modal region of a higher modalfrequency mode; means including a wall portion overlying a portion of the modal region and forming with it a segmental acoustic enclosure having vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged and proportioned so as toresonate at said higher modal frequency and to cause said diaphragm to be effectively excited at its fundaopening portions to said adjoining space; said vof the modal region.

'7. In a microphone having a vibratory mechano-electric energy translating element: a diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; said diaphragm having a fundamental iesonant frequency in the speech frequency range and having a segmental modal region of a higher modalfrequencymode; means including a wall portion overlying a portion of the modal region and forming with it a segmental acoustic enclosure having vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged and proportioned so as to resonate at said higher modal frequency, and to v cause said diaphragm to be effectively excited at straight border portion acoustically sealed to an inner portion of said diaphragm by a narrow slit space proportioned to lconstitute' anv effective acoustic inertance over frequency regions substantially outside the frequency region of the modal region.

8. In a' microphone having a vibratory mechano-electric energy translating element: a diaphragm one sidoof which is exposed to an adjoining air space inwhich sound is propagated; said diaphragm having a fundamental resonant fre-A quency in the speech frequency range and having a segmental modal region of a higher modal-frequency mode; means including a Wall portion overlying a portion of the modal region and forming with it a segmental acoustic enclosure having vent opening portions to said adjoining space; said 'enclosure and said vent opening portions being arranged and proportioned so as to resonate at said higher modal frequency and to cause said diaphragm to be effectivelyV excited at its fundamental and modal frequencies; said vibratory element being actuated by a portion of the diaphragm which is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of the diaphragm directly exposed to said exterior space; said Wall portion being shaped substantially as a segment of a circle having asubstantially straight border por-tion acoustically sealed to an inner portion of said diaphragm by a narrow slit space proportioned to constitute an effective acousticinertance over frequency regions 'subhaving a segmental modal region of a higherV modal-frequency mode; means including a wall portion overlying a portion ofthe modal region and forming with it a segmentalacoustic enclosure having vent opening portions to said ad- A joining space; said enclosure and said vent opening portions being arranged and proportioned so as to resonate at said higher modal frequency and to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; said vibratory element being actuated by a portion of the diaphragm which is set into vibration through excitation by the sound waves in said enclosure and by sound waves acting directly on f the part of the diaphragm directly exposed to said exterior space; said wall portion having a substantially straight border portion acoustically sealed to an inner portion 'of said diaphragm by a narrow slit space proportioned to constitute an effective acoustic inertance over frequency regions substantially outside -the frequency region of the modal region; the inner portion of said diaphragm at said acoustic seal constituting a substantially nodal region with respect to said higher mode. 4

l1. In a microphone having a vibratory mechano-electric energy translating element: a diaphragm one side of which is exposed to an adjoining air space in which sound is propagated; said diaphragm having a fundamental resonant frequency` in the speech frequency range and having a segmental modal region of a higher modal-frequency mode; means including a wall stantially outside the frequency region of thev modal region. l A i 9. In a microphone having a vibratory mecha ano-electric-'energy translating element: a diaphragm one side ofA which is exposed to an adjoining air space in which sound is propagated; said diaphragm'having a fundamental resonant frequency in the speech frequency range and portion voverlying a portion of the modal region and forming with it a segmental acoustic' enclosure having vent opening portions to said adjoining space; said' enclosure and said vent opening portions lbeing arranged and proportioned so as to resonate at said higher modal frequency and to cause said diaphragm to be effectively excited at its fundamental and modal frequencies; .f said vibratory element 'being actuated by a porf tion of the diaphragm which is set into vibration and forming with it a segmental acoustic en- Y closure having vent opening portions to said adjoining space; said enclosure and said vent opening portions being arranged'and proportioned so Vas to resonate at said higher modal frequency and to causesaid diaphragm to be effectively excited at its fundamental .and modal frequencies; said vibratory element being actuated by a por; tion of the diaphragm which is set into vibration through excitation by the sound waves in said v enclosure and by sound waves acting directly on the part of the diaphragm directly exposed to t asaid exterior space; said wall' portion having a through excitation by the sound waves in said enclosure and by sound waves acting directly on the part of theI diaphragm directly exposed to said exterior space; said wall portion being shaped substantially as a segment of a circle having a substantially straight border portion acoustically sealed to an inner portion of said diaphragm by a narrow slit space proportioned to constitute an effective acoustic ertance over frequency regions substantially out ide the frequency region of the modal region; the inner portion of said diaphragm at said acoustic seal constituting a substantially nodal region with respect to said higher mode.

122' In a microphone forming part of a hearing aid small enough for inconspicuous wear on lthe body of the user: a microphone unit comprising vtwo outwardly facing vibratory diaphragms mounted on opposite sides of said unit and vibroelectric translating means locatedbetween said diaphragms and actuated thereby for supplying Van electric output corresponding to sound vibra- A tions imparted to said diaphragms; a casing enfront of said two diaphragms and arranged to conne in front of said two diaphragms two narrow acoustic air spaces constituting an acoustic coupling between the peripheral air space and said diaphragms; said casing having vent'opening portions from its peripheral space to the exterior air space; said exterior vent opening portions of said peripheral acoustic space and said narrow acoustic spaces being designed and proportioned to constitute an acoustic system reso# nant in the range between about 1000 and 2500 cycles.

13. In a hearing aid designed to Ibe small enough for inconspicuous wear on the body of the user: a microphone unit; a casing forming an acoustic enclosure around said microphone unit and having an exterior surface subjected to mechanical vibrations caused by rubbing contact with a garment portion of the user or the like; connector elements held by a portion'of said casing and providing circuit `interconnections between said microphone unit and the other elements of said hearing aid; and supporting meansincluding sheet elements of exible metallic material carrying said microphone in a relatively movable position within said casing so as tov effec.-

ous wear on the body or the user, such as a hear" ing aid microphone, a microphonic unit, a casing having two facing casing shells forming an acoustic enclosure around said microphonic unit,

one oi said shells having an anchoring element for an electrical connecting cord, said shells having facing edgeportionsv and Junction elements at a plurality oi' spaced points along said edge portions for joining said shells into a self-supporting structure, said Junction portions being so arranged and placed at sumcient distances from said anchoring element as to substantially impede vibrations reaching the shell having said anchoring element and substantially suppress the transmission of said vibrations from the shell having said anchoring element to the other shell.

RICHARD W. CARLISLE.

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