US2399513A - Piezoelectric transducer - Google Patents

Description

vApril 30, 1946. H. B. sHAPlRo 2,399,513

' I PIEZOELECTRIC TRANSDUCER i Filled June 27. '1942 2 sheets-sheet 1 A' im #7mm/fr April30, 1946. H. B. sHAPlRo 2,399,513

PEZOELECTRIC TRANSDUCER Filed June 27, 1942 2 Sheets-Sheet 2 Qsponsewndb O 400A, FQEQUENCY 5 KC l Preferably,

Patented pr. 30,

Unirse stars menemismo 'mansiones-.n

E. @harsh-n, signor to @terreineny my mit..

.Such apparatus usually' comprises a carrying case wherein are disposed one or more ampiiiier tubes and the electric batteriesrequired for operatio one wail of the case beingprovided with Heights. @him as- The Benet Ghia, a corporation @him merit @eingangs an opening behind which is mounted a microphone. Resilient means are sometimes interposed between the microphone-housing and the wail of the carrying case, for the purpose of minimizing the transmission of noise vibrations from the case to the microphone-elements, by way of the microphone housing, but such means have not given entire satisfaction.

It is, accordingly, an object of this invention to provide a microphone that shall not require extraneous resilient supports.

Another object is to provide a microphone that shall be highly sensitive over a pre-determined remmer-range.

Another object is to provide amicrophone the response characteristic of 'which includes a plurality of peaks distributed over the useful audio frequency range.

Another vobject is to provide-a microphone that shall be particularly adapted for installationy into a hearing aid device.

Another object is to provide a microphone that shall be substantially immune to noise vibrations.

Another object is to provide a microphone that may be mounted rigidly in the carrying case of a hearing aid device. without strong microph'oaic A still further object is to provide a microphonev having improved low frequency response.

In the specific A embodiment oi' the invention I chosen for purposes of explanation, a piezoelectric crystal unit and an actuatingv diaphragm therefor are both ailixed t and are carried by a oating" element, having relatively high inertil. auch as a metallic disc or the like. The disc is supported within a rigid, cup-shaped housing by ilexible means, such as an annulus of rubber or the like, ailixed to its periphery.

the stiliness of the suspension annulus is so correlated to the total 4mass of the mounting ring. crystallunit and diaphragm that the assembly has a natural periodof vibration, -mitii-respect to the housing, at a low audio ireuueney. Eleetrieei connection is made to the crystal unit hv weer of tivo conductive rivets or the hize ch entend through an end wallof the housing. and to the inner ends oi which the lends from the unit, respectively, may be solm tiered or otherwise sinned. inasmuch as the ieads are protected against breakage they may be made incisi foil so thin, and having such slight stiiiness with respect to the inertia of the ring, that they cannot function to transmit vibrations to the crystal unit from the housing.

'ifiie novel features .considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organisation and method of operation, together vantages thereof, will be understood best from the following description of a preferred embodiment. taken in connection with the accompanying drawings, in which:

Figure 1 is a conventionalized front view in elevation of a hearing aid device embodying the invention,

Figure 2 is a. greatly enlarged. dimensionally exaggerated and conventionalised cross-sectional view or the device, taken along a line correspondingtothelineII-Ilinrigurei,

Figure 3 is an enlarged plan view of the mounting disc and the crystal unit ailixed thereto,

Figure 4 is a ygreatly enlarged and dimension-v ally exaggerated sectional view exemplifying the manner in which the mounting ring may be supv ported from the microphone housing,

Figure 5 ig a oonventionalised curve. exemplifying the improved low frequency response oi' a microphone constructed according to the invention, and

Figure 6 is a conventionalised circuit diagram illustrating the electrical microphone.

Figure 'I is a view somewhat like Figure 4, but showing a modification of the manner in which the mounting ring may be supported from the microphone housing. v

Beiore'passing to the detailed description of a specific embodiment of the invention, it is to be understood that the figures of the drawings are highly conventionalized for purpose ot'explanation land that no attempt has been made to i1.- lustrate a commercial embodiment of the invention, to scale.

In ail figures of the drawings, identical elements are similarly designated.

Referring now to the drawings, a preferred embodiment of the invention comprises a massive mounting disc or ring i having appreciable inwith further objects and adanalogo! the improved i o1' analogous material ertia, to one face of which is cemented `the flexible periphery 2 of a stili conical diaphragm 3 and to the opposite face of which are cemented or otherwise ahxed three corners of a Rochelle salt crystal unit 5 of the twister type. The crystal unit may be a Bimorph such, for example, as is disclosed in United States Reissue Patent 20,680, but it is to be understood that the invention is not limited to umts of any particular type.

In order that the disc shall have relatively large inertia, a metal such as brass is suitable therefor. However, any available material having high specic gravity and suiilcient stillness may be utilized.

The mounting ring I has a central opening 1 through which extends a drive link l that interconnects the apex of the diaphragm 3 with the free corner of the crystal unit I. The central opening may be proportionately larger or smaller than illustrated, and the crystal unit may be mounted on the same side of the disc as the diaphragm, if desired.

A flexible suspension annulus I0 of rubber or is cemented to the periphery of the mounting ring I and the outer edge of the annulus is held between the rim of a cupshaped microphone case II and the matching rim of a cover-plate I3 having an opening I5 therethrough concentric with the diaphragm. The rim of the cup-shaped case and the rim of the cover-plate may be stepped, as shown in Figure 4, for the purpose of tightly gripping the periphery of the suspension annulus and the periphery of the annulus may be cemented thereto. It also lies within the scope of the invention to extend the flexible rim oi' the diaphragm itself beyond the edge of the mounting ring and to utilize the extension in lieu of the suspension annulus, as is shown in Fig. 7. c

A plurality of conductive rivets Il or the like extend through the bottom oi' the cup-shaped case II, to the inner ends of which. respectively. the leads I9 from the crystal unit are electrically connected, as by soldering. Inasmuch as the leads are protected against mechanical injury by the case I I, they may be made from metal foil having a thickness of the order of .001 ot an inch and, because of their extreme ilexibility, they do not convey mechanical vibration from the case to the mounting ring or to the crystal unit and the diaphragm supported thereon.

A small opening 2I is provided through the bottom of the cup-shaped case II, whereby changes in atmospheric pressure on the diaphragm are compensated.

In use, the improved microphone may be rigidly mounted within a carrying case 23, wherein are also disposed the necessary thermionic amplifying tube or tubes, batteries, volume control devices, etc., (not shown). 'I'he carrying case, of necessity, is more or less open to the atmosphere at those points where the ,shafts of control devices, etc., extend through the walls thereof, thus permitting sound pressure to reach the back of the microphone diaphragm. The summation of the various openings is indicated, generally, by the numeral 25.

In order that a clear understanding of the operation of the invention may be had, attention is directed to Figure 6, which is a diagrammatic view of an electrical'network wherein the various components function electrically in a manner analogous to the mechanical and acoustic response ot the microphone and carrying oase lOlink In the actual design o t a microphone,

to sound forces. In the circuit diagram, inductance corresponds to mechanical mass, capacity to mechanical compliance, current to velocity. charge to displacement and alternating potential to alternating sound forces. The elements of the circuit are as follows:

L1 is an inductance representing the mass of air moving in the large opening Il.

La represents the mass of diaphragm I and In represents the mass of mounting ring I. L4 represents the effective mass of crystal unit 5 L5 represents the mass of air moving through the lower smallopening 2| in the bottom of microphone case II.

La represents the mass of air moving through opening 25 in the carrying case 23.

Lv represents the mass oi' air moving through opening 1.

Ci is a capacitance representing the compliance of the air between the outer surface oi' diaphragm 3 and cover-plate I3. Cz represents the co pliance of rim 2 of diaphragm 3.

C: represents the compliance of diaphragmmounting annulus I0.

C4 represents the compliance of the air trapped in microphone housing II. be\w the mounting ring.

Cs represents the compliance of crystal unit I.

Ca represents the compliance of the air trapped in carrying case 2l.

Cv represents the compliance of the air trapped between diaphragm 3 and mounting ring I.

The connection 21 exempliiies the fact that sound forces can reach the rear of the diaphragm by way of the openings 2l and 1.

From an inspection of the circuit diagram. it will be apparent that the system has a plurality of degrees oi freedom. Each degree, be-

cause of the strong coupling factors between the' elements of the circuit. gives rise to a peak of current through the capacitor Cs, and corresponds to a peak in the sensitivity of the system. by proper choice of the circuit constants, these responsepeaks may be caused to lie rather closely together within the range of voice-frequencies, as indicated in Figure 5, thus enhancing the overall sensitivity and particularly the sensitivity at the low end o1' the audio range. The circuit oonstants, after determinationthercof either by colculation or experiment. may be translated into mechanical masses, compliances and dimensions through utilization of conversion formulae wellknown to those skilled in the art. Although no damping elements are illustrated. it is to be understood that they may be included at strategic points, so to speak, such as in series with the capacity C4 by means of a material such as nannel or the like, or in series with Mx by dispoung a. foraminous plate (not shown) over or within the 4voice opening I l. The plate could be made from sintered metallic particles. such as is disclosed in the copending application of A. L. W. Williams et al., SerialNo. 429,896. now reilied as Ser. No. 541,694. or from any other material having analogous properties. y

In the preferred embodiment I support the microphone case ii within the carrying case 28 by suitable supporting means such as plastic cements, and so correlate the volume of the microphone case to the volume o! the carrying case that y escasas is enhanced by tic'ularly at a frequency on the order of 400eyclesf Der second.

A microphone constructed according to the -inventionhas-many advantages, among which'may be mentionedthe following:l

l. The microphone may berigidly mounted in a carrying case without being noisy.

2. The output from the microphone is not reduced provided the mounting ring has reasonablyl high inertia.

3. The low frequency response improved.

4. .Mechanical vibrationsv are not transmitted to the crystal unit or diaphragm by the leads interconnecting the microphone with an accom'- Danying amplifier.

Although a specific embodiment of the invention has been selected for purposesof explanation it is to be understood that many modifications lare possible and it is not intended that such selection shall be construed as limiting the scope thereof. The invention, therefore, is not to be restricted except insofar as is lnecesllitated by the prior art and by the spirit of the appended claims. 1

What is claimed is:

1. In combination. a cup-shaped microphone case having a substantially closed end, a rigid partition provided with a central opening, means for resiliently supporting the partition in the case substantially parallel to the end thereof., the supporting means also functioning to prevent air from flowing past the periphery of thelpartition upon movement thereof, a diaphragm resiliently affixed to said partition and sealing the opening therethrough, and a piezoelectric crystal unit mounted on said partition and operatively connected to said diaphragm for actuation thereby, the mass of the partition, the piezoelectric unit and the diaphragm forming with the resilient supporting means for the partition a system mechanically resonant to a low audio frequency.

2. In a hearing aid device, a carrying case denning substantially an enclosure andhaving ,at least one large and one small opening thereof, a microphone in a second case having at least one relatively large opening and one relatively small opening, and means for so supporting the microphone case within the carrying case that the said large openings substantially'coineide, the said cases together constituting an acoustic low-pass filter with respect to sounds entering the carrying case by way of the small opening in the wall thereof.

of the device is of points in a wall 3. The invention set forth in claim 2, characterlzed in this: that the said cases also function to enhance the response of the microphone, by

resonance phenomena, at a plurality of points within the range of audio frequencies.

4. The invention set forth in claim 2, characterized in this: that the microphone is resiliently mounted within the microphone case,

5. The invention set forth in claim 2, character-ized in this: that the microphone is resiliently supported within the microphone case, the supporting means being mechanically resonant to a low audio frequency.

6. In combination, a microphone unit case, a mounting device having appreciable inertia, means for resiliently supporting said device within said case for limited movement substantially axially thereof, a piezoelectric transducer and the'wall" of said case.

` unit caseestablishing an -on opposite sides of mounted on said device for movement therewith, and electrical having stiffness low comparedA to the stiffness of saidresilient supporting device. extendingbetween said transducer 7. The invention set forth in claim 6. characterized in this: that the mounting device and the resilient supporting means therefor constitute a substantially closed partition.`

8. In a hearing aid having a casing which when worn by a user is subject to mechanical noise vibrations, a microphone unit case rigidly mounted within said hearing aid casing and subject to the mechanical noise vibrations of the said hearing aid casing, mounting means within said microphone unit case, a diaphragm mounted on said mounting means and lying within said hearing air casing, a piezoelectric tranducer connected to said diaphragm and mounted on said mounting means, said piezoelectric transducer having an inertia small compared to the inertia of said mounting means, and resilient means supporting said mounting means from said microphone unit case whereby the. transmission of mechanical noise vibration lthrough said support from the microphone unit case to the diaphragm is min-- ixnal.'

9. In a hearing aid as set forth in claim 8, the further characterization that the inertia of the saidmounting means is large compared to the combined inertia' of said piezoelectric crystal means and said diaphragm.

l0. In a hearing aid as set forth in claim 8, the further characterization that the said resilient means comprises the annular edge portion of said diaphragm.

l1. In a hearing aid as set forth in claim 8, the further characterization that the said piezoelectric transducer includes thin flexible lead means connected to the said microphone unit case, and that the stiffness of said lead means is small compared to the stiffness of the said resilient means whereby the transmission of mechanical noise vibrations through said lead means from the microphone unit casing to the piezoelectric crystal means is minimal.

l2. In a hearing aid as set forth in claim 8, the further characterization of an opening through said mounting means into an enclosure defined by said mounting means and said diaphragm means, and of an opening through the wall oi' said microphone unit case, the saidtwo openings and the cavity between the two housings and the space between the-mormting means and the microphone acoustic resonant chamber.

13. In a hearingaid as set forth in claim 8, the further characterization that' the said diaphragm and the said piezoelectric transducer are mounted said mounting means anda coupling drive pin connects the piezoelectric transducer to said diaphragm through an opening in said mounting means, said opening being an acoustic vent tuned to establish a resonance within the frequency range of said hearing aid.

14. In a microphone adapted for installation within a housing for a hearing aid device by rigid connection thereto without introduction of excessive mechanical noise vibrations through said connection to said microphone, a microphone case adapted for rigid attachment to the hearing ald housing, a mounting device, resilient means supporting said mounting device within said case. and a piezoelectric transducer mounted on said mounting device for movement with respect thereto. the 15. In n microphone u let forth in clim 14, inertia. of said mounting device being 1arte comthe further characterization of exible leads ex pared to the inertia of the transducer, and the tending from said piezoelectric tnnsduoerto ter mass of the transducer and of the mounting deminals mounted on slid microphone cue. the vice and the stillness ot said resilient means de 5 stiffness of said lends bein: less than the stittneso termining a system mechanically resonant to a of resilient means. l

low audio frequency. HARRY B.

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