US3731124A

US3731124A - Electroacoustic transducer having improved transducing element supporting means

Info

Publication number: US3731124A
Application number: US00190207A
Authority: US
Inventors: H Bryant
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1971-10-18
Filing date: 1971-10-18
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01

Abstract

An electromechanical transducer element is supported within a transducer housing by at least one folded spring-like member having a plurality of generally annular ''''leaves'''' each having an annular width relatively large in comparison to its thickness. In operation, air resistance between the leaves or convolutions of the spring-like member serves to acoustically damp undesirable transducer resonances, thereby providing a transducer frequency response characteristic that is relatively uniform throughout the band of interest.

Description

.XFi iifililelZk UlllI/Cu Datum 1 aecu'e 1 1' Bryant 1 May 1, 1973 [54] ELECTROACOUSTIC TRANSDUCER HAVING IMPROVED TRANSDUCING ELEMENT SUPPORTING MEANS [75] Inventor: Herbert William Bryant, Middle town,N.J.

[73] Assignee: Bell Telephone Laboratories lncorp0rated,Murray Hill, NJ.

[22] Filed: Oct. 18, 1971 [21] Appl. No.: 190,207

[52] U.S. C1 ..3l0/9.4, 310/82 [51] Int. Cl .1101! 7/00, H041 17/00 [58] Field of Search ..310/3, 8.2, 8.3, 310/9.19.4', 91/505, 506; 417/222 [56] References Cited UNITED STATES PATENTS 3,619,672 11/1971 Nagata ..310/9.4

2,488,781 11/1949 Reeves ..3l0/9.2 X 2,453,435 [1948 Haustad ..3l0/9.3 X

3,622,816 11/1971 McGrew ..3l0/9.4

Primary Examiner.l. D. Miller Assistant Examiner-Mark O. Budd Attorney-R. J. Guenther et a1.

7 [5 7] ABSTRACT An electromechanical transducer element is supported within a transducer housing by at least one folded spring-like member having a plurality of generally annular leaves each having an annular width relatively large in comparison to its thickness. In operation, air resistance between the leaves or convolutions of the spring-like member serves to acoustically damp undesirable transducer resonances, thereby providing a transducer frequency response characteristic that is relatively uniform throughout the band of interest.

11 Claims, 5 Drawing Figures BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates generally to electroacoustic transducers utilized to convert sound energy into electrical energy, and vice-versa, and, more particularly, to such transducers having improved means for supporting an electromechanical transducing element within the transducer housing.

2. Description of the Prior Art While electroacoustic transducers of the piezoelectric ceramic type possess advantages of reduced size and increased stability when compared to other instruments serving the same function, various design problems have delayed their widespread adoption for use in telephone and other electronic systems. Notable among these problems has been the need for a suitable means for supporting the transducing element within the transducer housing that not only provides a system stiffness which yields a desired frequency response characteristic, but also meets design requirements in terms of stability with age and temperature variations. Conventional transducing element supports have included a pair of gaskets, at least one of which is rubber, for supporting opposite faces of the element. Typically, these gaskets, as for example rings, are compressed against the periphery of the faces, so that a pressure seal is provided, and the element held in the desired position. Unfortunately, while such supporting means are relatively simple and inexpensive to produce, several disadvantages are inherent in their use. First, and most importantly, the stiffness of the vibratory system is closely tied to the amount of compression applied to the rings. As a result, changes in the dimensions of the parts, assembly techniques, or in rubber composition produce varying amounts of compression, which in turn, may often adversely affect the transducer frequency response characteristics, or, at least cause unwanted variations in such characteristics between individual transducers of the same lot. Second, the use of a'rubber material in the element supporting structure could introduce an undesirable and virtually uncontrollable time dependent variation in the transducer frequency response characteristics, by virtue of changes that occur in the physical properties of the rubber as it ages. Additionally, the rubber stiffness is also quite prone to variations brought about by changes in ambient temperature, again possibly adversely affecting the overall performance of the instrument.

Accordingly, it is the broad object of the present invention to provide an electroacoustic transducer,

preferably of the piezoelectric ceramic type, having an tively immune to variations in ambient temperature and relatively stable over long periods of time.

SUMMARY or THE INVENTION Each of the foregoing and additional objects are achieved in accordance with the principles of the invention by the provision, in an electroacoustic transducer, of an improved means for supporting the transducing element within a housing which includes at least one folded spring-like member having a plurality of generally annular leaves each of which has an annular width relatively large in relation to its thickness. Briefly described, the aforesaid member may be fabricated by folding a thin strip of moistureproof paper or metal foil back upon itself a number of times, and by then forming a central hole in the folded member and appropriately trimming the outer edges thereof.

The advantageous provision of the supporting means described greatly facilitates the attainment of a desired transducer frequency response characteristic, since the stiffness of the transducing element support and the degree of damping provided thereby are easily controllable during design. For a given housing, theformer is simply a function of the mechanical properties of the material employed, the thickness of the strip and the number of leaves utilized, while the latter is adjusted simply by changing the size of the central hole,'and the number of leaves, thereby producing a corresponding variation in air resistance between adjacent leaves as the supporting means vibrates. Additionally, by virtue of the fact that, in accordance with the invention, the supporting means are constructed from a metal, moisture-proof paper, or other similar material, and do not require the use of a rubber material, excellent stability in transducer performance can be achieved despite changes in ambient overextended periods of time.

BRIEF DESCRIPTION OF THE DRAWING The aforementioned and other features and advantages of the instant invention will become more readily apparent to persons skilled in the art by reference to the following detailed description, when read in light of the accompanying drawing, in which:

FIG. 1 is a central cross-sectional view of an electroacoustic transducer constructed in accordance with the principles of the invention;

FIG. 2 is a fragmentary cross-sectional view of a piezoelectric ceramic transducing element for use in the transducer of FIG. 1;

FIG. 3 is a top view of the means used to support the transducing element in the transducer of FIG. 1;

FIG. 4 is a sectional front view of the supporting means of FIG. 3; and

FIG. 5 is a graph of a typical frequency response characteristic of a transducer constructed in accordance with the instant invention.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown in central cross-sectional view an electroacoustic transducer constructed in accordance with the principles of the invention. The transducer comprises a housing, designated temperature, and

generally at 10, defining an internal chamber 11, and a planar electromechanical transducing element 12 within the chamber. Means for supporting the element 12 within the chamber 11, and for spacing the element y from the top and

bottom walls

24 and 23 thereof, in-

clude first and second spring-

like members

13 and 14, respectively.

Transducing element 12 may be of the piezoelectric ceramic type, and may include a pair of oppositely poled discs of a polarizable ferroelectric ceramic material such as barium titanate, lead zirconate-lead titanate, or sodium potassium niobate, bonded together to form a composite bilaminate element, as shown greatly enlarged in FIG. 2. The individual discs may be fabricated in various ways, one of which is fully described in a concurrently filed patent application, Ser. No. 190,209, filed Oct. 18, 1971, by T. C. Austin and H. W. Bryant, entitled the same as the instant application and assigned to the same assignee. 7

After fabrication, electrodes are affixed to both faces of each ceramic disc, such as

electrodes

12c and 12d on disc 12a and electrodes 122 and 12f on disc 12b, shown .in FIG. 2 greatly exaggerated in thickness for the purposes of illustration. The electroded discs are then placed in alignment, and securely bonded together so that inner electrodes 12d and 12e are in electrical contact. A bonding technique found suitable for the purposes of the instant invention is described in the aforesaid copending application, but other well-known methods may be equally satisfactory.

The overall electrical response characteristics of the transducer of FIG. I depend upon both the acoustics of the housing 10 and the means utilized to support the transducing element 12 within the chamber 11 formed thereby. In certain transducer applications, it is particularly desirable to achieve a relatively flat or uniform characteristic curve over the frequency band of interest, although it is to be understood that other characteristics can also be achieved in accordance with the invention. If required, a low-frequency rolloff of the desired curve is relatively easy to obtain, in the case of a piezoelectric ceramic transducer, when it is realized that the ceramic transducing element represents a capacitance at low frequencies. Accordingly, by appropriately choosing the value of resistance connected across the transducer, as, for example, the input resistance of a preamplifier stage following. the transducer, the corresponding R-C corner frequency may be suitably located. The high frequency end of the transducer characteristic typically contains an undesirably high peak located at the first natural resonant frequency of the mounted transducing element 12, as well as higher frequency overtones, all of which must be suppressed or filtered out in order to obtain a flat response. This suppression is provided, in part, by the means used to support the element in accordance with the invention, to be more fully discussed hereinafter, and also by the proper engineering of the acoustics of housing 10.

Referring again to FIG. 1, housing 10 includes a cupshaped body member 15 and a disc-like frontal plate 16, both of which may be fabricated from a plastic material possessing suitable qualities of heat and moisture resistance. Frontal plate 16 has centrally drilled therethrough a port or center hole, such as port 17, which is covered by an acoustic damping member 18 either fastened across the hole atop the plate 16 or nested and fastened within a centrally bored recess 19 provided in plate 16, as shown in FIG. 1. Port 17 serves as a low-pass filter, and when suitably dimensional, selectively suppresses the high frequency overtones noted above. The acoustic resistance or damping member 18, which may conveniently be fabricated from porous sintered steel, helps to reduce the primary resonance and overtones to an acceptable level. In assembled form, frontal plate 16 and body member 15 are encased by a thin metal shell 20 having a generally U-shaped cross section. A coned metal spring washer 21 may be used to supply the pressure necessary to ensure positive contact between plate 16 and member 15 and to hold the unit in proper alignment.

As stated above, the proper design of means to support transducing element 12 within chamber 11 of housing 10 is extremely critical to the attainment of the desired transducer frequency response characteristic. Functionally, the means chosen, in order to be effective, must provide a first natural resonance of the mounted element at a desired frequency, and in addition provide a semirigid clamping action around the element periphery that serves to minimize planar or piston-like motion of the element while inducing or facilitating a cupping or spherical bending action in the element, the latter being the only source of electrical output. Additionally, the supporting means must be capable of providing a desired degree of mechanical damping, if required, to further reduce the resonant peak.

In accordance with the principles of the instant invention, means .for supporting element 12 within chamber 11 of housing 10 that meet the above requirements include first and second folded or accordian pleated spring-

like members

13 and 14, respectively, as depicted in FIG. 1. As will be discussed hereinafter, a single such member may, in certain circumstances, be advantageous.

Each spring-like member'13 and 14, such as member 13, shown greatly enlarged in top and sectional front views in FIGS. 3 and 4, respectively, maybe fabricated from a thin strip of moisture-proof paper, metal foil, or other suitable material, folded back upon itself a plurality of times to form a plurality of accordian-like interconnected leaves, such as

leaves

30, 31 and 32. A central hole 35, preferably round, is next punched or otherwise formed centrally through the member, and the outer periphery trimmed in the general shape'of a circle 36 having a diameter 37 slightly larger than the distance 38 between the folds. As shown in FIG. 1, the outer diameter 37 of the members 13 .and 14 is arranged to be slightly greater than the outer diameter of transducing element 12. Each leaf of

members

13 and 14 thus consists of a generally annular area having an annular width relatively large in relation to its thickness, said width being sufficiently great to provide a measurable amount of acoustic damping to the transducing element support system, due to energy dissipation in the air resistance between adjacent leaves during vibration thereof.

During assembly,member 13 is seated upon the inner face of cup-shaped body member 15, which forms the bottom wall 23 of chamber 11. One end of a first strip 25 of electrically conductive material is next attached to a terminal rivet 28 extending through the bottom portion of cup-shaped housing member 15, and the other end positioned atop a portion of the top leaf 30 of member 13. Transducing element 12 is next seated on member 13 so that electrode 12f makes electrical contact with strip 25. Cement may be used to fasten element 12 to member 13, thereby preventing misalignment due to mechanical shock. A second strip 27 of gold plated copper foil or other conducting material similar to strip 25 is then placed in electrical contact with a portion of the outer periphery of electrode 12c, that strip having been previously extended along the side wall 22 and bottom wall 23 of chamber 11 to an electrical connection with a second terminal rivet 26 similar to terminal rivet 28. Member 14 is next seated on the upper face of transducing element 12. To complete assembly, frontal plate 16 is then aligned with body member 15, and spring washer 21 is placed thereupon. Finally, the entire transducer is clamped together by appropriately bending shell 20, as shown in FIG. 1. Electrical connection between the transducer and an external load or source is accomplished by appropriately connecting leads to the external portions of

terminals

26 and 28. In the event that member 13 is fabricated from an electrically conductive material, electrical contact between electrode 12f and rivet 28 can be made directly through member 13, thereby obviating the need for strip 25. However, in this configuration, electrical contact between member 13 and the portion of strip 27 passing thereunder must be prevented by the provision of a small insulating member 29 of a suitable insulating material, as shown in FIG. 1. v

The stiffness of the spring-

like members

13 and 14 which support transducing element 12 is easily controlled during design by appropriately selecting the mechanical properties of the particular material employed, the thickness of the material, and the number of leaves utilized. The provision of a desired stiffness in turn controls the natural resonance frequencies of the entire support system. For the non-resonant response given in FIG. 5, the following combination of the above properties has been found satisfactory, in the case of supporting members constructed from metal foil:

Material: 61 ST Aluminum Thickness ofleaves 30, 31, 32: .0016 inch Youngs modulus: l0 lbs./in.

Number of leaves per member: In addition, changes in the degree of damping provided by the transducing element supporting means are effectuated by appropriately controlling the ratio of the annular width of

members

13 and 14 to the thickness of each leaf by changing the size of hole 35, adjusting the ratio R between the combined thicknesses of the transducing element and fully

compressed members

13 and 14 and the depth of transducer chamber 11. R can be varied by changing the number of leaves utilized and the dimensions of the transducer housing. The following dimensions and ratios have been found suitable in the transducer of FIG. 1:

Outer Diameter 37: .675 inches Distance 38 between folds: .615 inches Diameter of hole 35: .440 inches Ratio of annular width to thickness: 73

Space factor (1R): 31%

It is to be understood, of course, that other combinations of the aforementioned properties of members 13 and 141, depending upon the particular transducer characteristics desired, are intended to be within the scope of the invention.

From the above description of the transducer of FIG.

. 1, it can be seen that means to support transducing element 12 within chamber 11 have been provided in accordance with the invention which are quite stable and not prone to change due to the passing of time and ambient temperature variations. Since transducing element 12 is semirigidly supported between

members

13 and 14, it is buffered against mechanical damage normally possible with rigid clamping. In addition, the supporting means are easily tailored to meet desired response requirements, as evidenced by FIG. 5, which depicts the frequency response characteristic of a transducer constructed in accordance with the invention. As shown, the response is flat within i 1.5 db, in the band from 0.2 to 9.0 kHz.

While a pair of spring-

like members

13 and 14 are utilized to support transducing element 12 in the instrument of FIG. 1, it is to be noted that a single such member may on occasion be desirable, as, for example, with a metal-ceramic transducer element mounted on a greatly stiffer support member than previously described, but, having the same physical shape as

members

13 and 14. In this configuration, element 12 is simply secured to the uppermost leaf 30 of member 13, by means of cement, epoxy or any other suitable fastening arrangement, and appropriate clearance is maintained between the upper face of element 12 and the top 24 of chamber 11.

It is to be noted that a metal-ceramic transducing element, as described in the above-mentioned copending application, Ser. No. 190,209, filed Oct. 18, 1971, by T. C. Austin and H. W. Bryant, may be used in lieu of the bilaminar transducing element 12 of FIGS. 1 and 2, in certain circumstances, proper account being taken of the required element sizes and geometry. For example, in the transducer of FIG. 1, such a transducing element having a ceramic disc whose outer diameter is smaller than the diameter of hole 35 may be employed, or alternatively, a larger ceramic may be utilized in a transducer having a single supporting member 13.

In order to more fully appreciate the degree of com pactness of a transducer of the type depicted in FIG. 1, the following typical dimensions are given by way of illustration only, the instant invention not being limited to the sizes stated:

Outside diameter of complete transducer 0.822 inches Thickness of complete transducer 0.302 inches Diameter of transducing element 12 0.59 inches Thickness of transducing element 12 0.012 inches element is of an electromechanical transducer type other than a piezoelectric ceramic. Still further, while the shape of the transducing element has heretofore been described as round, or disc-like, it should be clearly understood that appropriate modifications to the peripheral shape of the transducer and/or its internal components may sometimes be required.

What is claimed is: l An electroacoustic transducer comprising an electromechanical transducing element, a housing defining an internal chamber having a top wall and a bottom wall for containing said element,

and means for supporting said element within said chamber, said supporting means including at least one spring-like member for spacing said element from one of said walls, said member having a plurality of annular leaves each having an annular width relatively large in relation to its thickness, said leaves being interconnected by hinge portions on the periphery thereof.

2. The invention defined in claim 1 wherein said electromechanical transducing element includes a polarizable ferroelectric ceramic material.

3. The invention defined in claim 2 wherein said ceramic material is selected from the group consisting of barium titanate, lead titanate-lead zirconate, and sodium potassium niobate.

4. The invention defined in claim 3 wherein said spring-like member is a metal foil.

5. The invention defined in claim 1 wherein said sup' porting means includes two spring-like members for spacing said element from said top wall and from said bottom wall.

6. The invention defined inv claim 5 wherein said electromechanical transducing element includes a polarizable .ferroelectric ceramic material.

7. The invention defined in claim 6 wherein said ceramic material is selected from the group consisting of barium titanate, lead titanate-lead zirconate, and sodium potassium niobate.

8. The invention defined in claim 7 wherein said spring-like member is a metal foil.

9. An electroacoustic transducer comprising an electromechanical transducing element,

a housing defining an internal chamber having a top wall and a bottom wall for containing said element, and

at least one annular spring-like member for spacing said element from one of said walls, said member comprising a plurality of interconnected generally circular leaves each having a thickness and an annular width determined by the size of a central hole formed therein, said width being large in relation to said thickness.

10. In an electroacoustic transducer including a transducing element and a housing for containing said element, means for supporting said element in 'said housing comprising at least one annular spring-like member having a plurality of interconnected leaves each having an annular width relatively large in relation to its thickness.

1 1. An electroacoustic transducer comprising an electromechanical transducing element,

a housing defining an internal chamber having a top wall and a bottom wall for containing said element and means for supporting said element within sald chamber, said supporting means including at least one accordion pleated member comprising a plurality of interconnected generally annular leaves each having an annular width sufficiently great to provide a measurable amount of acoustic damping.

Claims

1. An electroacoustic transducer comprising an electromechanical transducing element, a housing defining an internal chamber having a top wall and a bottom wall for containing said element, and means for supporting said element within said chamber, said supporting means including at least one spring-like member for spacing said element from one of said walls, said member having a plurality of annular leaves each having an annular width relatively large in relation to its thickness, said leaves being interconnected by hinge portions on the periphery thereof.

5. The invention defined in claim 1 wherein said supporting means includes two spring-like members for spacing said element from said top wall and from said bottom wall.

6. The invention defined in claim 5 wherein said electromechanical transducing element includes a polarizable ferroelectric ceramic material.

9. An electroacoustic transducer comprising an electromechanical transducing element, a housing defining an internal chamber having a top wall and a bottom wall for containing said element, and at least one annular spring-like member for spacing said element from one of said walls, said member comprising a plurality of interconnected generally circular leaves each having a thickness and an annular width determined by the size of a central hole formed therein, said width being large in relation to said thickness.

10. In an electroacoustic transducer including a transducing element and a housing for containing said element, means for supporting said element in said housing comprising at least one annular spring-like member having a plurality of interconnected leaves each having an annular width relatively large in relation to its thickness.

11. An electroacoustic transducer comprising an electromechanical transducing element, a housing defining an internal chamber having a top wall and a bottom wall for containing said element, and means for supporting said elemeNt within said chamber, said supporting means including at least one accordion pleated member comprising a plurality of interconnected generally annular leaves each having an annular width sufficiently great to provide a measurable amount of acoustic damping.