Electroacoustic Transducer

Abstract

An electroacoustic transducer is described which is especially useful in telephones. An essentially flat frequency response curve in the voice range is realized by setting up a resonance peak in the free section of the diaphragm. The diaphragm is supported at its periphery by elastic bodies so that the fundamental frequency of oscillation of the diaphragm will have a node line in the region where the diaphragm is supported. The support arrangement according to the invention sets up at least a second node line having a smaller diameter than the fundamental node line and in the free section of the diaphragm.

Description

BACKGROUND OF THE INVENTION

This invention relates to electroacoustic transducers, and particularly to such transducers which utilize a diaphragm which will vibrate according to the acoustic vibrations impinging thereon and produce corresponding electrical signals. The invention is particularly applicable to those diaphragms constructed of a material which requires that the diaphragm be stressed to the point of bending.

It is known in transducers of the above type that the frequency response curves thereof in the bandwidth or frequency range in which it is desired that the transducer operate meet certain predetermined requirements. These latter requirements particularly emphasize a constant or substantially flat frequency response curve in the desired area of operation of the transducer. Prior art transducers are usually equipped with appropriately adjusted resonators that influence the frequency response curve in the desired way by flattening the resonance peaks thereon. Such transducers, however, are complicated and expensive to manufacture.

Various forms of construction of transducers of this type have been used in an effort to improve their characteristics. The prior art demonstrates that diaphragms of electroacoustic transducers may be supported in their peripheral areas by the use of elastic intermediate layers. For example, in carbon microphones the diaphragm is often fixed between two thin sheets of rubber. These sheets serve to insulate the diaphragm from the case of the transducer. This mode of construction, however, has a somewhat derogatory effect on the operation of the diaphragm, and in no way can be used to enhance its operational characteristics, particularly its frequency response.

In electromagnetic transducers it is known to suspend the metal diaphragm at its peripheral area between two rubber members having little wall strength. The purpose of this suspension arrangement is to insure that the diaphragm in each of its many possible positions is as flat as when it is at rest. This mounting arrangement, of course, has no application to diaphragms which are generally stressed to the point of bending.

In constructing multiple layer diaphragms having several layers of electrostrictive material it is known to place the several layered diaphragm between a firm and a resilient support around the peripheral portion thereof. This mode of suspension is useful for increasing the sensitivity of the transducer but it has no useful effect in producing the desired frequency characteristics for the transducer.

It is, therefore, an object of this invention to provide a transducer of the type mentioned hereinabove which can have its frequency response curve easily adjusted to the desired flat characteristics in the frequency band of interest.

It is another object of this invention to provide a support means for such a transducer which will cause the diaphragm of the transducer to have a substantially flat frequency response curve in the frequency range in which it is desired that the transducer operate.

SUMMARY OF THE INVENTION

The aforementioned and other objects are realized in an electroacoustic transducer constructed according to the principles of this invention in which upon the diaphragm an additional resonance peak is set up in addition to the resonance peak of the fundamental frequency of oscillation of the diaphragm. This additional resonance peak is at a frequency which is numerically related to the fundamental frequency of the diaphragm and is within the desired operating range of the transducer, and the node line for the additional peak is in the free section of the diaphragm. This mode of operation of the diaphragm will produce a substantially constant frequency response curve in the frequency range in which the transducer is operating. In the preferred form of the invention the speech frequency range is of interest, and in prior art diaphragms operating in this frequency range a substantial loss is usually experienced at the upper end of the range. The mode of operation described hereinabove avoids such losses in an inexpensive and uncomplicated manner.

The mode of oscillation for the diaphragm described hereinabove can be simply realized by placing the diaphragm between elastic positioning bodies having corresponding elastic characteristics. It has been found that, for example, silicon rubber is a material which is very appropriate for this application. As discussed hereinabove, it is known to use elastic materials in various forms to suspend a diaphragm, but there is no suggestion in the prior art forms of the particular mode of suspension described herein which enhances the frequency characteristics of a diaphragm of the type here in question. That is, the prior art modes of suspension do not produce the additional oscillation node line in the free section of the diaphragm, and do not produce a flattening of the frequency response curve of the transducer in the range of interest.

The mode of elastic suspension of the diaphragm described herein produces a fundamental oscillation node line in the region of the suspension, i.e., the diameter of the fundamental node line substantially corresponds to the diameter of the diaphragm. However, the node line for the first harmonic of the fundamental frequency of oscillation of the diaphragm will be in the free section of the diaphragm in an area adjoining the supported area. The parts of the diaphragm which are separated by the latter oscillation node line move in opposite directions. The material of the bodies supporting the diaphragm is deformed by the movement of the diaphragm in the area of suspension. This means that these resonance peaks are flattened in a desirable way and that any other resonance peaks are flattened thereby smoothing the frequency response curve.

The transducer according to this invention is particularly easy to construct in that the positioning bodies which touch the diaphragm on both sides are extensions of a body of the same material that surrounds the edges of the diaphragm or of abutting bodies which meet outside of the edge of the diaphragm. This elastic body containing the diaphragm is then supported within the transducer case. Elastic bodies constructed according to the invention and used with round diaphragms have essentially the form of a ring split in its inside diameter and can be easily placed against the diaphragm edge. The diaphragm and its elastic supporting body can readily be positioned in a transducer case.

The elastic body supports the diaphragm by engaging with the portions thereon radially inward of the edge of the diaphragm so that the edge area of the diaphragm is free to move in a plane vertical of the plane of the diaphragm.

An advantageous mode of construction of the elastic supporting body is one that utilizes opposed projections extending inwardly of the elastic body which engage with the opposite sides of the diaphragm. In particular, in the case of a round or disc-shaped diaphragm it has been found advantageous to use at least two ring shaped projections whereby the radially outermost projection has a somewhat smaller diameter than the diameter of the diaphragm.

An elastic body having the desired elasticity can be realized independently from the size of such a body needed to fit within a transducer case by removing certain portions of the cross sectional area of the body so that these portions of the elastic body are in fact weaker.

It has been found that the projections extending from the elastic supporting body to support the diaphragm can be made in a variety of cross sectional shapes. Instead of using ring-shaped projections to engage the diaphragm, which are integral with the supporting body or bodies, it has been found that separated ring shaped supporting bodies made of elastic materials and having different diameters may be used.

The oscillatory behaviors of the diaphragm according to the invention is particularly appropriate for electroacoustic transducers in which the diaphragm is provided with at least one layer of a material which has piezoelectric or piezoresistive qualities. As is well known, such diaphragms are usually stressed to the point of bending according to the particular characteristics of the material used. Depending on the application involved it is often useful to use a diaphragm made of metal having a layer of such a material. The fundamental frequency of oscillation of such a diaphragm can be placed in the appropriate range according to the elastic qualities of the material used for the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

The principles of the invention disclosed and claimed herein will be best understood by reference to a description of the preferred embodiments given hereinbelow in conjunction with the drawings in which:

FIG. 1 is a cross sectional view of a piezoelectric microphone constructed according to the principles of this invention;

FIG. 2 is a partial cross sectional view of an alternative embodiment of the microphone illustrated in FIG. 1, and

FIG. 3 is a simplified cross sectional view of a suspended diaphragm according to the principles of the invention in the position for first harmonic vibration.

DETAILED DESCRIPTION OF THE DRAWINGS

The microphone in FIG. 1 includes a flat disc-shaped diaphragm 1 preferably made of titanium and which has pasted thereon a layer 2 of piezoelectric ceramic material. The materials used are, of course, a matter of choice depending on the application. The diaphragm 1 is contained in and supported by a ring-shaped elastic body 3, preferably made of silicon rubber. The elastic body 3 has two opposed pairs of inwardly projecting circular or ring-shaped projections 6, 7, 8 and 9, which engage with and support the diaphragm 1. The radially outermost projections 6 and 8 are of a diameter such that the edge surface 14 of the diaphragm is free to move vertically in a plane perpendicular to the plane of the diaphragm.

The elastic body 3 containing the diaphragm is supported within the microphone structure upon a horizontally positioned disc 15 having an upward turning flange 16 around the outer edge thereof. The supporting member 15 is itself supported by a case 18 of the microphone in the area of the supporting member 15 which is below the portion thereof which supports the elastic body 3. The assembly discussed above is covered by a cover 17. The electrodes of the piezoelectric ceramic layer are connected by leads 21 and 22 to an amplifier 23. This amplifier may be any of the well-known types conventionally used for such applications and as is conventional as well, a diode 24 may be needed. The amplifier and diode are suspended within a body of insulating material 25. This body of insulating material is fastened beneath the flange portions 16 of the above mentioned disc-shaped supporting member 15.

The elastic supporting body 3 for the diaphragm is constructed so that it extends upwardly to substantial engagement, or so that it at least adjoins, the cover 17. This portion of the elastic supporting body is indicated by a numeral 4. The portion of the elastic body which rests on supporting member 15 is indicated by the numeral 5. It has been found that by removing portions, such as portions 10 and 11, of the cross sectional area of the portion 4 of the elastic supporting body 3, and by removing cross sectional portions 12 and 13 of the portion 5 of the supporting body 3 that those portions of the elastic supporting body having the smaller cross sectional area will substantially follow the motion of the diaphragm. The portions of body 3 which have this smaller cross sectional area are vertically adjacent the projections 6-9. This mode of construction allows the achievement of the desired elasticity of the supporting body substantially independently of the amount or size of the material which is needed to form the elastic supporting body so that it fills the area between the diaphragm and the remaining portions of the microphone assembly.

In the preferred embodiment of the invention described hereinabove, which is a microphone for use in telephones, the diaphragm has a diameter of 43 millimeters, and the piezoelectric material therein is of diameter of 30 millimeters with a thickness of 0.15 millimeters. The opposed pairs of projections 6 and 8, and 7 and 9, of the elastic body 3, respectively, have diameters of 41.5 and 38 millimeters.

In FIG. 2 is illustrated an alternative embodiment of the microphone discussed in connection with FIG. 1. In this embodiment, an elastic supporting body 26 for the diaphragm is formed in substantially the same manner as is the body 3 in FIG. 1. However, the elastic body 26 does not have the portions removed from its cross sectional area so that the portions 31 and 32 of the elastic body 26 do not have weaker areas therein. The elastic body 26 includes projections 27-30 which are constructed in the manner described in FIG. 1, but in this case the projections have a trapezoidal cross section. It has been found that a number of cross sectional shapes are useful for this purpose.

FIG. 3 is a simplified cross sectional drawing illustrating the principles of operation of the invention. In this Figure, an elastic supporting body L is shown in which a diaphragm M is supported in the manner described in connection with FIG. 1. The diaphragm M has a piezoelectric layer P fastened to the surface thereof. The Figure illustrates that in the free section of the diaphragm a nodal line is formed adjoining the suspended portions of the diaphragm. It is further shown in this figure that the movement of the diaphragm deforms the elastic supporting body L in the illustrated manner during its operation.

The preferred embodiments of the invention described herein above are considered to be only exemplary, and it is contemplated that many modifications or changes to these embodiments may be made within the scope of the dependent claims.

Claims

We claim:

1. An electroacoustic transducer, comprising:

a diaphragm constructed from electrostrictive material and

means for supporting said diaphragm around the periphery thereof constructed of elastic material, said supporting means, in order to engage said diaphragm on both sides thereof, having at least an opposed pair of projections extending inwardly of said supporting means and around the perimeter thereof, the portions of said supporting means engaging said diaphragm on both sides thereof having corresponding elastic characteristics,

whereby a resonance peak corresponding to the fundamental frequency of oscillation of said diaphragm, in the desired frequency range of operation of said transducer, and at least one additional resonance peak are set up, said additional resonance peak having a node line in the free section of said diaphragm and being at a frequency which is in the frequency bandwidth in which it is desired that said transducer operate.

2. The electroacoustic transducer defined in claim 1 wherein said diaphragm is stressed to the point of bending.

3. The electroacoustic transducer defined in claim 1 wherein said elastic material is silicon rubber.

4. The electroacoustic transducer defined in claim 1 wherein said diaphragm is placed in said supporting means so that the edge area of said diaphragm which vibrates in a plane perpendicular to the surface plane of the diaphragm is free of said supporting means.

5. The electroacoustic transducer defined in claim 1 wherein said supporting means comprises at least two opposed pairs of said projections having a shape substantially corresponding to the peripheral shape of said diaphragm and wherein the radially outermost of said projections surround a surface area of said diaphragm smaller than the total surface area of said diaphragm.

6. The electroacoustic transducer defined in claim 5 wherein said supporting means has portions removed from its cross sectional area so that the portions of said supporting means having a smaller cross sectional area are vertically adjacent said projections.

7. The electroacoustic transducer defined in claim 1 wherein said projections have a trapezoidal cross sectional shape.

8. The electroacoustic transducer defined in claim 1 wherein said diaphragm includes at least a layer of piezoelectric material.