GB1565860A

GB1565860A - Microphone utilizing high-polymer piezoelectric membrane

Info

Publication number: GB1565860A
Application number: GB1350177A
Authority: GB
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1976-04-02
Filing date: 1977-03-30
Publication date: 1980-04-23
Also published as: NL176992B; NL7703582A; DE2714709C3; DE2714709A1; NL176992C; DE2714709B2

Description

(54) MICROPHONE UTILIZING HIGH-POLYMER PIEZOELECTRIC MEMBRANE (71) We, MATSUSHITA ELECTRIC INDUSTRIAL COMPANY, LIMITED, a corporation organized under the laws of Japan, of No. 1006, Oaza Kadoma, Kadoma City, Osaka, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electroacoustic transducers and in particular to such a transducer having a high-polymer piezoelectric membrane as a transducing element which is formed to take the shape of an arch to develop a mechanical stress in the circumferential direction.

In conventional microphone units, a highpolymer piezoelectric membrane or diaphragm is mounted on an elastic support member having an arcuate surface with which the membrane is in contact so that the membrane is arched and stressed in the direction of its circumference. To provide consistent frequency response and electroacoustic sensitivity over an extended period of time it is necessary that the curvature of the membrane be maintained. However, the elastic support member tends to deform because of aging and instability, so it is difficult to maintain the original curvature of the piezoelectric membrane over a long period of time. Moreover, as the elastic support vibrates in unison with the diaphragm in response to the variation of sound pressure, the acoustic performance of the prior art microphone is not satisfactory.

According to the present invention there is provided a microphone including a support structure having an opening defined by opposed first side edges and opposed second side edges each of said first side edges being curved, and a high-polymer piezoelectric membrane having electrically conductive, oppositely polarized surfaces and a single axis of elongation parallel to said surfaces, said membrane being secured in said opening and held in a part-cylindrical shape by reason of the curvature of said first side edges, the axis of elongation lying in the circumferential direction of the thus curved membrane and the membrane being stressed in the direction of the axis of elongation.

In another aspect the present invention provides a method of making a microphone by: providing a high-polymer piezoelectric film; elongating said piezoelectric film in one direction until a predetermined thickness is reached; coating each side of said piezoelectric film with a conductive film; polarizing said film in the direction of its thickness by applying an electrical potential across the metal coatings; adhesively securing a frame structure to one surface of said metal coated piezoelectric film; cutting said piezoelectric film along the edges of said framed structure; bending said framed structure together with said piezoelectric film to from a partcylindrical surface whose direction of circumference coincides with the direction of said elongation to produce a mechanical stress therein in the direction of the elongation; and securing said framed structure to a housing.

With the invention, the membrane is bent with the support structure to take the shape of an arch to develop circumferential mechanical stress in the direction of elongation.

The frame or support structure and the piezoelectric film attached thereto may have a tendency to become flat so that they are bent when mounted in a housing to take the shape of an arch to develop mechanical stress along the direction of elongation. The framed piezoelectric membrane or diaphragm is secured to the housing so that the edges of the membrane are rigidly secured and the curvature of the membrane is thus retained. The frame structure is preferably constructed of a material having a similar coefficient of linear expansion to that of the piezoelectric material to prevent deformation of the framed membrane unit due to the difference in thermal expansion between the two elements.

The microphone housing can include an acoustic cavity rearwardly of the piezoelectric membrane. In such a cavity is secured an acoustic absorber to damp the acoustic energy such that the resonant peaks may be suppressed to provide a flat frequency response.

The microphone can be constructed to be immune to mechanical shocks by adding a second framed membrane unit in opposition to the first framed membrane unit and electrically connected, for instance. by an intermediate metal casing which serves not only as an electrical connecting member but also as a cavity in which an acoustic absorber may be provided.

Thus, in particular embodiments the present invention provides a microphone including a support structure having opposed openings respectively defined by opposed first side edges and opposed second side edges, each of the first side edges being curved, first and second high-polymer piezoelectric membranes each having electrically conductive. oppositely polarized surfaces and a single axis of elongation parallel to said surfaces, said first and second membranes being secured in respective ones of said openings and held in part-cylindrical shapes in spaced relation by reason of the curvature of said first side edges, the axes of elongation lying in the circumferential directions of the thus curved membranes and each membrane being stressed in the direction of its axis of elongation the inner faces of the membranes being electrically interconnected and the directions of curvature of said first side edges and the directions of polarization of said membranes being such that voltages produced from the outer faces of said membranes in response to an impulse affecting the membranes reinforce each other to produce and enhanced output voltage when the membranes are flexed in opposite directions, and cancel each other out when the membranes are flexed in a same direction.

Preferably, a pair of identical framed structures is adhesively secured to a flat metal-coated piezoelectric film and cut along the edges of the framed structures to form a pair of framed membrane units connected together by connecting member.

These framed membrane units are then bent to form a pair of parallel part-cylindrical membranes with the direction of polarization of one membrane being opposite to the direction of polarization of the other.

The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is an exploded view of an embodiment of the invention; Figure 2 illustrates a typical example of a process for making a framed membrane unit of the embodiment of Figure 1; Figure 3 is an end view of the framed membrane unit with the arrow indicating the direction of elongation; Figure 4 is an enlarged cutaway view of a portion of the framed membrane unit in a preferred form; Figures 5 to 7 and 9 are modifications of the acoustic absorber of the previous embodiment; Figure 8 is a graphic illustration of the frequency response characteristic of the embodiment of the invention in comparison with the prior art device; Figure lOA is an exploded view of a modified acoustic absorber; Figure lOB is a cross-sectional view of the modified acoustic absorber of Figure 10A when assembled; Figure 11 is another embodiment of the invention incorporating a pair of framed membrane units mounted in opposition to each other to cancel the signals resulting from an application of a mechanical shock; Figure 12A illustrates the details of the construction of the framed membrane units within the housing of Figure 11 and Figure 12B illustratesd the operation of the embodiment of Figure 12A; Figure 13A is a modification of the embodiment of Figure 12A and Figure 13B illustrates the direction of polarization of two membranes; and Figures 14A to 14C show a series of processes for making the framed membrane units of Figure 12A.Figure 1 or the 12A.

Referring now to Figure 1 of the drawings, a cutaway view of an electroacoustic transducer or a microphone 10 embodying the present invention is shown. The microphone 10 comprises a housing 12 having an opening 14, and a framed membrane unit 16 which is disposed in grooves 18 formed on the side walls of the housing 12. An acoustic absorber 20 is provided in the space behind the membrane unit 16. The membrane unit 16 comprises a rectangular frame structure 15 and a high polymer piezoelectric film 17 secured to the frame structure 16. The piezoelectric membrane to be used in the present invention is one which has been prepared by elongating a film of high-polymer material such as polyfluoride vinylidene about three times its original length until a thickness of from 5.5 to 30 micrometers is reached. A metal coating is then deposited on each side of the piezoelectric film by evaporating the metal m a vacuum chamber. The piezoelectric film is then polarized in the direction of its thickness by setting up an electric field of about 1000 kilovolts per centimeter to impart a piezoelectric constant of from 20 x 10-12 to 30 x 10-12 Coulombs per Newton.

A plurality of frame structures 24 may be adhesively secured to the metal-coated piezoelectric film 22 in a manner as illustrated in Figure 2 to mass produce framed membrane units 16. The film is then cut along the edges of each frame to produce a plurality of such framed piezoelectric mem rane units.

When mounted in the housing 12, the framed membrane unit 16 is bent to take the shape of an arch to provide a curved surface with the direction of its circumference coinciding with the direction of elongation as indicated by the arrow in Figure 3. This mechanical stress is modulated as the film 17 is caused to flex by sound pressure acted thereupon and generate an electrical signal proportional to the difference in flexure between the opposite sides of the arched membrane. The metal coatings serve as electrodes to pick up the generated signal.

The frame structure 15 is preferably formed of a plastic material such as a high-polymer of acryl-nitril-butagen styline (ABS) having a similar coefficient of linear expansion to that of the material of the piezoelectric film 17 and a metal coating which may be provided by galvanizing the plastic frame structure.

Figure 4 illustrates in detail a portion of the structure of the diaphragm unit 16 having a plastic molded frame structure 26 with galvanized metal coatings 28 to which is adhesively secured the metal coating 17b deposited on the piezoelectric film 17a.

Because of the similar coefficients of linear expansion, the curvature of the film's cylindrical surface is retained over a substantial range of varying temperatures. This is particularly important in terms of frequency response and acoustical sensitivity since the resonant peak will occur at a frequency which is inversely proportional to the radius of the film's curvature while the sensitivity is proportional to the radius of the curvature.

The microphone 10 tends to have a resonant peak or peaks at frequencies in the higher frequency end of the audible frequency spectrum. In order to provide a flat frequency response it is necessary to suppress the resonant peaks by damping the acoustic waves generated in the cavity rearward of the piezoelectric membrane 17. In the example shown in Figure 1, the acoustic absorber 20 is adhesively secured to the bottom and side walls of the housing 10 so that the upper surface of the absorber 20 is spaced from the piezoelectric film 17 and prevented from contacting therewith under any environmental conditions.

Figure 5 depicts a modification of the previous embodiment. In the modified embodiment an apertured cylindrically curved metal structure 30 is mounted behind the piezoelectric film 17 with a predetermined spacing therefrom to provide a forward acoustic chamber 32 and a rearward acoustic chamber 34. The rearward chamber 34 is filled with the acoustic absorber 20.

With this arrangement the acoustic wave generated in the forward chamber 32 tends to concentrate in the aperture 31 formed in the rear structure 30. The air mass in the aperture 31 is forced downward by the sound pressure developed in the forward chamber 32 at a velocity which is inversely proportional to the cross-sectional area of the aperture 31. The air mass flows into the rearward chamber 34 at a higher speed than if the apertured member 30 is not otherwise provided and encounters the acoustic absorber 20 at a great speed. Therefore, the portion of the acoustic absorber 20 which is located at the immediate area of the aperture 31 acts as an acoustic resistance to the generated acoustic waves. The size of the aperture 31 and the spacing between the diaphragm 17 and the apertured member 30 are chosen in relation to the volume of rearward chamber 34 to provide a required degree of dampening to the resonant peaks.

The rear apertured member is formed of a metal and disposed in the housing 12 in electrical contact with the diaphragm support frame 15 so that the rear member 30 can also serve as an electrode. It is appreciated that an acoustic absorber 36 may be provided in an area adjacent to the aperture 31 as illustrated in Figure 6, or alternatively in the aperture as shown in Figure 7. In the latter case, the rear member 30 has a sufficient thickness to provide a required degree of dampening. It is to be noted that the apertured resistance member 30 is not necessarily shaped to form a cylindrical surface; it can be alternatively shaped to provide a flat surface, and that a plurality of such apertures 31 may be provided.

Figure 8 depicts a frequency response characteristic of the microphone of the invention as constructed in accordance with Figure 5. As clarly indicated by broken lines, the resonant peaks at the high frequency end of the audio frequency range are substantially suppressed to provide a flat frequency response which is compared favorably with the prior art microphone indicated by the solid line curve.

Figure 9 illustrates an alternative embodiment of the previous embodiments shown in Figures 5 to 7. In Figure 9 the apertured acoustic member 30 and the acoustic absorber 20 of the previous embodiments are replaced by a porous member 38 having a part-cylindrical surface spaced from the diaphragm 17. The member 38 may be constructed by a lamination of felt which is impregnated with a curing liquid agent.

During the curing process the lamination is shaped into a part-cylindrical form of a predetermined thickness with an identical curvature to that of membrane 17. The acoustic absorber 38 is thus given a certain degree of structural integrity while retaining its porosity, and mounted in the housing 12 so that its upper surface is equally spaced from the adjacent surface of the membrane 17.

Alternatively. the dampening action can also be provided by an arrangement shown in Figures 10A and 10B in which the porous structure 38 of the previous embodiment is replaced by a pair of identically shaped part-cylindrical members 40 and Q which are secured together by a pair of spacers 44 each having a thickness of about several tens of micronmeters. The upper member 40 is provided with a plurality of apertures 46 and the lower member 42 with a plurality of apertures 48 which are located such that when both members are secured together the apertures of each member are not in correspondence with those in the other member.

In operation, the air mass in the forward cavity is forced out of the apertures 46 of the upper member 40 by the sound pressure and its acoustic energy is dampened or attenuated as it flows through the narrow space between the upper and lower members 40 and 42. In this case, the air mass between the space flows as a laminar flow which increases the loss of acoustic energy due lo the increased contacting surface of the adjacent members. Thus, the upper and lower apertured members offer acoustic resistance of such magnitude that the acoustic absorber as used in the previous embodiment can be dispensed with.

Figure 11 depicts another embodiment of the present invention in which the microphone 50 comprises a pair of identical piezoelectric diaphragm units 52 which are mounted in opposite sides of a microphone housing 51. Figure 12 illustrates the details of the construction of the microphone 50. In Figure 12A, a'metal frame 54 is provided having a pair of oppositely concaved surfaces on each of which respective piezoelectric diaphragm units 52a and 52b are secured in electrical contact therewith. When assembled, the inside surfaces of the diaphragms 52a and 52b are electrically connected together via the intermediate casing 54 and electrical connections 56 and 57 are made to the outer side of the diaphragms. It is necessary that the direction of polarization of each diaphragm is such that the individual signals add to each other when identical sound pressure is applied to both diaphragms in opposite directions. In Figure 12B, the diaphragm 52a is shown as positive on the outer side while the diaphragm 52b is negative on the outer side. Upon inward flexure of both diaphragms caused by the oppositely applied acoustic waves as indicated by the arrows P, the polarity of the voltages so generated is in conformity with the polarity of the diaphragms as indicated by the signs in the Figure so that the individual signals add to each other and the combined output is double the amplitude of each signal.

Assume that a mechanical shock is applied to the microphone 50 in a direction as indicated by the arrow M in Figure 12B, the diaphragms 52a and 52b will be caused to flex in the same direction as indicated by broken lines due to their tendency to remain stationary. Under these circumstances, the polarity of the voltages developed across both diaphragms is such that they cancel out each other at the output terminals 56 and 57. and thus no voltage will develop.

Figure 13A illustrates a modification of Figure 12A which is preferable in terms of mass production. Identical piezoelectric membranes 66 and 68 are adhesively secured to metal frames 60 and 62 respectively which are integrally connected together by members 64. Both membranes are arched in the same direction as clearly shown in Figure 13B. In this modification the directions of polarization are opposite to each other so that in this example the outer side of both membranes are poled positive with respect to the inner side. Upon inward flexure of both membranes in response to an acoustic signal, the voltage developed across membrane 66 is of the same sign as indicated in Figure 13B while the voltage across the membrane 68 is opposite to the indicated sign.

The microphone of Figure 13A can be fabricated in a series of processes as depicted in Figures 14A to 14C.

Since the outer sides of the membranes 66 and 68 are poled at the same polarity, the frames 60 and 62 can be adhesively secured to one side of a polarized piezoelectric film 74 as shown in Figure 14A. The film is then cut along the edges of the frames (Figure 14B), pressed to form a pair of cylindrical surfaces and bent at right angles at the junctions between the frames and connect mg members 64 in the directions as indicated by the arrows in Figure 14C.

Patent No. 1361371 claims, in claim 1, a piezoelectric electroacoustic transducer comprising two piezoelectric polymer films each having electrodes on the two opposite surfaces thereof, said polymer films being arranged to face one another, means being arranged between said films to support them in covex or concave configurations and the electrodes of said polymer films being so electrically interconnected that when an electrical potential is applied thereto the resulting electric fields between the respective pairs of electrodes causes one of the piezoelectric polymer films to elongate and the other to shrink.

WHAT WE CLAIM IS: 1. A microphone including a support structure having an opening defined by opposed first side edges and opposed second side edges each of said first side edges being curved, and a high-polymer piezoelectric membrane having electrically conductive, oppositely polarized surfaces and a single axis of elongation parallel to said surfaces, said membrane being secured in said opening and held in a part-cylindrical shape by reason of the curvature of said first side edges, the axis of elongation lying in the circumferential direction of the thus curved membrane, and the membrane being stressed in the direction of the axis of elongation.

2. A microphone as claimed in claim 1, including an acoustic cavity rearwardly of said membrane, and means provided in said acoustic cavity and spaced from said membrane for damping the acoustic energy in said acoustic cavity.

3. A microphone according to claim 2, wherein the acoustic cavity is formed between the membrane and a part cylindrical surface spaced from and parallel to the membrane.

4. A microphone according to claim 3, wherein the part cylindrical surface is a surface of a body of porous material.

5. A microphone according to claim 3, wherein the the part cylindrical surface is a surface of a first apertured member, there being a second apertured member spaced from said first apertured member, each of said apertured members having a plurality of apertures located such that the apertures of each member do not coincide with those in the other member, said first and second apertured members being spaced to provide a laminar air flow through the spacing between said first and second members.

6. A microphone according to claim 3, wherein said part cylindrical surface of an apertured member, there being a body of acoustic absorbing material behind the aperture.

7. A microphone according to claim 3, wherein said part cylindrical surface is a surface of an apertured member, there being a body of acoustic absorbing material disposed in the aperture.

8. A microphone according to any preceding claim, wherein said support structure is formed of a material having a similar coefficient of linear expansion to that of said piezoelectric membrane.

9. A microphone acccording to claim 8, wherein said frame structure is coated with a conductive film.

10. A microphone according to any preceding claim, wherein the membrane has a thickness of from 5.5 to 30 microns.

11. A microphone including a support structure having opposed openings respectively defined by opposed first side edges and opposed second side edges, each of the first side edges being curved, first and second high-polymer piezoelectric membranes each having electrically conductive, oppositely polarized surfaces and a single axis of elongation parallel to said surfaces, said first and second membranes being secured in respective ones of said openings and held in part-cylindrical shapes in spaced relation by reason of the curvature of said first side edges, the axis of elongation lying in the circumferential directions of the thus curved membranes and each membrane being stressed in the direction of its axis of elongation, the inner faces of the membranes being electrically interconnected and the directions of curvature of said first side edges and the directions of polarization of said membranes being such that voltages produced from the outer faces of said membranes in response to an impulse affecting the membranes reinforce each other to produce an enhanced output voltage when the membranes are flexed in opposite dirrections, and cancel each other out when the membranes are flexed in a same direction.

12. A microphone according to claim 11, wherein the support structure is electrically conductive and provides said electrical interconnection.

13. A microphone according to claim 11 or 12, including an acoustic absorber disposed in said structure to provide a substantially flat frequency response characteristic.

14. A microphone according to claim 11, 12 or 13, wherein said first and second membranes are arched in opposite directions to each other and are electrically polarized in the same direction.

15. A microphone according to claim 11, 12 or 13, wherein said first and second

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. to one side of a polarized piezoelectric film 74 as shown in Figure 14A. The film is then cut along the edges of the frames (Figure 14B), pressed to form a pair of cylindrical surfaces and bent at right angles at the junctions between the frames and connect mg members 64 in the directions as indicated by the arrows in Figure 14C. Patent No. 1361371 claims, in claim 1, a piezoelectric electroacoustic transducer comprising two piezoelectric polymer films each having electrodes on the two opposite surfaces thereof, said polymer films being arranged to face one another, means being arranged between said films to support them in covex or concave configurations and the electrodes of said polymer films being so electrically interconnected that when an electrical potential is applied thereto the resulting electric fields between the respective pairs of electrodes causes one of the piezoelectric polymer films to elongate and the other to shrink. WHAT WE CLAIM IS:

1. A microphone including a support structure having an opening defined by opposed first side edges and opposed second side edges each of said first side edges being curved, and a high-polymer piezoelectric membrane having electrically conductive, oppositely polarized surfaces and a single axis of elongation parallel to said surfaces, said membrane being secured in said opening and held in a part-cylindrical shape by reason of the curvature of said first side edges, the axis of elongation lying in the circumferential direction of the thus curved membrane, and the membrane being stressed in the direction of the axis of elongation.

15. A microphone according to claim 11, 12 or 13, wherein said first and second

membranes are arched in the same direction and are electrically polarized in opposite directions to each other.

16. A microphone including: a housing having opposed first and second apertures; a spectacles-like support structure having interconnected first and second conductive frames each having the shape of an arch and in said housing adjacent to said first and second apertures, respectively the frames being arched in the same direction; first and second high-polymer piezoelectric membranes each of which has been prepared by elongation in one direction until a desired thickness is attained and electrically polarized in the direction of its thickness and coated with an electrically conductive film on its opposite surfaces, said first and second membranes being adhesively secured to said first and second frames, respectively to take the arched shapes with their directions of polarization being opposite to each other, and their directions of elongation being in the circumferential directions of the arches and being stressed in their directions of elongation, the directions of arches and polarization of said membranes thereby being such that when the membranes are caused to flex in opposite directions there developes an output signal which is substantiallv twice the amplitude of the signal developed from each membrane, and, when said membranes are caused to flex in the same direction. there developes substantially no output signal.

17. A method of making a microphone by: providing a high-polymer piezoelectric film; elongating said piezoelectric film in one direction until a predetermined thickness is reached; coating each side of said piezoelectric film with a conductive film; polarizing said film in the direction of its thickness by applying an electrical potential across the metal coatings; adhesively securing a frame structure to one surface of said metal coated piezoelectric film; cutting said piezoelectric film along the edges of said framed structure; bending said framed structure together with said piezoelectric film to form a partcylindrical surface whose direction of circumference coincides with the direction of said elongation to produce a mechanical stress therein in the direction of the elongation; and securing said framed structure to a housing.

18. A method as claimed in claim 17, wherein said framed structure comprises a pair of identical frames connected together by a connecting member and in that said frames together with said piezoelectric film are each bent to take the shape of a pair of identical arches which are spaced from, and in parallel with, each other.

19. A microphone constructed substantially as described hereinabove with reference to and as illustrated in Figures 1 to 4, 11 and 12 or 13 and 14 of the accompanying drawings or as modified substantially as hereinbefore described with reference to and as illustrated in any one of Figures 5 to 7, 9 and 10 of the accompanying drawings.

20. A method of making a microphone substantially as described hereinabove with reference to Figures 2 and 14 of the accompanying drawings.