Electroacoustic Transducers

Abstract

An electroacoustic transducer uses a bilaminar vibratile element having at least two plate members which are bonded together in face-to-face relationship, at least one of the plates being made from a piezoelectric material. Flexible electrical conductors freely support the bilaminar vibratile plate members on a frame. Any flexural vibrations of the bilaminar element apply an alternating voltage to the electrical conductors. A sonic energy mask is positioned over and spaced away from an exposed surface of the vibratile element to prevent sound radiation from the masked surface portion of the vibratile element.

Parent Case Text

This is a continuation of application Ser. No. 17,430, filed Mar. 9, 1970 and now abandoned.

Description

This invention relates to electroacoustic transducers, and more particularly to transducers employing a resonant bilaminar plate operating at its fundamental flexural mode of vibration.

In general, many bilaminar transducer elements operate in a flexural mode. For example, many widely used loudspeakers and microphones have a bilaminar piezoelectric plate which is pivoted or clamped at specific regions. A diaphragm is coupled to the unclamped vibratile portion of the bilaminar plate. Examples of such prior art constructions are shown, for example, in FIGS. 4, 5, 7 and 8 of U.S. Pat. No. 2,518,993.

The invention is not limited to use in any particular frequency region. However, the improvements described herein are particularly valuable at the higher audible and ultrasonic frequencies where it has generally been extremely difficult to achieve a high efficiency operation. If the prior art structures are used in the ultrasonic frequency region, it is extremely difficult to operate the transducer efficiently because of the mass of the diaphragm and the relatively high compliance at any unsupported free corner of the mounted bilaminar plate. Therefore, to improve the high frequency operation, as taught by this invention, the diaphragm is removed, and the bilaminar element is suspended to vibrate freely without any restraint or added mass. By removing the mounting restraints, the fundamental resonant frequency of the bilaminar plate is increased. The high frequency vibrations are sustained more efficiently.

To enable a design of a practical transducer employing a resonant free supported bilaminar plate, it is necessary to remove all restraints from the surfaces of the vibratile plate. Further, it is necessary to couple the vibrating surface of the plate in a manner to drive a maximum radiation of sound energy into the medium.

Accordingly, an object of this invention is to design new and efficient low-cost electroacoustic transducers utilizing a bilaminar plate operating at a fundamental free resonant mode.

A further object of this invention is to suspend a bilaminar piezoelectric plate within a frame-like mounting structure. Here, an object is to position the bilaminar plate accurately and without imposing mechanical restraints at the fundamental resonance mode of vibration.

Yet another object of this invention is to mount a piezoelectric bilaminar rectangular plate within an opening in a frame-like structure. Here an object is to use thin ribbon-like conductor leads (mounted at right angles on opposite sides of the rectangular piezoelectric plate) as a suspension means to hold the plate with negligible restraint.

An additional object of this invention is to provide a sound opaque mask in a fixed spatial relationship with respect to the vibratile surface of a bilaminar disc operating at its fundamental free resonance mode. Another object is to shape the mask so that only a portion of the surface of the resonant plate is exposed to the medium.

A still further object of this invention is to provide a unitary mounting structure for a bilaminar transducer element and simultaneously to provide a sound opaque masking area which prevents the exposure of a portion of the vibratile plate surface to the medium.

Still another object of this invention is to provide a very simple mounting and housing structure for a bilaminar piezoelectric plate in order to achieve an efficient operation at its fundamental free resonant mode.

Other objects, features and advantages will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of one embodiment of the inventive transducer construction, with the outer transducer housing removed;

FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1 (with the outer housing in place);

FIG. 3 is another cross-sectional view taken along the line 3--3 of FIG. 1 (with the outer housing in place);

FIG. 4 is a plan view of the top of another embodiment of the inventive transducer element assembly, this embodiment using fewer parts as compared with the transducer element assembly of FIG. 1;

FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 4;

FIG. 6 is a plan view of the bottom of the transducer element assembly shown in FIG. 5; and

FIG. 7 is a plan view showing a one-piece frame structure before the bilaminar plate is mounted therein.

The reference character 10 designates a rigid flat plate, preferably made of an electrical insulation material such as Bakelite. A square piezoelectric bilaminar plate assembly 12 is suspended in a square opening formed in the center of the rigid plate.

The bilaminar plate 12 could take either of two forms. In one form, the plate may include a pair of polarized piezoelectric ceramic elements, such as barium titanate or lead zirconate titanate, for example. Alternatively, other suitable piezoelectric materials may also be used, as is well known in the art. Metalized electrodes are formed on the opposite surfaces of the two piezoelectric elements. The bilaminar plates are arranged with their common positive potential surfaces, marked (+), bonded together in face-to-face relationship. In the second form, the bilaminar assembly 12 could also include an inert plate (such as aluminum) which has a piezoelectric element bonded thereto. However, it should be understood that other inert materials could also be used.

Thin ribbon-like electrical conductors 13 and 14 are conductively bonded to the two opposite exposed electrode surfaces of the bilaminar plate assembly. These conductors lie at right angles to each other and along the center lines of these two surfaces. The conductive bond may be made by means of conducting cement, by welding, or by other suitable means. The ribbon conductor 13 is attached to the bottom surface electrode of the bilaminar assembly (as viewed in FIG. 1). The ribbon conductor 14 is attached to the top electrode surface.

Means are provided for suspending the square bilaminar piezoelectric assembly 12 with a free-free suspension. More particularly, during assembly, these two ribbon conductors 13, 14 are crossed over so that the ribbon 13 is cemented to the top surface of the plate 10 (FIG. 2), and the ribbon 14 is cemented to the bottom surface of plate 10 (FIG. 3). By thus crossing the ribbon leads, the bilaminar element 12 is effectively held in a very accurate alignment with a free-free clamping. The bilaminar element 12 is thus free to vibrate without restraint in its fundamental resonance mode. In the free fundamental resonance mode, the four corners of the square bilaminar transducer element 12 vibrate together in phase. All four corners are opposite in phase to the center area of the bilaminar element.

To improve the radiation efficiency of the transducer, a sound opaque mask 15 is suspended over the center portion of the bilaminar piezoelectric element 12. The outline contours of the mask 15 follow the shape of the nodal line on the surface of the piezoelectric plate 12. Thus, the area of the mask is about one-half the area of the piezoelectric element. Only the four corners of the piezoelectric element 12 are exposed to the medium, as illustrated in FIG. 1. The mask prevents the out of phase radiation in the center area from neutralizing the radiation from the four corners of the element.

The mask 15 may be made from any suitable sheet of metal or plastic which is sufficiently thick to remain practically stationary during the operation of the transducer. The proper thickness for any material may be easily determined by noting the sensitivity of the transducer at its frequency of operation. The optimum thickness is where no additional increase in sensitivity results when extra thickness is added to the masking plate.

To complete the transducer assembly, a U-shaped metal bracket 16 is notched at the tips of the arms forming the U. These notches lock into mating slots 17 at the periphery of the plate 10. At its base, an eyelet 18 attaches the bracket 16 to a metal lid 19. The eyelet 18 is part of the insulated terminal 20. The ribbon conductor 14 has its free end soldered to the terminal pin 20. The surface of the ribbon conductor 13 is soldered to the contacting surface of the U-bracket 16. Thus, the complete electrical circuit extends from eyelet 18, through bracket 16, conductor 13, the element 12, conductor 14, and the pin 20.

The somewhat cup-shaped metallic housing 21 has a protective screen 22 welded or cemented therein to cover an open surface 23. The open lip of the cup housing is crimped at 19a over the edge of the lip 19 to complete the outer shell of the transducer. For operating the transducer, electrical connections are made to external equipment via the terminal 20 and the metallic plate 19.

A second embodiment of the transducer element assembly is illustrated in FIGS. 4-7. Here (FIG. 5), one molded piece of rigid plastic provides a frame structure 24 with a square cavity 25 partly recessed in the bottom thereof. The surface at the base of the cavity 25 has four corner sections which are perforated or pierced. For easy identification, one of these perforated areas is shown as cross-hatched at 26. It is somewhat triangular, lying over a corner of the bilaminar element.

The bilaminar piezoelectric element 12 is mounted within the frame structure 24 by means of the ribbon conductors 13 and 14. These ribbon conductors 13, 14 may be cemented or otherwise fastened to the surface of the frame structure 24. In this assembly, only one ribbon conductor 14 crosses the edge of the piezoelectric element 12, as shown in FIG. 5. The ribbon conductor 13 is drawn along the plane of the rear surface of the piezoelectric element 12 and the frame structure 24. This arrangement of conductor 13 is most clearly illustrated in FIG. 6, which shows a bottom view of the assembly. In the top surface of the molded piece 24, the solid center portion 28 serves the same function as the masking plate 15 serves.

After the completion of the assembly of FIGS. 4-7, it may be substituted for the assembly held by the U-shaped bracket 16 in FIG. 2. The remainder of the transducer assembly may be completed as previously described in connection with FIG. 2.

To obtain an efficient operation of the bilaminar transducer structure, it is necessary for the four corners of the piezoelectric element 12 to be free to vibrate with minimum restraints, during operation at the fundamental resonance mode. The ribbon-like conductors 13, 14 suspend the piezoelectric members at the four nodal points in the centers of the four sides of the square bilaminar plate. This suspension achieves minimum restraints in the mounting of the element.

We have also found that, if the thickness of the bilaminar piezoelectric element is greater than approximately 1/20th the outer periphery of the element, the free fundamental resonance mode of vibration of the element becomes restricted, and the vibrational efficiency is reduced. Therefore, the dimensions of the bilaminar elements should preferably be such that its periphery is more than 20 times greater than its thickness.

While several specific embodiments of the present invention have been shown and described, it should be understood that modifications and alternative constructions may be made. Therefore, the appended claims are intended to cover all equivalents falling within their true spirit and scope.

Claims

We claim:

1. An electroacoustic transducer comprising a bilaminar vibratile element comprising two square piezoelectriplates bonded together in face-to-face relationship, electrode surfaces on the two exposed outer surfaces of said bonded plates, a first ribbon-like electrical conductor conductively bonded along a center line of one exposed electrode surface on one side of said bilaminar element, a second ribbon-like electrical conductor conductively bonded along a center line of the opposite exposed electrode surface on the other side of said bilaminar element, said first and second ribbon conductors being oriented to lie at right angles to each other, said ribbon conductors having a length sufficient to project beyond the edges of said bilaminar element, a structural frame member having an opening which is larger in size than the size of said bilaminar element, and means including said ribbon-like conductors for freely supporting said vibratile bilaminar element within said opening without any restraint or added mass whereby the free fundamental resonant mode of vibration is established for said vibratile element.

2. The transducer of claim 1 and masking means spaced parallel to an exposed surface of said vibratile bilaminar plate, and said masking means having an area which is less than the area of said bilaminar plate.

3. The transducer of claim 2 and a housing structure enclosing said bilaminar element, said housing having a sound transparent opening located in a noncritical spaced proximity to said masked means and said vibratile bilaminar plate member.

4. The transducer of claim 2 wherein said masking means is approximately square and has an area which is approximately equal to one-half the area of said bilaminar element, said square masking means being located in spaced relationship over the center portion of said vibratile bilaminar element, said masking means being rotated with reference to the bilaminar element so that the diagonal axes of said masking means are in alignment with the orthogonal axes of the bilaminar element.

5. The transducer of claim 1 further characterized in that said bilaminar element has a periphery which is greater than twenty times its thickness.

6. An electroacoustic transducer comprising a bilaminar vibratile member including at least two plate members bonded in face-to-face relationship, at least one of said bonded plates being a piezoelectric material with opposing electrode surfaces, a pair of perpendicularly extending flexible electrical ribbon-like conductor means connected to the nodal points on said opposite electrode surfaces, said piezoelectric material vibrating in out-of-phase modes when said member is energized by an alternating current signal applied via said conductor means, support means comprising a structural plate member having a recessed cavity formed therein, said recessed cavity being larger than said vibratile member, and means including said conductors for freely supporting said bilaminar vibratile member within said recessed cavity and in spaced relationship with respect to the peripheral edge and the recessed surface of said cavity, said electrical conductors having a flexibility which enables said vibratile member to vibrate in a free mode of vibrations, a portion of said recessed surface being perforated opposite a portion of the area of said vibratile member to form sound opaque energy masking means positioned over at least one portion of said member which is vibrating out of phase with respect to other portions of said member.

7. The transducer of claim 6 wherein both said recessed cavity and said bilaminar vibratile member are square, and there are four of said perforations, each perforation being approximately triangular in shape with each of said triangular perforations being located at a corresponding one of the four corners of the square recessed surface, whereby the four corner sections of said supported bilaminar vibratile plate member lie opposite said perforations.

8. The transducer of claim 6 further characterized in that the shape of the unperforated portion of said recessed surface is approximately square and at the center of the four triangles, the area of said center square being approximately one-half the area of said bilaminar vibratile member, and the diagonal axes of said unperforated center square portion of said recessed surface being aligned with the orthogonal axes of said vibratile member.

9. The transducer of claim 6 further characterized in that said bilaminar member has a periphery which is greater than twenty times its thickness.