Abstract:
A piezoelectric audio transducer suitable for underwater use comprises a piezoelectric ceramic plate within a housing having open front face and back faces exposing the front and back faces of the ceramic plate to ambient pressure. The ceramic plate can be supported in spaced relation from the housing by, for example, open cell foam, so as to be vibrationally isolated from the housing.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a piezoelectric audio transducer, such as a microphone. 
     Piezoelectric microphones suited for underwater use are known. For example, in U.S. Pat. No. 5,218,576 to De Chico a piezoelectric microphone has a piezoceramic transduction layer and a metal substrate with a thin film of viscous fluid between them. The viscous film allows the transduction layer and substrate to expand and contract relative to each other when the laminate bends under increasing hydrostatic pressure as the transducer descends in a body of water. While this arrangement reduces hydrostatic stresses, such stresses are not eliminated. 
     U.S. Pat. No. 4,013,992 to Bewberry et al. describes a microphone intending to expose both sides of the piezoelectric ceramic plate to ambient pressure. This would tend to avoid bending under increased hydrostatic pressure. However, it is believed that the design may be improved. 
     Accordingly, the present invention seeks to provide an improved piezoelectric audio transducer. 
     SUMMARY OF INVENTION 
     A piezoelectric audio transducer suitable for underwater use comprises a piezoelectric ceramic plate within a housing having open front face and back faces exposing the front and back faces of the ceramic plate to ambient pressure. The ceramic plate can be supported in spaced relation from the housing by, for example, open cell foam, so as to be vibrationally isolated from the housing. 
     According to the invention, there is provided, a piezoelectric audio transducer comprising: a piezoelectric ceramic plate; a housing for said ceramic plate, said housing having an open front face exposing a front face of said ceramic plate to ambient pressure and an open back face exposing a back face of said ceramic plate to ambient pressure. 
    
    
     Other features and advantages of the invention will be apparent after a review of the following description in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate an example embodiment of this invention, 
     FIG. 1 is a plan view of a microphone made in accordance with this invention, 
     FIG. 2 is a cross-sectional view along the lines II—II of FIG. 1, 
     FIG. 3 is a view of a portion of FIG. 2, and 
     FIG. 4 is a plan view of a microphone made in accordance with another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIGS. 1 to  3 , a piezoelectric microphone  10  comprises a piezoelectric plate  12  within a housing  14 . The housing may be fabricated of metal or metalised plastic and electrically grounded. The plate comprises a ceramic or crystal piezoelectric wafer  16  with a front face coated with a metal layer  18  and a back face attached face-to-face to a conductive vibratile membrane  20  by a layer of conductive glue  22 . However, wafer  16  is assumed to be ceramic so that plate  12  is referred to as a ceramic plate. The conductive vibratile membrane may be fabricated of metal, or a metalised plastic. An insulating layer is formed over the ceramic plate. 
     The membrane  20  may extend beyond a periphery of the back face of the piezoelectric wafer  16  to support a pair of inertial weights  24   a ,  24   b  that are attached to the vibratile membrane  20  adjacent opposite sides  26   a ,  26   b  of the piezoelectric wafer  16 . 
     A damping body may be attached to the ceramic plate. The damping body may comprise rubber disk  28  attached to the back face of the vibratile membrane. The ceramic plate  12 , with rubber disk  28 , is enveloped by an envelope  30  of open cell foam such that the plate is spaced from the housing. The foam, being rigid enough, locates the ceramic plate within the housing. Being open celled, the foam is highly porous. 
     The housing  14  has a large front opening  34  exposing the entire front face of the ceramic plate  12 . The housing also has a large back opening  36  exposing substantially the entire back face of the ceramic plate  12 . A metal front screen  38  extends across the front opening  34  and a metal back screen  40  extends across the rear opening  36 . These screens, which may be made of stainless steel, are connected into the housing  14 . 
     An output wire  44  is connected to the conductive membrane  20  and another  46  is connected to the metal layer  18 . These wires  44 ,  46  may connect to a pre-amp  48  which, in turn, outputs to dual signal wire  50 . 
     The microphone may be used as a lip microphone in air. When the microphone is submerged in water, water enters the housing through the front and back openings  34 ,  36  and flows through the open cell foam envelope  30 . This exposes the front and back faces of the ceramic plate  12  to the ambient pressure. Thus, no matter what the ambient hydrostatic pressure, this pressure is applied equally to both the front and back faces of the ceramic plate. In consequence, hydrostatic stresses on the ceramic plate are avoided. 
     When the microphone emerges from water, the water within the housing will readily and quickly drain from the open cell foam envelope  30  through the front and back openings in the housing  14 . 
     Since water moves freely into and out of the microphone, the microphone will be available for use within moments of leaving the water. 
     The open cell foam envelope  30  also provides good vibrational isolation between the housing  14  and the ceramic plate  12 . With vibrations in the housing substantially isolated from the ceramic plate, a potential source of noise is substantially reduced. 
     When a user speaks into the front opening  34  of the housing  14 , sound waves impinge on the front face of the ceramic plate  12 . Provided the user speaks directly into the front opening and the housing is close to the user&#39;s lips less sound energy will impinge on the back face of the ceramic plate. This difference in the sound energy impinging on the front and back faces of the ceramic plate results in plate vibration and an output from the microphone. On the other hand, for distant sound sources, there will be little, if any, difference between the energy impinging upon the front and back faces of the plate. In consequence, background noise is substantially, or completely, cancelled at the microphone. Thus, another potential source of noise is miniminized. The inertial weights proximate the periphery of the ceramic plate are relatively immobile in the presence of sound energy compared to the ceramic plate due to their much higher mass. Thus, the inertial weights help ensure the ceramic plate bends in the presence of the sound energy rather than simply be displaced. The bending of the ceramic plate distorts the structure of the wafer  16  producing a voltage proportional to the bending of the wafer. Thus, the impinging sound waves are converted into electrical signals which pass to the pre-amplifier  48  and output on output dual wire  50 . 
     The screens  38 ,  40  with the grounded housing  14  shield the output signal from stray voltages. The rubber disk  28  dampens the vibration produced by sound wave pressure changes and suppresses resonant frequencies. 
     In FIG. 4, which illustrates a further embodiment of this invention, like parts have been given like reference numerals. Microphone  100  differs from microphone  10  (FIGS. 1 and 2) in that the pair of opposed inertial weights  24   a ,  24   b  (FIGS. 1 to  3 ) has been replaced by an inertial ring weight  124 . The inertial ring weight serves the same purpose as the pair of opposed weights. However, the ring weight will tend to stiffen the ceramic plate which lowers its sensitivity to sound energy. 
     In the embodiment of FIG. 4, the open cell foam envelope  30  (FIGS. 1 and 2) has been replaced by ligaments  130  attaching the ring weight  124 , and hence the ceramic plate  12  to the housing  14 . The ligaments locate the ceramic plate within the housing but leave both faces of the ceramic plate exposed to the ambient pressure. Additionally, because the ligaments provide only a small area of connection between the housing and the ceramic plate, the ligaments also help minimize the passage of vibrational energy in the housing to the ceramic plate. 
     In further embodiments of the invention, the open cell foam envelope  30  (FIGS. 1 to  3 ) may be replaced by any other material that is highly porous and capable of locating the ceramic plate within the housing. For example, a course rubber sponge or flexible spider suspension may provide a suitable support. 
     As will be appreciated by those skilled in the art, with appropriate electronics, microphone  10  (FIG. 1) or  100  (FIG. 4) could be converted into a sound projector. Thus, the apparatus of the invention is a sound transducer, rather than being solely a microphone. 
     Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.