Piezoelectric electroacoustic transducers of the bi-laminar flexural vibrating type

Claim 1

In combination in an electroacoustic transducer, a flexurally vibratile plate assembly comprising a plurality of bonded plates at least one of which is capable of changing its lateral dimensions upon being subjected to the influence of an electrical signal, support means for supporting said vibratile plate assembly, said support means characterized in that it does not significantly impede the free flexural displacement of the vibratile plate assembly when it is operated at its fundamental free flexural resonant mode of vibration, said vibratile plate assembly characterized in that when it is operating at its fundamental free resonant frequency mode, the displacement of the peripheral area of the vibrating surface of said vibratile plate assembly is of opposite phase compared with the displacement of the central area of said vibratile plate assembly, a cup-shaped housing structure having a closed bottom portion and an axially-aligned open end portion, said housing structure adapted for enclosing said vibratile plate assembly, said support means characterized in that it holds said vibratile plate assembly in axial alignment with said housing structure when it is enclosed in said housing, a lid, means for attaching said lid to said open end of said housing structure whereby to close said open end, said lid characterized in that it has an opening, and further characterized in that said opening is concentric with the axis of said cup-shaped housing when said lid is attached to close said housing, spacing means for locating the vibratile surface of said vibratile plate assembly in close spaced proximity to the inner wall surface of said lid when said lid is attached to said housing structure, said spacing means characterized in that the total contact area of said spacing means is negligibly small compared with the surface area of said vibratile disc, said spacing means further characterized in that it includes at least three separate small surfaces located in a plane perpendicular to the axis of said cup-shaped housing, said spacing means further characterized in that the central axis of said vibratile plate assembly is held in alignment with the central axis of said housing structure, said opening in said lid further characterized in that the area of the opening does not exceed 50% of the total area of said vibratile plate assembly, said lid further characterized in that the radial dimension between the outer peripheral edge of the opening in said lid and the outer peripheral edge of said closely spaced vibratile plate assembly is greater than approximately 1/4 wavelength and less than approximately 3/4 wavelength of the sound in the medium at the frequency of operation whereby said lid acts as an acoustic delay line for reversing the out-of-phase acoustic vibrations generated by the outer peripheral area of the vibratile disc before they reach the opening in said lid whereby the delayed peripheral vibrations add constructively to the vibrations generated by the central area of said vibratile disc.