GB899474A - Improvements in or relating to electroacoustic transducers - Google Patents

Abstract

899,474. Piezo-electric instruments. MASSA, F. Sept. 17, 1958 [Sept. 17, 1957], No. 29824/58. Class 40 (4). [Also in Group XL (b)] A piezo-electric microphone or receiver which operates at audio or ultrasonic frequencies comprises a flat piezo-electric bender element having one face bonded to a flexible biasing plate with a first electrode disposed between the plate and the element, and a pair of further electrodes on the opposite face of the element connected to respective leads, the element being pre-polarized in one sense between the first electrode and one of the pair of electrodes, and in the opposite sense between the first electrode and the other of the pair of electrodes. The piezo-electric element comprises barium titanate or lead metaniobate in the form of a disc 3, and has either a single electrode 28, Figs. 8 and 9, on one face and double electrodes 31, 32 on the other face, or the two faces have two and four electrodes 31, 32 and 34, 35, 36, 37, Figs. 9, 12 and 13, respectively. The electrodes 35, 36 are linked by a conductive portion 38, and the pre-polarization circuits are shown in Figs. 10 and 14. A transducer of the kind shown in Fig. 8 is used in a microphone or receiver T, Fig. 2, the surface of the piezo-electric element associated with a single electrode being secured by cement 2 to a sound-receiving or radiating plate 1, which also functions as a biasing plate. The transducer is enclosed in a housing 13 by securing the plate 1 with cement 14. Electrical connections to the electrode pair 31, 32 are effected by the twin conductors 11 of a screened cable which passes through an opening in the housing and is sealed by waterproofing 15. The transducer shown in Figs. 9, 12 and 13 is used in an alternative embodiment, Fig. 4, in which a biasing plate 30 is secured by cement 2 to the element face having the fewer electrodes 31, 32. The transducer assembly is mounted in a housing 16 between a cork ring 17 and a foam rubber ring 24, the outer edge of the housing base being crimped over an end plate 20. A metallic screen 21 permits the passage of sound waves into the housing. A concentric cable 25 provides connections in this case to two electrodes 34, 37 in the four-electrode groups; and the resonant frequency of the transducer is lowered by a small weight 23 cemented to the biasing plate 30. The biasing plate enables the transducer to operate in a bending mode and is made of aluminium, nickel or copper with a thickness equal to or exceeding that of the piezo-electric element. The Specification refers to known two-electrode piezo-electric elements, Figs. 5 and 6 (not shown), using X-cut quartz, L-cut ammonium dihydrogen phosphate and Y-cut lithium sulphate, and the use of slated or evaporated electrodes of silver, gold and copper.