GB592217A - Improvements in or relating to piezoelectric transducer means - Google Patents

Abstract

592,217. Piezo - electric telephone instruments. BRUSH DEVELOPMENT CO. Nov. 4, 1943, No. 18357. Convention date, Feb. 18, 1942. [Class 40 (iv)] [Also in Group XL] A piezo-electric compressional. wave sender or receiver comprises a base plate, a diaphragm, a plurality of 45‹ X-cut crystals of the Rochelle salt type mounted between and in contact with the base plate and diaphragm, the two crystal faces which are in contact with the base plate and diaphragm being defined by a dimension which does not change due to piezo-electric activity and by a shorter dimension which does change due to piezo-electric activity when the element is subjected to an alternating electric field, and means for subjecting the crystals to an alternating electric field. In Figs. 1 and 2 two sets of 45‹ X-cut crystals 11 are mounted radially on edge between a base plate 21 and a diaphragm 22. The dimension at right angles to the base plate has the maximum piezo -electric displacement so as to impart the most drive to the diaphragm. The faces in contact with the base and the diaphragm are defined by a long X dimension along which there is no piezoelectric displacement, and a short dimension along which there is some displacement. This gives only a small relative displacement between the crystal surfaces and the base and diaphragm thus reducing frictional heat and spurious frequency effects. The ends of the crystals are provided with electrodes. The outermost and innermost electrodes are in contact with circular spring contacts 32, 33 connected to a lead 34, while the intermediate ends of both sets of crystals are connected to a serrated spring contact 35 connected to a lead 36, thus placing all the elements electrically in parallel. Leads 34, 36 are formed into a cable 37 which passes through base 21 in watertight manner. The diaphragm 22, crimped annularly at 26, 27, is fastened to the rim 30 and raised part 28 of base 21 to form a sealed enclosure suitable for subaqueous signalling. In the modification shown in Fig. 4, the crystal elements 11 are arranged radially in slots in a composition spider 53 of low elasticity such as sponge rubber or cork. The casing is closed by a thick rubber diaphragm and the whole assembly is filled with castor oil. The spider 53 absorbs vibrations radiated from the side faces of the crystals, and a layer of cork or sponge rubber 68 absorbs radiation from the lower face of the casing, so improving the disectional characteristics. In a further modification, Figs. 5 and 6 (not shown), the crystals are mounted in four parallel rows to form a square block in contact with the diaphragm. The casing is air-filled, and a tube of silica gel is provided to absorb condensation and to prevent dehydration of the crystals. In a further modification, Figs. 7 and 8 (not shown), the crystals are arranged in two parallel rows in a long thin casing to give desired directional characteristics. Specification 592,222 is referred to.