GB477623A - Improvements in or relating to acoustic rectifying devices and to alternating current signalling systems incorporating such devices - Google Patents

Abstract

477,623. Piezo electric sound - reproducers. STANDARD TELEPHONES & CABLES, Ltd. July 17, 1936, No. 19888. Convention date, Aug. 1, 1935. [Class 40 (iv)] [Also in Group XL] A modulated electrical carrier wave is converted into a mechanical vibration having a corresponding carrier frequency and modulation by a piezo-electric plate which is tuned to the frequency of the carrier wave (or to a frequency derived therefrom), and produces (in the surrounding air) sound waves corresponding to the voice currents with which the carrier wave is modulated. As shown in Fig. 1, a piezo-electric plate 8 is clamped between metal strips 6 attached to an arm 3 slidable on a stand 1. The plate is dimensioned to have a resonant frequency corresponding to the carrier frequency and is coated on its vibrating surfaces with a thin layer of silver or with layers of silver and platinum. The selective characteristic of the plate may be slightly broadened by mechanical damping introduced by the clamping effect of the spring clips 6 and /or by means of the series resistance 14 and the shunt resistance 15. In a modified arrangement, Fig. 3, the piezo-electric plate 16, coated as described in connection with the plate 1, is clamped at its corners by two metallic frame members one of which is indicated at 17, arranged relative to the plate 16 to provide gaps 27. Receiving circuit arrangements, described in connection with Figs. 5, 6 (not shown) include a superheterodyne arrangement by which a piezo-electric plate of relatively low frequency may be used with carrier waves of different frequencies. The resonance band of the piezo-electric plate may be broadened by an inductance in place of the resistances 14, 15 of Figs. 1, 3, in which connection U.S.A. Specification 1,921,035 is referred to.