Patent GB-184314-A (GB)
Title: Improvements in piezo-electric devices
IPC: H04R17/00
Filed: 1921-06-09
Abstract: 184,314. Russell, E. W. C., and Cotton, A. F. R. June 9, 1921. Receiving and detecting audible signals. - In a piezo - electric device employed for the reception or reproduction of sound or pressure waves, for example, in telephones or gramophones, the active crystal is subjected to pressure along an axis and to double torsiona.l effects at the ends of the axis, one or more torsional arms providing the medium for the passage of waves to or from the crystal in reception or transmission respectively. In one form, Fig. 2, the crystal a having the usual contact plates or circumferential bands b is mounted between two oppositely-projecting parallel torsion arms d, and the mounted crystal is supported on an axis perpendicular to the basal planes and the torsional arms. These arms may be made of metal with roughened faces, and are provided with contact pads e of tinfoil &c. interleaved with cellulose acetate or similar material which has the property of converting mechanical sound waves into pressure variations in the material itself. The cellulose acetate may be softened before assembly for example, by dissolving in the acetone, so as to take up the shape of the basal planes of the crystal. The arms d may be crossconnected by a cross-bar h to which a diaphragm g acting as a transmitting or receiving source is rigidly secured; or, alternatively the diaphragm is secured directly to the two arms d. Constant pressure is applied to the system by means of a cradle f, the ends of which carry spring plates adjustable by nuts, intermediate damping and bearing plates of cellulose acetate being employed. The whole device may be supported from the diaphragm g; or alternatively it may be supported in a container, the constant pressure being supplied from uprights attached to the base or sides of the container. In order to prevent pressure waves from passing to the main frame when the diaphragm is gripped at its outer edge, the diaphragm may have a reduced thickness at its edges, for example, it may have an annular groove around the central active area, which may also be lens-shaped. The volume of the members conducting the pressure wave is made proportional to the capacity of the crystal, and the collecting surface or diaphragm should be made proportional to the capacity of the conducting members. Two or more crystals in tandem or in pairs may be employed. each subjected to double torsion. Continuous external sounds may be eliminated differentially by employing an additional receiver subject only to the external sounds. Sounds above audibility may be rendered audible by beat methods. According to one of the Provisional Specifications, the axial support for the crystal may comprise hard steel balls set in ebonite fixed to the outer faces of the torsional bars and fitted in the cupped ends of two axiallyaligned screws, one of which may be made adjust. able by being carried by a spring of whalebone or irinoid, whilst the cross-bar h is provided with points or knife-edges engaging recesses in the torsion bars, the centre of the cross-bar carrying a reed which is connected at its other end to the vibratory diaphragm.