GB670586A - Improvements in or relating to granule-type electro-acoustic transducers - Google Patents

Abstract

670,586. Microphones. STANDARD TELEPHONES & CABLES, Ltd. April 22, 1949 [April 23, 1948], No. 11184/48. Class 40 (iv). In a granule-type transmitter, a casing member on or in which are mounted all other parts and which forms the peripheral wall of the granule chamber has seatings for an acoustic diaphragm and for two diaphragm-type electrodes. A sub-assembly comprising a mica disc 27, the back electrode 15, a nickel silver cover 26 and an insulating ring 28 of resin-impregnated canvas are held in a recess in the die-cast zinc or aluminium casing 1 by a lead ring 29, as described in Specification 594,082. Similarly held in the casing 1 is a domed front electrode 12, the distance between the two electrodes being least along the axis of the transmitter. Both electrodes are made of carbonized nickel as described in Specification 594,116. A coating of white cellulose enamel insulates the casing and the annular portion of the front electrode from the granules, or an annulus of silk may be inserted between the front electrode and the granules as described in Specification 670,585. The diaphragm 21 is similar to those of Specifications 616,158 and 631,970, being a plastic annulus with a serrated metal flange bonded thereto and frictionally engaging the domed portion of the front electrode 12. The diaphragm 21 is peripherally clamped between the casing 1 and a die-cast disc 24 which are held together by a plastic or brass ring 25 having spun over flanges. The central aperture in the diaphragm is protected by a moisture cover 23. An aluminium alloy plate 17 secured in the casing is pierced in three places 20, the resulting upturned metal prongs being compressed to provide the correct amount of damping for the diaphragm 21. A plug secured with clay shellac cement, closes the granule chamber filling hole 32. The back of the casing 1 is coated with a colloidal suspension of carbon or a silver paste to give a good contact surface. In a modification, Fig. 2, the filling hole is in the cover-plate 35 and the back electrode 37 is apertured, the space between being used as a granule reservoir.