GB471270A - Improvements in or relating to granular microphones - Google Patents

Abstract

471,270. Microphones. SIEMENS BROS. & CO., Ltd., and SUTTON, G. W. April 17, 1936, No. 11075. Addition to 383,348. [Class 40 (iv)] In a two-electrode microphone or transmitter having a substantially rigid moving electrode coupled to a diaphragm by means of a light rod or pin and adapted to move as a whole in the granules in which it is immersed in such manner that its motion in either direction is opposed by granules, the front interior wall of the granule chamber has a substantially rigid outer portion adapted to provide a mechanical reaction against the movement of the granules which lie between the moving electrode and this wall and an inner portion composed of flexible non. resonant material forming an absorbing surface adapted to minimize the formation of unwanted standing waves in the granules and to assist in making the mechanical reaction to movement of the moving electrode more nearly the same for movements in either direction. As shown, the diaphragm 1, which is clamped between a casing member 4 and a spun-over ring 5, is connected by a rod 8 to a moving electrode 9, preferably of carbon, which is shaped so that a part of its surface is approximately parallel to the associated surface of a fixed electrode 12 formed with a central bore 20 and clamped to a brass member 22 forming one of the terminals of the microphone. The wall of the granule chamber is partly constituted by a brass member 17 having the outer part of its front wall of frusto-conical shape and formed with a hole closed by a composite washer 15 of a number of discs of Swiss silk. The washer 15 is clamped between the member 17 and an apertured brass disc 16 seated in a brass member 14 provided with a flanged portion 26 which is clamped between the member 4 and a threaded member 24 carrying an insulating block 23 in which is mounted the fixed electrode 12. The bore 20 is closed by a felt ring 21 which acts as a mechanical counterpart to the flexible washer 15, and also serves to minimize the formation of standing waves in the granules. The second terminal of the microphone is formed by a spring ring 11, and the surface of the granule chamber is coated with a layer of insulating varnish or enamel.