GB400629A - Improvements in or relating to condenser microphones - Google Patents

Abstract

400,629. Microphones. TELEPHONE MANUFACTURING CO., Ltd. and BURRIDGE, B. J., Martell Road, West Dulwich, London. April 27, 1932, No. 12156. [Class 40 (iv).] A condenser microphone comprising in combination a casing and fixed and movable electrodes, one of which is the diaphragm, is characterized in that the diaphragm is substantially untensioned, and the air space enclosed behind it is such that the pressure therein is not appreciably modified by movements of the diaphragm in normal operation. Preferably, the fixed electrode has a surrounding air space or spaces between it and the casing and there is no "breather hole" in the casing. In the construction shown, the fixed electrode consists of a large number of pins 16 extending from a plate 17, which is mounted on an insulating block 14 provided in a casing 10. The outer ends of the pins may all lie in a plane parallel to that of the diaphragm 20, which in the device shown is a flat sheet of gold foil. The diaphragm is clamped between a ring 21 and the upper end of the casing, which is formed with a lip 24 that serves to stretch the diaphragm slightly, but only sufficiently to prevent excessive slackness. The gap between the diaphragm and the back electrode is comparatively large, e.g. from À01 to À008 of an inch. The thickness of the diaphragm is preferably 2À5 X 10<-6> inches, and its "effective weight" is less than the estimated pressure produced by sound waves in normal use. A shield 25 may be provided to protect the diaphragm and also to serve as a sound filter which may be used to modify the frequency response. The shield may consist of two layers of wire gauze with an intermediate layer of cotton-wool. The diaphragm may be of conical form, and the back electrode may comprise a number of strips instead of pins.