GB915811A - Improvements in or relating to microphones - Google Patents

Abstract

915,811. Microphones. STANDARD TELEPHONES & CABLES Ltd. Sept. 11, 1959, No. 31044/59. Class 40 (4). A pressure gradient directional microphone has the rear portion of its diaphragm D connected by tubes L1 and L4 to a cavity C3 having access to the external air via shallow holes R3 and a tube L8 and to a closed chamber C2 via a further tube L2. Tubes L4 are held in gaskets R4 of acoustic resistance material such as porous metal. The tube L2 contains acoustic resistance material such as lambs' wool. The shallow holes R3 are also covered with acoustic resistance material such as silk. For producing the phase change necessary to produce a cardiod polar diagram the rear of the diaphragm is fed, in the case of signals below 300 c./sec. via a low-pass system comprising the tube L8; and in the case of signals above 300 c./sec. via the resistance capacitance system R3, C3. The directional effect is arranged to fall off between 3000 c./sec. and 6000 c./sec. after which the " obstacle " effect is relied upon to produce the directional characteristics. The L2, C2 system increases the low frequency response. The high frequency response of the microphone may be reduced by the provision of an annular ring attached to the diaphragm. The response may be boosted in therange 3000-4000 c./sec. by the provision of a tube in a position similar to that of L4, said tube being closed at the end remote from the diaphragm and containing acoustic resistance material at its other end. The chamber C3 may contain thin spiralled layers of acoustic resistance material.