GB1003835A - Improvements in or relating to condenser microphones - Google Patents

Abstract

1,003,835. Electrostatic microphones. STANDARD TELEPHONES & CABLES Ltd. July 27, 1962, No. 28977/62. Heading H4J. In a capacitor microphone arrangement comprising two transducers responsive respectively to the pressure gradient and the pressure of an incident sound wave, the electrical outputs of the microphones are combined in such a way that the directionality of the arrangement varies with the frequency of the received sound. The pressure gradient transducer has a cardioid shaped directional pattern above a frequency of 50 c.p.s. and its response falls-off fairly rapidly below this frequency. The pressure transducer has an omni-directional pattern below 50 c.p.s. and its response falls off above 50 c.p.s. The combined response of the microphones is thus substantially level over a wide frequency range but the directional pattern varies over this range. The " cut-off " frequencies of the microphones may be obtained by varying their electrical or their acoustical characteristics. Thus in the former case they may be connected in parallel to a conventional amplifier (Figs. 1 and 2, not shown) and by adjusting the values of resistors and capacitors in parallel and series therewith the appropriate cut-off frequencies for each can be obtained. Alternatively the stiffness of the diaphragm of the pressure gradient type can be increased, thus raising the cut-off frequency and the reactance of the diaphragm and the cavity between the diaphragm and a fixed electrode of the pressure type together with the resistance of an acoustic resistance e.g. polyvinylchloride placed in the cavity can be varied to lower the cut-off frequency of the latter type. In the alternative case, the outputs are combined by placing the capacitor microphones in adjacent arms of a high-frequency (¢-10 Mc/s.) bridge circuit (Fig. 3, not shown). The polarizing potentials for the electrodes may be derived from the output of a transistorized high-frequency oscillator feeding the bridge circuit. A pressure gradient microphone G is mounted in the open end of a tube 6 and has its diaphragm exposed to the sound field on both sides by means of holes 8 provided in tube 6. The diaphragm 1 of a pressure microphone P is exposed to the sound field only via holes 8 although a small aperture may be provided in sealing member 5 whereby the sensitivity is decreased at very low frequencies. In Fig. 6 (not shown) an alternative form of tube for holding the microphones is depicted.