GB594646A - Improvements in or relating to directional microphones - Google Patents

Abstract

594,646. Circuits for microphones. WESTERN ELECTRIC CO., Inc. June 9, 1944, No. 11116. Convention date, June 24, 1943. [Class 40 (iv)] A directional microphone comprising a diaphragm with both of its surfaces in communication with the atmosphere, one substantially directly and the other through acoustic phaseshifting means so that the resultant acoustic force exerted on the diaphragm due to the incident sound waves is dependent on the angle of incidence of the sound waves, is provided with means for applying to the diaphragm a controlling force which is derived from the microphone output and which is varied with frequency so as to equalize the response of the microphone at different frequencies. As shown in Fig. 1, the diaphragm 14 of a condenser microphone is clamped between a ring 15 and a shoulder 13 on a casing 10 closed at the rear by a plate 11 and forming a chamber with access to the atmosphere through peripherally spaced holes 12 covered by acoustic resistance material 41, such as silk. The perforated metallic electrodes 16, 17 of the microphone are spaced from the diaphragm 14 by washers 20 and are mounted within rings 18 by means of insulating supporting rings 19, preferably of polystyrene, the electrodes being clamped against the washers 20 by externally threaded rings 21. In the circuit arrangement shown in Fig. 3, the output from the electrode 16 of the microphone and an associated amplifier 43 is partly fed back to a circuit 44 comprising a capacitance 45 and a shunt resistance 46 connected to the second electrode 17 of the microphone. In a microphone with a cardioid response, the arrangement can be such that the total phase shift due to the acoustic network and to the difference in length of the sound-wave path to the opposite faces of the diaphragm is proportional to frequency with the result that the driving force applied to the diaphragm decreases as the frequency increases. The response curve for such a microphone in a direction normal to the diaphragm and without feedback is somewhat as indicated in curve A of Fig. 4, the peak occurring at the resonant frequency of the diaphragm. It is analytically shown that a flat response, as indicated by curve B of Fig. 4, is obtained if the feedback network 44 has a transmission characteristic increasing with frequency to compensate for the decrease of acoustic driving force as the frequency increases. The curve C indicates the equalized response for sound incident at an angle to the normal axis. The diaphragm 14 provides a shield between the electrostatic fields on its opposite sides so that the output to the amplifier is modified solely in accordance with the mechanical controlling influence exerted on the diaphragm by the feedback voltage. As the feedback circuit can be designed to provide a level response, an improved signal-to-noise ratio can be obtained by suitable modification of the microphone characteristics to this end, for example in the choice of dimensions and material of the diaphragm.