GB498435A - Electrostatic microphone system - Google Patents

Abstract

498,435. Microphone systems. REISZ, E. Feb. 14, 1938, No. 4544. [Class 40 (iv)] In an electrostatic microphone system in which condenser microphones of different sizes and having different frequency-response characteristics deliver their audio-frequency outputs in parallel to an output impedance coupled to the input circuit of an amplifier, an ohmic resistance in series with the microphone having the lower frequency response characteristics serves to reduce the amplitude of response at resonance frequency substantially to the level of the amplitude of response to other frequencies. The condenser microphones G1, M1, a and G2, M2, a, responsive to low and high frequencies respectively, are connected in parallel to the primary winding p of a transformer having a blocking condenser C1 to prevent the discharge of the microphones. The circuit including the microphone capacities and the inductance of the primary winding p has a resonant frequency less than 50 cycles per sec. The secondary winding s has a number of turns corresponding to the characteristic of the line L and is connected at the point b to a condenser C2 and a battery B from which biassing potential is applied to the microphones through a resistance R. The resistance r included in the circuit of the microphone G1, M], a, reduces the amplitude of response at the resonance frequency, and up to frequencies at which r can be neglected current variations are supplied from the low-frequency microphone. At higher frequencies with the increased effect of the resistance r, current changes are mainly provided by the high-frequency microphone G2, M2, a. The battery B is connected to the centre point of the winding of the input transformer A, whilst the sheathing E. of the cable and the casing G of the microphone are earthed. The microphones G1, M1 and G2, M2 together with the transformer T are combined to form a unitary structure with the casing G.