GB1188679A - Photoelectric Microphone - Google Patents

Abstract

1,188,679. Photo-electric microphone. CESKOSLOVENSKA AKADEMIE VED. 22 Nov., 1967 [22 Nov., 1966], No. 53092/67. Heading H4J. [Also in Division G1] A photo-electric microphone comprises two mirrors, at least one of which is concave, and a light source and a photo-electric detector mounted between them, at least one of the mirrors acting as a diaphragm responsive to acoustic vibrations. In one arrangement, Fig. 1 (not shown), one mirror is plane and the other concave, the light source and detector being located side by side at the focal point of the latter. The plane mirror is inclined at a slight angle with respect to the normal to the axis of the concave mirror, which is attached at its circumference through corrugations to a support, so that it can act as a diaphragm. In the arrangement of Fig. 2, the light source 8 is at the focal point of one concave mirror 6 and the detector 9 at the focal point of the other concave mirror 7. The mirrors hermetically seal the ends of a cavity 10, the pressure inside which can be varied by piston 12 to change the sensitivity of the microphone. Perforated end caps protect the mirrors 6, 7 which are of thin resilient foil and vibrate in response to acoustic vibrations. The cavity 10 may be divided by a transparent window 18. To provide a directional microphone one end cap 6 may be imperforated. The source 8 may be an incandescent lamp or electroluminescent device, e.g. a light excessive semi-conductor diode of gallium arsenide or phosphide, which may be pulse operated. The detector may be a photo-resistor, a germanium or silicon photo-diode or photo-transistor, or a gallium arsenide photo-diode. Alternatively, the detector may be formed by a number of photo-transistors, e.g. a Darlington pair. The photo-detector may be a variable capacitance diode, when it forms part of an oscillator circuit, the amplitude or frequency of whose output varies as a result of the mirror vibrations. Tuned amplifier circuitry may be provided to reduce noise when amplifying the microphone output.