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Home  Electroacoustic Devices The Condenser Microphone |
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The Condenser MicrophoneThe condenser microphone consists essentially of a fixed plate and a thin, tightly stretched diaphragm, the two being insulated and separated by about 0.001 inch. The basic circuit is shown in Fig. 10. When sound waves strike the diaphragm it moves and the capacitance varies causing an electric current corresponding to the sound waves to flow through the high resistance. The resulting voltage drop is then amplified. The direct polarizing voltage is about 200 volts. The action of condenser transmitters and receivers was studied11 by Dolbear in 1881, These microphones became of practical importance because of the improvements made by Wente and the development of vacuum-tube amplifiers.13,14 It has been stated15 that modern acoustics began with the development, by Wente, of this microphone.
A condenser microphone is made small to minimize distortion of the sound waves; the diaphragm is about 1 inch in diameter. The capacitance is about 50 micromicrofarads. This requires (a) that very short leads must be used between the microphone and the amplifier and (b) that the input or grid resistor of the amplifier must be of high resistance. In a typical amplifier this is 100 megohms. Free-field response curves of the microphone and amplifier are shown in Fig. 11. Pressure calibration curves are also available.16 The output impedances of the amplifier commonly are 25 to 50 and 150 to 250 ohms.
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Last Update: 2011-06-06