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HIGH-FREQUENCY ELECTROSTATIC TRANSDUCERS FOR USE IN GASES
HARVARD UNIV CAMBRIDGE MASS ACOUSTICS RESEARCH LAB
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Electrostatic transducers have been developed which serve as sources and receivers of sound in gases and which are useful, at atmospheric pressure, as frequencies as high as one mcsec. When operated as microphones, these instruments have sensitivities of the order of -90 db re 1 vdynesq cm up to .5 mc and corresponding rise times of less than one microsecond for transient signals. A comprehensive treatment is presented of the theory and design of these high-frequency electrostatic transducers, and the results of an investigation of their behavior in air under normal laboratory conditions are included. The theoretical analysis assumes that the trapped air film has a uniform thickness of the order of one micron, and that the compliance of this air film and the mass per unit area of the diaphragm determine the resonance frequency of the transducer. Response characteristics at ultrasonic frequencies are found by measuring the voltage gain of transmitter-air-microphone systems with acoustic path lengths of about six in., and the transient behavior is verified with acoustic impulses from small sparks. Author
APPROVED FOR PUBLIC RELEASE