Abstract
A method has been developed for measuring the acoustical impedance of a microphone's diaphragm at various frequencies. The impedance is derived from a series of voltage ratios and the known acoustical impedance of the coupler used for the measurements. In place of the usual reciprocity procedure of matching open-circuit microphone voltages, diaphragm motion is matched with the carrier-frequency detection system described elsewhere in this program. Combination of the acoustical impedance with other known parameters of the microphone permits accurate sound pressure measurements with carrier-frequency microphones. Calibrations of condenser microphones have been extended to very low frequencies, near 1 c/sec, by taking account of the cooling effects of the walls on the acoustical impedance of conventional cylindrical couplers. Analytical expressions for the temperature distribution in cyindrical enclosures have been obtained, and computations of the effect on the impedance are given in the form of plotted correction factors. The results of calibrations of condenser microphones at very low frequencies will be presented.
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