Plane Wave Cavities for Pressure Response of Condenser Microphone above 10 kc

Abstract

Low frequency calibration of condenser microphones by the reciprocity technique utilizes a reciprocity parameter, equal to the acoustic admittance of the cavity and valid with the conditions that the wavelength is small compared to the dimensions of the cavity and that the admittance of the microphone diaphragm is small compared to that of the cavity. The frequency range of calibration may be extended by (a) reducing the size of the cavity, (b) use of a gas with a higher sound velocity such as hydrogen, (c) proper dimensioning of the cavity. If the size of the cavity is reduced to extend the frequency range of calibration, the reciprocity parameter becomes dependent upon the unknown impedance of the microphone diaphragms. The frequency range may be extended somewhat by proper choice of cavity dimensions so that the effect of a radial mode cancels that of a longitudinal mode. A further extension of frequency range has been attempted using a cylindrical cavity having the radius of the microphone diaphragms. If plane waves are assumed and the diaphragm is effectively rigid with respect to the cavity, a reciprocity parameter can be calculated. Results up to 40 kc are compared with results obtained from theoretical calculations using electrical impedance data and with results obtained using an electrostatic actuator. Agreement within usual calibration error is obtained up to 20 kc.