Calibration and rating of microphones

Calibration of a microphone consists of measuring its response to some known characteristic of a sound field under specified conditions. Usually the open circuit voltage for a one-microbar sound pressure is determined. Calibrations in an anechoic chamber give plane wave response, while those in a reverberant room give the response to sound arriving from all directions-at random incidence. Techniques have been developed for measuring response with considerable efficiency. Calibrations can be made with pure tones or with wide-band signals such as noise or warbled tones. Complete calibration includes measurements of directivity and impedance as well as linearity of response. From the calibrations it is possible to calculate ratings which give quick and relatively fair comparisons between microphones. The RETMA Rating is particularly effective for the commercial types with impedance below 200,000 ohms. For small crystal and condenser microphones a statement of the noise threshold is more indicative of true performance capability. Although complete specification of microphone performance requires considerable information, the ratings dispel the main ambiguities in response figures arising from differences in impedance, circuits and test sound pressures.