Free-field reciprocity calibration of microphones

Abstract

Standardized methods for the primary free-field calibration of laboratory standard microphones deal with Type L (ANSI S1.10-1967, R1977) “one-inch” diameter microphones. However, the use of “1/2-inch” diameter microphones for measurement of the sound pressure level in acoustic fields is increasing. Consequently, the NBS has developed a fixed-cost measurement service for the free-field calibration of these microphones by the reciprocity method over the range 2.5 kHz to 20 kHz. For this service, the apparatus and procedures, including essential properties of the anechoic chamber in which the calibrations are performed, are described. Opportunities for improvements are noted. The frequency-dependent positions of the apparent acoustic centers of the microphones were obtained. The overall uncertainty estimate for free-field calibration, expressed as the sum of the magnitude of credible bounds on the systematic component (s) and the random component (2σ, where σ is the standard deviation) is 0.16 dB or better (s=0.06 dB, 2σ=0.10 dB) at frequencies 1.25 kHz ⩽f⩽ 5kHz, and 0.07 dB or better (s=0.02 dB, 2σ=0.05 dB) for 5 kHz ⩽f⩽ 20 kHz. Comparison for given microphones of the measured difference between free-field and pressure response levels with the difference calculated by diffraction theory (derived by Matsui) indicates agreement of 0.16 dB or better in the low-frequency range (1.25 kHz to about 4 kHz) where free-field reciprocity measurements encounter the greatest experimental difficulties. This agreement is consistent with the estimated uncertainties of free-field and pressure calibration by the reciprocity method.