Determination of microphone acoustic center from sound field projection measured by optical interferometry

This article presents a method for determining the acoustic center of a microphone from a sound field measured by optical interferometry. The acoustic center defines the equivalent point source position of a microphone serving as a sound source where the spherical waveform starts to diverge. The value is used to determine the effective distance between microphones for free-field reciprocity calibration. Conventionally, it is determined from the inverse distance law properties of a point source using the transfer function method. In this study, the acoustic center was determined from the projection of the sound field of the microphone. Parallel phase-shifting interferometry was used to measure the line integration of the sound pressure from a microphone. The acoustic center is determined as the position where the squared error between the measured data and the projection model of a point source is minimized. Experiments with the B&K 4180 (Brüel & Kjær, Nærum, Denmark) microphone were performed for frequencies from 10 to 50 kHz. The best acoustic center estimation was obtained at a microphone distance of 0 mm, with a difference of 0.17 mm to the IEC 61094-3 value and 0.36 mm to the Barrera-Figueroa et al. [J. Acoust. Soc. Am. 120(5), 2668-2675 (2006)] result at a measurement frequency of 20 kHz.