Absolute calibration of microphones by a laser‐Doppler technique
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A new technique for the absolute calibration of microphones is described. This is based on the measurement of acoustic particle velocity in a traveling‐wave tube by means of a laser‐Doppler system. From this measurement and a knowledge of the specific acoustic impedance of the air in the tube, the sound pressure is determined at a point in the tube where a microphone is mounted and, by measuring the output voltage of the microphone, its sensitivity is found. At 500 Hz, the method is accurate to within ±0.03 dB. No significant discrepancy was found between a calibration by this method and one by a precise reciprocity technique, although there is some residual uncertainty arising from the fact that one calibration is made in a small coupler and the other in an essentially free field. The result is seen as validating the assumption of reciprocity on which most precise microphone calibrations rely.