The development of a microphone calibration technique using photon correlation spectroscopy

Laser Doppler Anemometry (LDA) measurements of acoustic particle velocity, utilizing the photon correlation method of signal processing, provide the basic for a working microphone calibration system. The calibration is performed using a standing wave generated inside a glass tube attached to a loudspeaker. From the photon correlation LDA velocity measurement and a knowledge of the characteristic acoustic impedance of the air inside the tube, the pressure amplitude at the end is calculated. The frequency response of a probe microphone fitted in the end of the tube is characterized separately, relative to a measurement microphone system supplied by the National Physical Laboratory (NPL). The LDA apparatus has been used to calibrate the measurement microphone system. For frequencies between 600 Hz and 2 kHz, the sensitivity of the microphone is determined to within ±0.2 dB of the sensitivity obtained by reciprocity calibration. While this is not state-of-the-art in terms of microphone calibration, it marks the first stage of an on-going project to establish the next generation of primary standards for sound pressure. Further work will be conducted with the emphasis on free-field measurements.