The development of a microphone calibration technique using laser doppler anemometry

The details of a working system for the calibration of microphones based on the direct measurement of acoustic particle velocity using Laser Doppler Anemometry (LDA) are presented and discussed. The calibration is carried out using a standing wave generated inside a glass tube attached to a loudspeaker. The probe microphone used to monitor pressure amplitude is fitted in the end of the tube. The pressure amplitude at the end is calculated from velocity measurements using the LDA system and the characteristic acoustic impedance of the air inside the tube. The frequency response of the probe is characterized separately. A measurement microphone system supplied by the National Physical Laboratory (NPL) has been calibrated using this apparatus. The sensitivity of the microphone is determined to within ±0.1 dB of the sensitivity obtained by reciprocity calibration for frequencies between 600 Hz and 2 kHz.