Towards a future primary method for microphone calibration: Optical measurement of acoustic velocity in low seeding conditions

Laser Doppler anemometry (LDA) is conventionally used for flow velocity measurements of particles in liquids and gases. The application of LDA to acoustic fields provides the potential for non-intrusive and direct measurement of acoustic velocity. This paper describes a dual-beam LDA system developed for the measurement of acoustic particle velocity in minimal photon scatter/seeding environments using a photon-correlation method and low optical power. The system has been used to measure the acoustic pressure in a standing-wave-tube with a 0.2 dB agreement with that of a standard 12-in microphone at low frequencies. The optical method offers the potential for the direct calibration of microphones against acoustic pressure, rather than an indirect method such as reciprocity that is the current internationally accepted method.