Abstract
Currently standards for the measurement of the S.I. derived unit of sound pressure, the pascal, are based upon microphone reciprocity. These methods require microphones of specific geometry and performance characteristics, effectively artefacts, and are traceable through standards for electrical units. Measurement of acousto-optic interactions can provide an alternative approach to measuring sound pressure. One acousto-optic interaction is the periodic scattering of photons caused by particles moving with sinusoidal acoustic particle velocity across an optical ruler formed at the intersection of two coherent laser beams. The sequence of these scattered photons, which is collected using telescopic optics and a single photon counting device, can be auto-correlated to give the periodicity of the photon events. Through mathematical analysis of the auto-correlation function it has been shown that acoustic particle velocity is inversely proportional to the time shift of the first minima. This has effectively been shown for measurements in acoustic standing wave tubes and has been developed in to a method which can be applied in an acoustic free field chamber. This paper describes the design and implementation of such a system which allows for a comparison between optical measurements and those of a microphone.
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