Sensing Sound Pressure in an Anechoic Chamber Using Backscattered Laser Light

Current standards for the measurement of the SI derived unit of sound-in-air pressure, the pascal, are based upon microphone reciprocity calibration and are achieved indirectly through microphone sensitivity. These methods require microphones of specific geometry and performance characteristics, effectively artefacts, and are traceable through standards for electrical and dimensional units. Measurement of acousto-optic interactions can provide a direct approach to measuring sound pressure. One acousto-optic interaction is the periodic scattering of photons caused by particles moving in a sinusoidal manner due to propagating sound across interference fringes formed at the intersection of two coherent laser beams. The sequence of these scattered photons, which is collected using telescopic optics and generated by a single photon counting device, can be autocorrelated to yield the periodicity of the photon events. Through mathematical analysis of the autocorrelation function it has been shown that acoustic particle velocity is inversely proportional to the time of the first minimum. This has effectively been shown for measurements in acoustic standing wave tubes and has been developed into a method which can be applied in an anechoic chamber. This paper describes the design and implementation of such a system which allows for a comparison of sound pressure measurements using optical and microphone based techniques.