Laser detection of sound

A differential laser Doppler velocimeter (LDV) has been assembled and tested to provide noninvasive absolute measurements of acoustic particle displacements of standing waves generated in a water-filled tube. The principle of the technique [see K. J. Taylor, J. Acoust. Soc. Am. 59, 691–694 (1976)] is to measure the Doppler shift of laser light scattered from colloidal microparticles oscillating under the action of an acoustic field. The system tested is capable of detecting particle displacements of the order of a few nanometers with a bandwidth of several kilohertz. The performances and limitations of the system are discussed. In particular, the effect of Brownian motion is shown to produce only negligible broadening of the spectral density of the signal of interest. The sensitivity of the present LDV system is estimated to be very close to the shot noise limit of the photomultiplier tube used to detect the Doppler shift of the scattered light. Experimental results are obtained under controlled laboratory situations with calibrated microparticles of radii 10, 1, 0.1, and 0.01 μm in suspension in very pure (reverse osmosis) water.