Possibility of improved laser‐Doppler detection of sound by tracking moving phase fronts

Abstract

Laser‐Doppler anemometry applied to acoustics presents special problems because of the small velocities of fluid flow associated with sound and because of the relatively rapid oscillations. The analysis of Hanish, attributed to General Electric Company (1967) and demonstrated by K. J. Taylor in 1976 and 1981, shows such problems can be overcome in major part for single frequency sound using the spectral density of the photomultiplier tube output. Comparable success is unlikely for broadband sound; an “uncertainty principle” requires averaging over time intervals comparable to the reciprocal of the Doppler shift in frequency; no black box electronics can beat this principle. Difficulties appear insurmountable if characteristic acoustic frequencies are much higher than the Doppler shift. A possible way of overcoming this is to take advantage of the constancy of the sound speed and of the omnidirectionality of incoming sound. Optical systems can be designed which move the effective optical scattering volume along nearly rectilinear paths at the sound speed such that the laser tracks a moving phase front. The fluid velocity sensed by the laser thus appears to be constant rather than oscillating, so one has the opportunity to examine meaningful averages over sufficiently long time periods. [Work supported by ONR.]