The effects of an acoustic standing wave on the laser beams of a laser-Dopp
ler-anemometry set-up have been investigated. The resulting changes in refr
active index give rise to oscillating fringes rather than stationary ones a
s is generally assumed. The theory has been developed to produce the form o
f the fringe movement due to this acousto-optical effect. A theoretical exp
ression for the ratio of the apparent motion of the particles in the fringe
s to the actual motion has been derived and compared with experimental resu
lts for the case of an underwater laser-Doppler-anemometry system. It has b
een shown that, over long propagation distances of the laser beams or for h
igh sound frequencies, the acousto-optical effect is dominant over any part
icle movement due to the acoustic wave. This means that the apparent motion
of the particles seen by the detector may have a larger amplitude than exp
ected and, although it will be easier to detect, the analysed signal need n
ot accurately represent the region of the flow of interest.