When laser Doppler anemometry (LDA) is used to measure sound fields in wate
r it is important to take into account the refractive index variations in t
he water due to the sound wave. These have the effect of creating a phase d
ifference between the two laser beams in the LDA setup so that when they in
tersect they create moving fringes. In some situations this acousto-optic e
ffect can dominate over the movement of the particles due to the sound wave
, thereby influencing the Doppler signal. This article determines in which
situations the acousto-optic effect can be ignored and in which situations
it has a dominant effect. Theoretical expressions are derived for the magni
tude of the acousto-optic effect on the LDA signal in terms of the distance
of the laser beam propagation and acoustic wave number. The results show t
hat varying the value of the wave number, the distance the laser beams have
traveled in the direction perpendicular to the optical axis, or the angle
of the sound wave, has an effect on the amplitude, gamma(amp), of the fring
e movement. For low wave number values, an angle of 0, +/- pi from the opti
c axis (gamma direction) corresponds to the situation in which the laser be
ams have been affected to the same degree by refractive index changes in th
e water and thus the path difference is zero and the value of gamma(amp) is
a minimum. A maximum value of gamma(amp) is obtained for an angle of +/- p
i/2 from the gamma direction for low wave number values. It is shown that t
he ratio of the acousto-optic effect to the amplitude of the particle movem
ent due to the sound wave varies as the square of both the acoustic wave nu
mber and the distance of the laser beam propagation for low wave number val
ues. An equation for the apparent motion of the particles in the fringes is
determined and is used to show that the acousto-optic effect dominates as
the value of the wave number and the distance of propagation of the beams i
ncreases. Also, for very low wave numbers and short distances, the acousto-
optic effect is negligible. This is because at low wave numbers, correspond
ing to low sound frequencies, the refractive index gradients, and hence the
phase changes along the lengths of the laser beams, are small. (C) 1998 Am
erican Institute of Physics. [S0034-6748(98)03112-8].