This paper deals with the effect of phase variations along the paths of the
laser beams due to refractive index changes caused by the acousto-optic ef
fect, when an LDA system is used to measure sound waves. Theoretical and ex
perimental results will be discussed. A theoretical expression will be deve
loped for the movement of the fringes in terms of the angle of the sound wa
ve, the wavenumber and the distance of propagation of the laser beams. Resu
lts will show that as the wavenumber increases, the angle at which the maxi
mum amplitude of the fringe movement occurs tends away from pi/2 to the las
er beam bisector, and towards the angle of the laser beams. It will also be
shown that the phase modulations detected experimentally are in reasonable
agreement with those predicted theoretically. Crown Copyright (C) 1999 Pub
lished by Elsevier Science Ltd. All rights reserved.