Laser-Doppler-anemometry measurements of instantaneous acoustic particle ve
locity are presented. The Doppler signals, from measurements in air, displa
y quasi-periodic amplitude modulation with a fundamental frequency equal to
the frequency of the acoustic field. This effect can cause signal-processi
ng problems. Periodic amplitude modulation is investigated by studying the
power spectral densities of the envelopes of measured Doppler signals. Vari
ous seeding and acoustic conditions are considered. It is shown that the pe
riodic amplitude modulation is much more significant with water-droplet see
ding than it is with smoke-particle seeding. Random amplitude modulation re
places periodic amplitude modulation when a significant steady flow is supe
rimposed on the acoustic field. The harmonic content of the Doppler-signal
envelope increases with the intensity of the acoustic field. A simple compu
tational model is used to simulate Doppler-signal envelopes. The simulation
is in good qualitative agreement with many experimental observations. Howe
ver, there is is some discrepancy with experimental measurements, notably a
90 degrees phase difference between the positions of measured and simulate
d envelope maxima. The paper briefly considers the possibility of exploitin
g the periodic amplitude modulation effect in a new type of anemometer for
acoustic velocity measurement in sound fields with low acoustic frequency a
nd high acoustic particle velocity amplitude.