LDA measurement of sound: amplitude modulation of laser Doppler signals

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.