ACOUSTIC VELOCITY MEASUREMENTS IN THE AIR BY MEANS OF LASER DOPPLER VELOCIMETRY: DYNAMIC AND FREQUENCY RANGE LIMITATIONS AND SIGNAL PROCESSING IMPROVEMENTS

Although laser Doppler velocimetry (LDV) has been used for measurements in fluid mechanics and even high-level acoustics for a number of years, much uncertainty remains concerning its ability to measure the velocities encountered in the field of more usual linear acoustics. Specific aspects of the acoustic velocity have to be taken into account in order to achieve measurements, namely to deal with low levels or high frequencies, which requires some optimization of both the optics, the mechanical supports, and the signal processing. This paper presents a LDV set-up dedicated to acoustics in air, and presents the performances obtained using commercially available hardware. It also illustrates the main limitations, especially concerning the signal processing, and presents a new approach for the signal processing, optimized for a sine wave forced excitation of the acoustic field.