Impact of tracer particle size on optical measurement of sound fields

This note deals with the effects of tracer particle size in laser Doppler measurement of the acoustic velocity in audio-frequency range sound fields in air. We use a computer model to study the role of particle size and find that both large and small particles can lead to systematic errors in the measured velocity, the former due to slippage and the latter due to diffusion. We show that for typical seeding materials and acoustic intensities the particle diameter needs to lie between 0.5 and 0.8 μm if the acoustic velocity is to be measured over the full frequency range with an uncertainty of less than 1%. We then report measured size distributions for two common seeding materials and demonstrate experimentally the need to weight the particle distribution by the optical scattering efficiency. We show that it is possible through light scattering to monitor the particle size distribution simultaneously with the acoustic measurement.