Acoustic diffusion wave in turbid media

The acoustic diffusion wave in a turbid medium including randomly dispersed wave scatterers was observed. Ultrasonic wave at 15 MHz was modulated in amplitude and emitted into water suspension of polystyrene beads with 0.2 mm in diameter. Ray trace of the wave takes a random walk in the multiple scattering, and the acoustic energy flows in a diffusional manner. The spatial distribution of the oscillating acoustic energy was observed with an optical probe based on the Raman-Nath diffraction. The wavenumber and the spatial damping of the diffusion waves were determined at the modulation frequency of 4-20 kHz range. The results were well described by a modified diffusion theory including the effect of ultrasonic absorption. The diffusion constant and the lifetime of the acoustic energy were determined, which led to the transport mean free path and the ultrasonic absorption. The experiment was made at different volume fractions of the suspension, and l* and the lifetime were found to decrease with the number density of the scatterer.