Observation of Acoustic Diffusion Wave

Abstract

The acoustic diffusion wave in a turbid medium containing randomly dispersed wave scatterers was observed for the first time. Ultrasonic wave at 15 MHz was modulated in amplitude and emitted into an aqueous suspension of polystyrene beads of 0.2 mm diameter. The 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 wave number and the spatial damping of the diffusion waves were determined at the modulation frequency range of 4–20 kHz. The results were well described by a modified diffusion theory that included the effect of ultrasonic absorption. The diffusion constant D and the lifetime of the acoustic energy were determined, which led to the transport mean free path l* as D=l*ν/3, with ν being the ultrasonic velocity. The experiment was conducted at different volume fractions of the suspension, and l* and the lifetime were found to decrease with the number density of the scatterers. These results were discussed in correlation with a theoretical prediction of the geometrical mean free path and other factors. This study is to show the potential of using acoustic diffusion waves in the characterization of materials with strong ultrasonic attenuation due to scattering.