EXPERIMENTAL TEST OF THE DIFFUSION-APPROXIMATION FOR MULTIPLY SCATTERED SOUND

We have critically tested the application of the diffusion approximati on to describe the propagation of ultrasonic waves through a random, s trongly scattering medium. The transmission of short ultrasonic pulses has been measured through a concentrated suspension of glass beads im mersed in water. The transmitted sound field is found to exhibit tempo ral fluctuations with a period determined by the width of the incident pulse. Provided that appropriate boundary conditions are used to acco unt for the reflectivity of the interfaces, the time dependence of the ensemble-averaged transmitted intensity is shown to be well described by the diffusion equation. This enables us to determine both the diff usion coefficient for the sound waves as well as the inelastic absorpt ion rate. The consistency of these results is established by varying t he experimental geometry; while the transmitted pulse shape changes ma rkedly, the values for the diffusion coefficient and absorption rate o btained through a description using the diffusion approximation remain unchanged. We have also measured the absolute transmitted intensity o f the sound as the sample thickness is varied; this provides an accura te measure of the transport mean free path and thus also the energy tr ansport velocity. These results convincingly demonstrate the validity of using the diffusion approximation to describe the propagation of so und waves through strongly scattering media.