Phonons in porous media at intermediate frequencies

Using ultrasonic techniques, the phonon diffusion coefficient and density of states have been measured at intermediate frequencies in a porous solid network of sintered glass beads. A transition to a very strong scattering regime is observed, whereupon the diffusion coefficient becomes independent of frequency. Near crossover, the density of states is enhanced and exhibits a broad maximum. These data are in good overall agreement with a lattice-based percolation model, and give additional insight into the effects of strong scattering on phonon transport in disorderd media.