Sound propagation in dense granular materials

Two distinct types of small-amplitude sound propagation have been identified in a confined granular medium composed of non cohesive particles: one corresponds to coherent waves, the other to the waves scattered by the inhomogeneous force networks within the granular systems. Over long distances of propagation, the multiply scattered elastic waves exhibit a diffusive character of transport after configurational averaging. We studied elastic properties of dry and wet granular materials by measuring coherent elastic wave velocities as function of externally applied load and wetting liquid content. Experimental data associated with coherent waves agree quite well with the effective medium theory based on the Hertz-Mindlin contact mechanics. Also we have investigated the compaction of the granular medium induced by cyclic loading-unloading. The irreversible rearrangement of grains was measured by using a correlation technique of the multiple ultrasonic scattering which is very sensitive to the configuration change of granular media.