Ballistic wave in correlated disorder media

We present an experimental and theoretical study of the influence of disorder correlations on the behavior of the ballistic coherent wave traveling through a strongly scattering medium. We considered two types of samples: on the one hand, a 3-D granular suspension of randomly packed submillimeter glass beads and, on the other hand, 2-D assemblies of randomly distributed millimeter copper rods immersed in water. In the 3-D case, for broadband emission in the MHz range, the transmitted signal exhibits strong dispersion, leading to a signal composed of a low-frequency ballistic wave (not observed in previous analogous experiments) followed by a higher-frequency multiply scattered wave, known as the Coda Wave. With increasing thickness, the Coda tends to disappear due to absorption, so that only the ballistic wave remains. Experiments on 2-D samples provide information on its origin: we compared the ballistic transmission through a dilute sample (well described in the ISA approximation) and through a compact stack of rods. A low-frequency ballistic wave only appears in the latter case. A parallel numerical study indicates that this effect does not require contact between the scatterers but appears at high surface fractions, typically of the order of 50% for which disorder correlations must be considered.