Sound wave acceleration in granular materials

The goal of this study is a better understanding of the numerous sound propagation mechanisms in granular materials. In a static, regular (crystal), three-dimensional packing, a small perturbation is created on one side and examined during its propagation through frictionless and frictional packings. The perturbation can be applied in longitudinal and shear directions in order to excite different modes of information propagation, including rotational modes. The wave speed and dispersion relation derived from simulation data are compared to those given by a theoretical approach based on a micro-macro transition. The detailed analysis of the wave velocity reveals an interesting acceleration close to the source. Finally a step towards real packings is made by introducing either friction or a tiny (but decisive) polydispersity in the particle size.