Sound propagation in a constrained lattice of beads: High-frequency behavior and dispersion relation
Christophe Coste and Bruno Gilles

We report on acoustic wave propagation in an regular array of nominally identical beads under isotropic static stress. The weak polydispersity of the beads makes the contact lattice random. Time-frequency analysis of the acoustic signal is performed and allows measurement of the full lattice dispersion relation. Comparison with the theoretical prediction for a perfect triangular lattice gives an indication of the level of randomness in the contact lattice. The results extend in a consistent way a previous study restricted to long wavelength propagation \cite{gilles03}: The contact lattice is ordered by increasing the stress, and the smaller the wavelength, the higher the stress required to get regular lattice behavior. Measurements involving ballistic propagation of the coherent wave, whatever its frequency, evidence reversible lattice behavior under compression-decompression. Nevertheless, correlations of short wavelength incoherent waves are a sensitive probe of disorder, and allow us to exhibit a small irreversible evolution of the lattice. PACS~: 45.70.-n , 43.58.+z , 43.60.+d

© 2008 The American Physical Society.