  Experimental results and their interpretations are presented on the nonlinear
acoustic effects of multiple scattered elastic waves in unconsolidated granular
media. Short wave packets with a central frequency higher than the so-called
cut-off frequency of the medium are emitted at one side of the statically
stressed slab of glass beads and received at the other side after multiple
scattering and nonlinear interactions. Typical signals are strongly distorted
compared to their initially radiated shape both due to nonlinearity and
scattering. It is shown that acoustic waves with a deformation amplitude much
lower than the mean static deformation of the contacts in the medium can modify
the elastic properties of the medium, especially for the weak contact skeleton
part. This addresses the problem of reproducibility of granular structures
during and after acoustic excitation, which is necessary to understand in the
non destructive testing of the elastic properties of granular media by acoustic
methods. Coda signal analysis is shown to be a powerful time-resolved tool to
monitor slight modifications in the elastic response of an unconsolidated
granular structure.
