Abstract
Probing of a granular medium by a parametric nonlinear emitting antenna is reported. Shear waves are used for this purpose for the first time. Strong anisotropy of nonlinearity for shear waves with different polarizations is observed. Excitation of longitudinal acoustic wave at the second harmonic by sinusoidal shear acoustic wave and subharmonics generation caused by propagation of the acoustic wave have been observed. The ensemble of the experiments confirms that the nonlinear interactions of acoustic waves in granular assemblages are highly sensitive to the fraction of weakly loaded (and weakly “unloaded”) contacts. Evidence is given that a significant portion of weak contact forces is localized below 0.01 of the mean force — a range previously inaccessible by any other techniques.
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