Luxemburg-Gorky effect in a granular medium: Probing perturbations of the material state via cross-modulation of elastic waves
A nonlinear effect consisting of transfer of modulation from one amplitude-modulated elastic wave, the "pump" wave, to the second initially monochromatic probe wave has been recently introduced in nonlinear acoustics by analogy with radio waves. For the first time, this effect is applied to probe perturbations of the state of a granular material induced by shocks, "seismic events". The experiments indicate a much stronger variability of the nonlinearity-induced modulation sidelobes in comparison with changes in the components at the fundamental frequencies of the probe and pump waves. Another new feature revealed in the experiments is that the transitional shock-induced variations in the amplitudes of the modulation sidelobes are several times stronger than the relaxed, residual values of the changes. The effects observed suggest interesting possibilities to application in active acoustic/seismic monitoring schemes.