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Evolution of granular packings by nonlinear acoustic waves

We investigate the nonlinear response of pulsed high-amplitude sound transmission in weakly compressed granular materials, simultaneously probing sound amplitude, time-of-flight velocity and harmonic generation. We observe that weakly compressed packings can both exhibit weakening and strengthening when driven by high-amplitude sound, and that weakening/strengthening of the sound velocity and transmission amplitude go hand in hand. We find that strengthening is associated with the generation of second harmonics, whereas for weakening, no appreciable second harmonics are generated. All these findings point to changes in the contact network; effective medium theory can describe these effects qualitatively, but fails to account for them quantitatively.