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We report the simultaneous softening and compaction of a confined densegranular pack in acoustic resonance experiments. Elastic softening ismanifested by a reduction of the shear-wave speed, as the wave amplitudeincreases beyond some threshold. No macroscopic rearrangement of grains ordilatancy is observed; instead, elastic softening is accompanied by a tinyamount of compaction on the scale of grain asperities. We explain theseapparent contradictory observations using a theoretical model, based on sheartransformation zones (STZs), of soft spots and slipping contacts. It predicts alinear shear stress-strain response with negligible macro-plastic deformationdue to the small-amplitude acoustic oscillation. However, these waves reducethe interparticle friction and contact stiffness through the acousticlubrication of grain contacts, resulting in an increase in the structuraldisorder or compactivity and softening of dynamic modulus. The compactionassociated with this microscopic friction decrease is consistent with theprediction by an Ising-like correlation between STZs in the subyield regime.

Shear-wave-induced softening and simultaneous compaction in dense granular media through acoustic lubrication at flow heterogeneities