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We study the reflection of an acoustic plane wave from a steadily slidingplanar interface with velocity strengthening friction or a shear band in aconfined granular medium. The corresponding acoustic impedance is utterlydifferent from that of the static interface. In particular, the system beingopen, the energy of an in-plane polarized wave is no longer conserved, the workof the external pulling force being partitioned between frictional dissipationand gain (of either sign) of coherent acoustic energy. Large values of thefriction coefficient favor energy gain, while velocity strengthening tends tosuppress it. An interface with infinite elastic contrast (one rigid medium) andV-independent (Coulomb) friction exhibits spontaneous acoustic emission, asalready shown by M. Nosonovsky and G.G. Adams (Int. J. Ing. Sci., {\bf 39},1257 (2001)). But this pathology is cured by any finite elastic contrast, or bya moderately large V-strengthening of friction.  We show that (i) positive gain should be observable for rough-on-flatmulticontact interfaces (ii) a sliding shear band in a granular medium shouldgive rise to sizeable reflection, which opens a promising possibility for thedetection of shear localization.Comment: 13 pages, 10 figure

Anomalous acoustic reflection on a sliding interface or a shear band