  It has been conjectured by Bagnold [1] that an assembly of hard
non-deformable spheres could behave as a compressible medium when slowly
sheared, as the average density of such a system effectively depends on the
confining pressure. Here we use discrete element simulations to show the
existence of transverse and sagittal waves associated to this dynamical
compressibility. For this purpose, we study the resonance of these waves in a
linear Couette cell and compare the results with those predicted from a
continuum local constitutive relation.
