The propagation of a steady wavefront in a medium with three levels of plastic deformation is considered: (a) dislocation, (b) meso-scale and (c) macro-scale. To take into account collective mechanisms of microplasticity, a relaxation term describing the momentum exchange between meso- and macro scales is incorporated into a dislocation-based constitutive law. This leads to a nonlinear second-order differential equation. Analytical and numerical analyses of the equation are performed. Using as example D16 aluminum alloy, we determine the parameters that provide a satisfactory correspondence between calculated and experimental profiles of particle velocity.