  Hadamard-Papapetrou method of field discontinuities is here employed in order
to determine the effective metric that describes the propagation of acoustic
perturbations in isentropic fluids. It is shown that, when dissipative effects
are present, small perturbations in fastly moving fluids have a natural
description in terms of an effective acoustic geometry. As an application of
our results, a model for an acoustic black hole in viscous fluid is
investigated.
