  The present work addresses the analogy between the speed of sound of a
viscous, barotropic, and irrotational fluid and the equation of motion for a
non--massive field in a curved manifold. It will be shown that the presence of
viscosity implies the introduction, into the equation of motion of the
gravitational analogue, of a source term which entails the flow of energy from
the non--massive field to the curvature of the spacetime manifold. The
stress-energy tensor is also computed and it is found not to be constant, which
is consistent with such energy interchange.
