  We explore the behaviour of barotropic and irrotational fluids with a small
viscosity under the effect of first-order acoustic perturbations. We discuss,
following the extant literature, the difficulties in gleaning an acoustic
geometry in the presence of viscosity. In order to obviate various technical
encumbrances, when viscosity is present, for an extraction of a possible
acoustic geometry, we adopted a method of double perturbations, whereby
dynamical quantities such as the velocity field and potential undergo a
perturbation both in viscosity and in an external acoustic stimulus. The
resulting perturbation equations yield a solution which can be interpreted in
terms of a generalised acoustic geometry, over and above the one known for
inviscid fluids.
