  Acoustic metafluids are defined as the class of fluids that allow one domain
of fluid to acoustically mimic another, as exemplified by acoustic cloaks. It
is shown that the most general class of acoustic metafluids are materials with
anisotropic inertia and the elastic properties of what are known as pentamode
materials. The derivation uses the notion of finite deformation to define the
transformation of one region to another. The main result is found by
considering energy density in the original and transformed regions. Properties
of acoustic metafluids are discussed, and general conditions are found which
ensure that the mapped fluid has isotropic inertia, which potentially opens up
the possibility of achieving broadband cloaking.
