  Fundamental features of rotationally symmetric acoustic cloaks with
anisotropic inertia are derived. Two universal relations are found to connect
the radial and transverse phase speeds and the bulk modulus in the cloak.
Perfect cloaking occurs only if the radial component of the density becomes
infinite at the cloak inner boundary, requiring an infinitely massive cloak. A
practical cloak of finite mass is defined in terms of its effective visible
radius, which vanishes for perfect cloaking. Significant cloaking is obtained
when the effective visible radius is subwavelength, reducing the total
scattering cross section, and may be achieved even as the interior radius of
the cloak is large relative to the wavelength. Both 2D vs. 3D effects are
compared as we illustrate how the spatial dependence of the cloaking parameters
effect the total cross section.
