Acoustic cloaking in 2D and 3D using finite mass
  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. 
Andrew N. Norris