Homogeneous and compact acoustic ground cloaks

We present the design, architecture, and detailed performance predictions for a class of ground-plane acoustic cloaking shells. The design begins with a coordinate transformation which, in contrast to a quasiconformal design, yields a homogeneous but anisotropic material and a shell size that is comparable to the size of the object to be hidden. We apply the general approach to the design of a broadband acoustic cloak in water, in which the desired material parameters are realized through acoustic metamaterials composed of blocks of steel, aluminum foam, and silicon carbide foam. Since metallic and ceramic foams are prone to sound absorption, we discuss the effects of loss inside the two types of foam. An important part of this design consists in reducing the shear wave effects inside the solids by isolating these solids from each other through narrow channels of background fluid. Numerical simulations of the entire device, as composed of the discrete and basic material building blocks, demonstrates good performance and shows that such a device can be physically realized through the assembly of available materials in a relatively simple form.