Metal Water: A Metamaterial for Acoustic Cloaking

A generic metamaterial is described that is suitable for making acoustic cloaking devices. The fundamental property is that it mimics the acoustic properties of water, yet can be modified to display anisotropic elastic properties suitable for cloaking. It has the important property that the amount of void space is conserved: a conservation of cloaked space. Acoustic cloaking can, in theory, be achieved using widely different material properties. The reason is that for a given mapping function the transformed version of the original scalar wave equation can be interpreted in terms of different material constitutive theories 1 . In this sense acoustic cloaking is distinct from its counterpart for electromagnetic waves, for which the material corresponding to the transformed Maxwell equations is uniquely defined by the transformation function. The first mechan- ism proposed for acoustic cloaking 2,3 was based on the concept of anisotropic density, and a single bulk modulus. Compressible fluids with anisotropic density are physically permissible 4 , by layering homogeneous fluids, for instance. The range of inertial properties required for achieving cloaking is great, however, and could be very difficult to achieve by layering. Significant cloaking of a cylinder was shown to be possible 5 using hundreds of homogeneous fluids to reproduce the smoothly varying properties of the transformed fluids. The number of independent fluids can be reduced to three 6 but