Metal based acoustic metamaterials

Abstract

Two apparently distinct types of acoustic metamaterial are considered: a metallic phononic lattice structure and an array of metal shells in water. The unifying feature is that metal acts primarily as a stiffness, and by adding material one arrives at a desired effective density. The metal provides a reservoir of stiffness in the sense that a little bit goes a long way toward the effective stiffness of water, or properties close to water. We first describe the Metal Water structure proposed as a generic metamaterial for transformation acoustics, in both 2D and 3D. The structures have isotropic elastic properties with low shear modulus, hence mimicking water. While designed for long-wavelength effective properties the structures also display interesting finite frequency effects, such as negative index properties. The thin shell metamaterial elements achieve the bulk modulus of water at a specific thickness/radius ratio. Simultaneous matching of effective bulk modulus and density is obtained using an internal mass. By design, both types of metamaterials separate stiffness and density, allowing for simple lumped parameters modeling. The use of metal also has implications for optimal cloaking properties, which result from the fact the metallic structure is non-causal. [Work supported by ONR.]