Anisotropic Acoustic Metamaterials Research Articles

Bending a periodically layered structure for transformation acoustics

Anisotropic acoustic metamaterials have been proved very useful for their high potential in guiding and manipulating sound energy. In this letter, we further develop the idea by using periodically layered structures for transformational acoustics. Such a simple scheme periodically inserts identically bent solid plates in a background fluid. It forms a metamaterial with high refractive index normal to the curved plates and an index near to one along the plates. We show that the periodically layered structure, combined with transformation approach, can be cut into particular device shapes for acoustic cloaking and illusion.

Anisotropic inertial acoustic metamaterials for use in aqueous environments.

The concept of using multiple scattering to create an effective anisotropic mass density/bulk moduli for a composite inertial metamaterial has been around for a few years. Designs for use in aqueous environments face several distinct challenges in comparison to designs useful for airborne applications. The main difficulties lie in that the types of sturdy material components with which an inertial metamaterial can be created are effectively limited to relative impedances of less than a few tens of times that of water. Simply put, the density of water and sound speed comprising the background wave propagation media are much larger than their gaseous counterparts, creating an upper bound on such systems. Further complications can arise from the fact that approximations typically used to describe the scattering elements in homogenized fluid‐like systems for airborne applications can become inaccurate for aqueous environments. Within this context, we present the current state of our experimental verification and validation of a multiple scattering based gradient index lens constructed for frequencies lower than 20 kHz, as well as theoretical/numerical investigation into this and related phenomenon for an orthotropic metamaterial system in an aqueous environment and consider the implications on more complex 2‐ and 3‐D inertial metamaterial systems for use therein. [Work supported by the Office of Naval Research.]