Numerical Analysis of Three-dimensional Acoustic Cloaks and Carpets

TL;DR

This paper reviews mathematical foundations of acoustic cloaks, proposes multilayered approximations with reduced singularities, and analyzes their effectiveness and bandwidth, including spherical and carpet cloaks, advancing practical acoustic invisibility technologies.

Contribution

It introduces multilayered approximations of acoustic cloaks that reduce material singularities and analyzes their performance and bandwidth in spherical and carpet configurations.

Findings

Multilayered cloaks exhibit low reflection and broad bandwidth.

Non-singular cloaks are feasible with layered approximations.

Carpet cloaks effectively mimic flat ground reflections.

Abstract

We start by a review of the chronology of mathematical results on the Dirichlet-to-Neumann map which paved the way towards the physics of transformational acoustics. We then rederive the expression for the (anisotropic) density and bulk modulus appearing in the pressure wave equation written in the transformed coordinates. A spherical acoustic cloak consisting of an alternation of homogeneous isotropic concentric layers is further proposed based on the effective medium theory. This cloak is characterised by a low reflection and good efficiency over a large bandwidth for both near and far fields, which approximates the ideal cloak with a inhomogeneous and anisotropic distribution of material parameters. The latter suffers from singular material parameters on its inner surface. This singularity depends upon the sharpness of corners, if the cloak has an irregular boundary, e.g. a polyhedron cloak becomes more and more singular when the number of vertices increases if it is star shaped. We thus analyse the acoustic response of a non-singular spherical cloak designed by blowing up a small ball instead of a point, as proposed in [Kohn, Shen, Vogelius, Weinstein, Inverse Problems 24, 015016, 2008]. The multilayered approximation of this cloak requires less extreme densities (especially for the lowest bound). Finally, we investigate another type of non-singular cloaks, known as invisibility carpets [Li and Pendry, Phys. Rev. Lett. 101, 203901, 2008], which mimic the reflection by a flat ground.