Tunable cylindrical shell as an element in acoustic metamaterial

TL;DR

This paper presents a tunable cylindrical shell with an internal mechanism that allows precise control of its effective acoustic properties, enabling applications like wave steering and cloaking in acoustic metamaterials.

Contribution

It introduces a novel internal mechanism for cylindrical shells that can be tuned to achieve desired acoustic properties and suppress flexural waves, advancing metamaterial design.

Findings

Effective tuning of bulk modulus and density in shells

Near-zero scattering cross section in water over broad frequencies

Design of a cylindrical-to-plane wave lens using property variation

Abstract

Elastic cylindrical shells are fitted with an internal mechanism which is optimized so that, in the quasi-static regime, the combined system exhibits prescribed effective acoustic properties. The mechanism consists of a central mass supported by an axisymmetric distribution of elastic stiffeners. By appropriate selection of the mass and stiffness of the internal mechanism, the shell's effective acoustic properties (bulk modulus and density) can be tuned as desired. Subsonic flexural waves excited in the shell by the attachment of stiffeners are suppressed by including a sufficiently large number of such stiffeners. Effectiveness of the proposed metamaterial is demonstrated by matching the properties of a thin aluminum shell with a polymer insert to those of water. The scattering cross section in water is nearly zero over a broad range of frequencies at the lower end of the spectrum. By arranging the tuned shells in an array the resulting acoustic metamaterial is capable of steering waves. As an example, a cylindrical-to-plane wave lens is designed by varying the bulk modulus in the array according to the conformal mapping of a unit circle to a square.