A high transmission broadband gradient index lens using elastic shell acoustic metamaterial elements

TL;DR

This paper demonstrates a broadband underwater acoustic lens using elastic shell metamaterial elements, combining simulations and measurements to achieve effective wave focusing over a wide frequency range.

Contribution

It introduces a novel elastic shell-based metamaterial design for broadband acoustic lenses, validated through simulations and underwater experiments.

Findings

Broadband operation from 20 to 40 kHz confirmed

Positive radiation pattern gain in four directions

Effective cylindrical-to-plane wave transformation achieved

Abstract

The use of cylindrical elastic shells as elements in acoustic metamaterial devices is demonstrated through simulations and underwater measurements of a cylindrical-to-plane wave lens. Transformation acoustics (TA) of a circular region to a square dictates that the effective density in the lens remain constant and equal to that of water. Piecewise approximation to the desired effective compressibility is achieved using a square array with elements based on the elastic shell metamaterial concept developed in [30]. The size of the elements are chosen based on availability of shells, minimizing fabrication difficulties. The tested device is neutrally buoyant comprising 48 elements of nine different types of commercial shells made from aluminum, brass, copper, and polymers. Simulations indicate a broadband range in which the device acts as a cylindrical to plane wave lens. The experimental findings confirm the broadband quadropolar response from approximately 20 to 40 kHz, with positive gain of the radiation pattern in the four plane wave directions.