Extraordinary focusing of sound above a soda can array without time reversal

TL;DR

This paper demonstrates that sharp acoustic focusing above a soda can array can be achieved without time reversal by exploiting a locally resonant metamaterial formed by the cans, resulting in sub-wavelength focusing near resonance.

Contribution

It introduces a method for focusing sound using a soda can array without time reversal, leveraging guided waves in a resonant metamaterial to achieve sub-wavelength focus.

Findings

Focusing improves as frequency approaches the Helmholtz resonance.

Focal spot size becomes smaller than a single can near resonance.

Guided waves in the metamaterial support sub-wavelength focusing.

Abstract

Recently, Lemoult et al. [Phys. Rev. Lett. 107, 064301 (2011)] used time reversal to focus sound above an array of soda cans into a spot much smaller than the acoustic wavelength in air. In this study, we show that equally sharp focusing can be achieved without time reversal, by arranging transducers around a nearly circular array of soda cans. The size of the focal spot at the center of the array is made progressively smaller as the frequency approaches the Helmholtz resonance frequency of a can from below, and, near the resonance, becomes smaller than the size of a single can. We show that the locally resonant metamaterial formed by soda cans supports a guided wave at frequencies below the Helmholtz resonance frequency. The small focal spot results from a small wavelength of this guided wave near the resonance in combination with a near field effect making the acoustic field concentrate at the opening of a can. The focusing is achieved with propagating rather than evanescent waves. No sub-diffraction-limited focusing is observed if the diffraction limit is defined with respect to the wavelength of the guided mode in the metamaterial medium rather than the wavelength of the bulk wave in air.