Optimal Sound Absorbing Structures

TL;DR

This paper introduces a design method for creating optimal sound-absorbing structures using acoustic metamaterials, achieving near-perfect broadband absorption with minimal thickness, supported by theoretical and experimental validation.

Contribution

It presents a novel design strategy combining acoustic metamaterials with conventional absorbers to approach the causal limit of sound absorption, verified through experiments and theory.

Findings

Achieved broadband absorption starting at 400 Hz with 12 cm thickness

Designed structures nearly attain the causal absorption limit

Experimental results closely match theoretical predictions

Abstract

Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted optimal. Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.