Sound Absorption Structures: From Porous Media to Acoustic Metamaterials

The recent advent of acoustic metamaterials has initiated a strong revival of interest on the subject of sound absorption. The present review is based on the physics perspective as the coherent basis of this diverse field. For conventional absorbers, viscous dissipation and heat conduction at the fluid-solid interface, when expressed through micro-geometric parameters, yield an effective medium description of porous media and micro-perforated panels as effectual sound absorbers. Local resonances and their geometric and symmetry constraints serve as the framework for surveying a variety of acoustic metamaterial absorbers that can realize previously unattainable absorption spectra with subwavelength-scale structures. These structures include decorated membrane resonators, degenerate resonators, hybrid resonators, and coiled Fabry-Pérot and Helmholtz resonators. As the acoustic response of any structure or material must obey the causality principle, the implied constraint—which relates the absorption spectrum of a sample to its required minimum thickness—is presented as a means to delineate what is ultimately possible for sound-absorbing structures. The review concludes by describing a recently reported strategy for realizing structures that can exhibit custom-designed absorption spectra, as well as its implementation in the form of a broadband absorber with a thickness that is close to the minimum value as dictated by causality.