Multiple slow waves in metaporous layers for broadband sound absorption

Sound absorption for a broad frequency range requires sound dissipation. The mechanics of acoustic metamaterials for non-dissipative applications has been extensively studied, but sound absorption using dissipative porous metamaterials has been less explored because of the complexity resulting from the coupling of its dissipative mechanism and metamaterial behavior. We investigated broadband sound absorption by engineering dissipative metaporous layers, which absorb sound by the mechanism of multiple slow waves, and combined local and global resonance phenomena. A set of rigid partitions of varying lengths was elaborately inserted in a hard-backed porous layer of a finite thickness. An effective medium theory was used to explain the physics involved; high performance at a low-frequency range was found to be mainly due to the formation of global resonances caused by multiple slow waves over the thickness of the metaporous layer, while enhancement at a high-frequency range was attributed to the combined effects of the global resonances and the local resonances directly related to the sizes of the inserted partitions.