Design of a slow sound based meta-poro-elastic material with enhanced absorption capabilities

Poroelastic foams are generally efficient to absorb sound. Thanks to their typical extensive air-solid interface, these materials are able to dissipate energy through viscous and thermal interactions. However, because of their absorption mechanisms, they have a low absorption efficiency at wavelengths much larger than the layer thickness. Works on the optimization of the pore design have shown that little improvement can be obtained at low frequencies. Meso-scale inclusions embedded in foam-like materials have been employed to address this problem and broaden the working frequency ranges, mainly through resonance phenomena. Although the subwavelength resonators effectively absorb sound in the low frequency range, the sizes needed to achieve low frequency performance frequently prohibit practical use. In this work we demonstrate how compact sound absorbing panels can be designed by exploiting the slow sound propagation achieved in flexible rubber tubes. An axisymmetric FEM formulation is developed and an optimization of the Biot parameters is carried out. The designed meta-poroelastic material, obtained by embedding rubber quarter wave resonators in a poroelastic foam, is able to absorb well the low frequency noise while keeping good absorption capabilities at high frequency.