Design of sound absorbing metamaterials by periodically embedding three-dimensional resonant or non-resonant inclusions in rigidly backed porous plate

Abstract

Air saturated porous materials, namely, foams and wools, are often used as sound absorbing materials. Nevertheless, they suffer from a lack of absorption efficiency at low frequencies, which is inherent to their absorption mechanisms (viscous and thermal losses), even when used as optimized multilayer or graded porous materials. These last decades, several solutions have been proposed to avoid this problem. Among them, metaporous materials consist in exciting modes trapping the energy between the periodic rigid inclusions embedded in the porous plate and the rigid backing or in the inclusions themselves. The absorption coefficient of different foams is enhanced both in the viscous and inertial regimes by periodically embedding 3D inclusions, possibly resonant, i.e., air filled Helmholtz resonators. This enhancement is due to different mode excitation: a Helmholtz resonance in the viscous regime and a trap mode in the inertial regime. In particular, a large absorption coefficient is reached for wavelengths in the air 27 times larger than the sample thickness. The absorption amplitude and bandwidth is then enlarged by removing porous material in front of the neck, enabling a lower impedance radiation, and by adjusting the resonance frequencies of the Helmholtz resonator.