Broadband and low-frequency sound absorption by a slit-perforated multi-layered porous metamaterial

A novel slit-perforated multi-layered porous metamaterial (SMPM) is proposed in this paper, which exhibits a broadband and low-frequency sound absorption characteristic. The proposed SMPM is composed of periodic porous matrix layers and periodically distributed adjacent second-type porous layers containing slits. The theoretical model is established by applying the double porosity (DP) theory for the slit and the porous layer which form an equivalent inclusion layer, and then the homogenization method is used for the unit-cell of the SMPM composed of the equivalent inclusion layer embedded in the porous matrix layer. The theoretical results are validated by the numerical results obtained by the finite element (FE) software COMSOL Multiphysics, and a broadband and low-frequency sound absorption performance of the SMPM is achieved. Furthermore, the acoustic impedance of the proposed SMPM, the time-averaged power dissipation density and the acoustic energy dissipation ratio of the two constituent porous materials are investigated in detail. The effects of the slit-width and the material composition ratio on the sound absorption performance are also explored. The results demonstrate that the introduction of the periodic slits can promote the acoustic wave propagation in the SMPM to enhance the sound absorption especially at low frequencies, and the porous matrix layers and the second-type porous layers play different roles in the sound energy dissipation process. To be more specific, the porous material with a lower air-flow resistivity contributes more to the energy dissipation at low frequencies while that with a higher air-flow resistivity becomes more involved in the sound energy dissipation process at middle to high frequencies.