Band frequency selective enhancement of sound absorption in a thin porous layer with a reactive metasurface

When a porous layer is installed on an acoustically hard wall, sound absorption performance is mainly determined by thickness of the layer. Although material parameters of porous materials are strongly dependent on frequencies, the thickness limitation related to the quarter wavelength of incident sound wave has been a key factor in the treatment of porous layers for noise reduction. This implys that a thicker porous layer is required to absorb lower-frequency sound effectively. To overcome the thickness limitation, metaporous layers, which are named as a compound of sound absorbing porous layers with the concept of metamaterials have received much attention for alternative implentations of porous layers. Recently, we proposed a new type of metaporous layer for enhancing sound absorption performance in a broad band of frequencies. The proposed metaporous layer is constructed with a thin porous layer backed by a reactive metasurface consisting of an array of bent channels. Analytical and numerical investigations show that the sound absorbing band is formed by the characteristic of the scattered sound field from the metasurface realizing anomalous sound reflection. The high sound absorption in the metaporus layer is resulted by interaction between the thin porous layer and multiple channels in the amplification of sound dissipation.