Enhancement of sound absorption in a band frequency using thin porous layer-lined metasurfaces

When a porous layer is installed on a 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 implies 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 attentions for alternative implementations of porous layers. Recently, we proposed a new type of metaporous layer for enhancing sound absorption performance in a specified broad frequency band. The proposed metaporous layer is constructed with a thin porous layer backed by a reactive metasurface consisting of an array of bent channels. Formation of sound absorption band is directly determined by the characteristics of scattered sound field from the proposed metaporous layer. Analytical and numerical investigations show that the metasurface is considerably responsible for the enhanced sound absorption in the proposed metaporous layer, while sound dissipation occurs only in the thin porous layer.