Sound Absorption and Metamaterials: A Review

The paper reviews and analyzes the state of the art of the problem on sound absorption in the linear formulation and methods for solving it. It is shown that the majority of publications are reduced, from the viewpoint of absorption efficiency, to the realization of one of two ideal absorbers: a Kirchhoff blackbody and an optimal absorber. These two absorbers have fundamentally different properties: the relative cross section of the blackbody absorber is equal to unity, whereas for the optimal absorber, this is the largest possible value, which can be much larger than unity. The practical problems they solve also differ. Ways of creating modern efficient absorbers are discussed, the most promising of which are absorbers made of acoustic metamaterials, which realize ideal bodies in the optimal way and require developed additive technologies for their manufacture.