Acoustic Metamaterial Absorbers

Porous and fibrous materials provide effective and broadband acoustic absorption at mid/high audible frequencies. However, these traditional treatments result in thick and heavy layers when designed for low frequency audible sound. To overcome these limitations, in the recent years metamaterials have been proposed as an alternative to design sub-wavelength and efficient absorbing structures. In the current chapter, we review the recent advances in metamaterial absorbers and their underlying physics based on the analysis of the scattering matrix. Particularly, we exploit the physical interpretation of the eigenvalues and eigenvectors of the scattering matrix in the complex frequency plane to design efficient absorbers. We start by the discussion of the reflection problem, where these eigenvalues and eigenvectors collapse to the reflection coefficient. We state the conditions to design efficient and perfect metamaterial absorbers. Then, we increase the complexity of the scattering considering the transmission problem, where additional conditions are needed for designing perfect absorbers. Through this chapter we discuss different acoustic metamaterials based on metaporous absorbers, arrays of Helmholtz resonators and air cavities, as well as membrane and elastic-plate resonators.