The effect of sound-absorbing walls in the design of cavity- based metamaterials

Abstract

Cavity-based metamaterials are a type of acoustic metamaterials, usually designed for sound absorbing or sound scattering performance. They consist of an arrangement of ducts and slits, forming acoustic resonators: Helmholtz resonators and quarter-wavelength resonators. Designing perfectly absorbing structures is usually a problem of matching the impedance of the structure with the impedance of the surrounding medium, which is realized through the manipulation of the open area of the unit cell and the viscothermal losses inside the ducts. When the structure is perfectly matched, complete sound absorption is observed for a given frequency range. However, the structures are designed under the assumption that the walls are perfectly rigid, i.e., they do now show sound-absorbing properties. The paper discusses the influence of the sound absorption of the walls on the sound absorption of the whole metamaterial unit in the case when the designed structure is and is not perfectly matched. Large differences are observed between the results, which is reflected by the measurement results.