Multiple resonances in lossy acoustic black holes - theory and experiment

Acoustic properties of the metamaterial graded absorber, also known as ``acoustic black hole'', are studied in linear regime. The absorber consists thin metallic circular plates, each with a central perforation, separated by annular air cavities. Radius perforation plate is gradually decreasing distance from to front surface, forming channel staircase radius profile. A semi-analytical equivalent fluid model accounting for variations both effective density and compressibility inside this developed, which incorporates assumes motionless plates. viscous thermal losses side cavities accounted using well-established Johnson-Champoux-Allard-Lafarge model. It demonstrated that high absorption coefficient values achieved wide range frequencies starting few hundred Hz or less. At low frequencies, resonances along length structure, i.e. global resonances, responsible sound attenuation. higher lateral cavities, local play major role. upper boundary frequency determined resonance plate. validated against impedance tube measurements on five samples different geometry FEM models. predicts elasticity plates affects behaviour at well below resonances.