Enhancing the acoustic absorption of vegetation with embedded periodic metamaterials

Abstract The sound absorption of vegetation with embedded periodic metamaterials is investigated numerically and experimentally. Acoustical property of the vegetation can be described as that in a porous material. The physical parameters of vegetation in the numerical study are obtained by fitting the measured reflection and transmission coefficients with the simulated results. The embedded metamaterial is composed of two-layer oppositely oriented split tubes, which can achieve two resonant absorption peaks at wavelengths much larger than its thickness. Both absorption peaks can be easily controlled by adjusting the width of the metamaterial while keeping its thickness unchanged. It is demonstrated that the absorption performance can be significantly enhanced by embedding periodic metamaterials into the vegetation. The results can be explained by coupling the viscous and thermal losses of vegetations with the resonance phenomenon arising from the metamaterials. Moreover, sound absorption increases with increased incidence angles and vegetation thickness. When more than one metamaterial per spatial period is considered, additional absorption peaks are observed which can further broaden the effective absorption bandwidth.