Perfect absorption of sound using curved metasurface

Abstract

An acoustic metasurface is one of the sound-absorbing structures for a perfect absorption of sound with a thickness of a sub-wavelength scale. Previous studies on the acoustic metasurface are based on periodic arrangements of unit cells with the invariant incidence conditions along the plane. In practical applications such as military technology to avoid sonar detection or noise reduction techniques in architectural acoustics, studies on the curved metasurface that can be placed along curved surfaces are required. To design the curved metasurface for the perfect absorption, the arrangement of the unit cells in which the incident conditions locally vary along the curved surface needs to be investigated. In this study, we investigate the perfect absorption for a one-dimensional unit cell arrangement in which the angle of incidence locally varies along unit cells, and propose a design procedure of a perfectly sound-absorbing curved metasurface at the target frequency. First, we design an angle-dependent unit cell, which perfectly absorbs sound for each angle of incidence at a target frequency, consisting of two different Helmholtz resonators with sub-wavelength scales. Next, the curved metasurface is designed by arranging N unit cells (N≥2) according to the distribution of angles of incidence of the given curved surface. As a result of the proposed design procedure, we obtain $\lambda$/24-thick circular-curved metasurfaces exhibiting the perfect absorption at 1000Hz. By increasing the number of unit cells N, we examine the perfect absorption of the circular-curved metasurface with one or more joint angles between unit cells. In order to consider different types of the curved surface, we design $\lambda$/24-thick elliptical-curved metasurfaces exhibiting the perfect absorption at 1000Hz. By changing the eccentricity e from 0 to 0.98, we examine the perfect absorption of the elliptical-curved metasurface with different distributions of angles of incidence.