Broadband acoustic meta-porous layer for reflected wave manipulation and absorption

Acoustic metasurfaces (AMs) used for reflected sound wavefront manipulation are generally designed based on the generalized Snell's law (GSL) at a single frequency, suffering from inferior broadband properties. Herein, an acoustic meta-porous layer (AMPL) with periodic structures is designed to realize reflected wavefront manipulation and effective sound absorption over a wide frequency band. The AMPL is constructed by four periodically arranged units, each consisting of porous elements inserted by acoustically rigid partitions, forming a linear reflected phase-shifting within 0 to 2π maintaining in a target frequency range of [1000,3000]Hz. To predict the reflected response of the element of the AMPL, an analytical model is proposed based on the transfer matrix method and is numerically validated. The sound reflection and absorption performance of the AMPL is investigated numerically and experimentally. The scattering sound pressure fields under normally and obliquely incident sound waves at different frequencies demonstrate the broadband reflected wavefront manipulation capability, including negative reflection and surface wave conversion. Compared to a uniform porous foam, an effective absorption is achieved by the AMPL due to the surface wave excitation at an identical thickness of 40 mm, with an averaged absorption coefficient greater than 0.9 in [800,3000]Hz. The results suggest the proposed structure could be a promising candidate of broadband noise absorption metamaterial for practical applications.