Broadband, wide angle of incidence sound absorption enhancement using rigid-backing-free periodic composite structure via wave manipulation

Abstract

Based on generalized Snell's law, a composite structure is proposed which is composed of periodically arranged porous elements of varied thickness in the in-plane direction with rigid separations in between to obtain broadband and wide angle of incidence absorption. Without a rigid backing, the proposed design is suitable for applications where both reflection and transmission need to be tackled simultaneously. Firstly, an analytical model to calculate the phase response of reflection and transmission is derived. Moreover, based on the analytical model, a detailed design is made and validated numerically and experimentally: an absorption coefficient of over 0.9 is achieved between 1500 Hz and 3000 Hz for a range of angles of incidence of over 120 degrees. Our work shows a way to achieve high reflection and transmission coefficients simultaneously using regular foams, showing the potential for real-world applications.