Broadband low-frequency sound absorption by Helmholtz resonators with embedded rings

This study proposed a novel acoustic metasurface composed of densely arranged HRs with periodically embedded neck rings to enhance low-frequency absorption. A hybrid theoretical model was developed, incorporating cross-sectional mutation and end effect corrections with the key coefficient β calibrated via FEM, ensuring high fidelity. Mechanism analysis reveals that the embedded rings modulate acoustic reactance and resistance while amplifying frictional dissipation between air and neck walls, thereby improving the sound absorption coefficient (SAC). Parametric investigations demonstrate that the ring outer radius (the dominant factor), ring width, and neck length significantly influence the absorption peak frequency. A 7-unit parallel array is designed and experimentally validated, achieving quasi-perfect sound absorption (SAC ≥ 0.9) over the frequency range of 170–230 Hz. This work provides valuable guidance for the design of low-frequency broadband acoustic absorbing metasurfaces.