A sector-shaped acoustic metamaterial for broadband and wide-angle sound absorption

Maintaining stable absorption performance at large oblique incidence while achieving broadband sound absorption remains a major challenge for conventional sound-absorbing materials. To address this issue, a sector-shaped acoustic metamaterial composed of two-dimensional gradient waveguides and a porous backing cavity is proposed to achieve broadband and wide-angle sound absorption. By arranging the entrances of the two-dimensional gradient waveguides in a fan-shaped configuration spanning different azimuthal directions, incident sound waves from various directions can be efficiently guided into the structure, which reduces the dependence of absorption performance on the angle of incidence. Meanwhile, the multiple narrowband resonant responses induced by the gradient waveguides are coupled and smoothed through the damping effect of a porous backing cavity, enabling continuous broadband absorption. Numerical simulations demonstrate that the proposed structure maintains stable and high absorption over a broad frequency range of 500–3400 Hz for incidence angles from 0° to 75°, and remains effective even under near-grazing incidence at 80°. Comparative results further demonstrate that, under large oblique incidence, the proposed metamaterial maintains higher absorption levels across a wide frequency range than conventional porous absorbers and sonic black hole structures. These findings provide an effective design strategy for compact broadband sound absorbers with enhanced angular robustness.