Enabling broadband low-frequency sound absorption with a stepped Helmholtz-honeycomb metamaterial

To address the challenge of achieving efficient low-to-mid frequency noise absorption under strict weight and thickness constraints, this study proposes a novel honeycomb structure incorporating stepped multi-cavity Helmholtz resonators (SHRs). The SHR configuration enhances sound absorption in targeted frequency bands through tailored resonance coupling. By integrating theoretical modeling, numerical simulation, and experimental validation, we systematically investigated the influence of step gradient angle, micropore geometry, pore positioning, and structural coupling on acoustic performance. Parametric optimization yielded a half-absorption bandwidth of 350–650 Hz, with the lower cutoff frequency reduced by 39.6% compared to conventional coupled structures (580 Hz). Notably, increasing the gradient angle to 33.7° within a 15 mm cavity depth lowered the resonance frequency by 14.3%, demonstrating the efficacy of gradient tuning for low-frequency control. The proposed structure maintains minimal thickness while significantly improving low-to-mid frequency absorption, offering a promising approach for noise mitigation in complex operational environments.