A Hybrid Metamaterial for Broadband, High-Performance Sound Absorption via Helmholtz Resonators Incorporating Porous Media.

Finite-thickness acoustic absorbers often face a trade-off between low-frequency resonance and broadband dissipation. The porous material (PR) provides stable mid-to-high-frequency losses through viscous and thermal effects, whereas Helmholtz resonators (HRs) offer compact low-frequency resonance but usually operate over narrow bands. Accordingly, an HR-PR hybrid metamaterial absorber is developed by coupling a central air neck and a sealed back cavity with a surrounding porous layer described by the Johnson-Champoux-Allard (JCA) rigid-frame equivalent-fluid model. A surface impedance model is formulated for the HR branch, the PR branch, and their area-weighted parallel admittance and is validated against COMSOL 6.4 unit-cell simulations. Complex-frequency reflection zero-pole analysis separates the HR-dominated localized resonance from the broadband dissipative contribution of the porous branch. Nine-unit cells with different back-cavity volumes are then arranged into a 3 × 3 composite material to distribute the HR resonances across the target low-frequency range. Using a peak placement optimization strategy to identify a broad continuous absorption band satisfying a sound absorption coefficient (α) ≥ 0.7, the equivalent model predicts an absorption band of 360-563 Hz with a bandwidth of 203 Hz. Air impedance tube measurements show the same overall absorption trend as the theoretical and numerical results, supporting the effectiveness of the optimized HR-PR hybrid metamaterial absorber.