Design of ultra-thin broadband sound-absorbing metamaterials based on Helmholtz resonators

Achieving efficient low-frequency sound absorption within limited thickness remains a persistent challenge in acoustic engineering. In this work, an ultra-thin broadband absorber is developed by exploiting the weak coupling interaction among Helmholtz resonant units. Instead of relying on bulky cavity enlargement, the proposed design establishes a gradient distribution of resonant frequencies through deliberate modulation of geometric parameters, enabling broadband performance via spatially compact arrangements. Numerical simulations reveal that the two configurations exhibit high absorption efficiency (sound absorption coefficient > 0.8) over 480–1023 and 300–500 Hz, respectively. Despite their compact geometries, the overall thicknesses are restricted to 32 and 42 mm, corresponding to only 1/10.6 and 1/27 of the minimum operating wavelengths. To verify the predictive model, impedance tube measurements were carried out independently for both structures. The experimental results closely follow the simulated trends, confirming the reliability of the analytical and numerical framework. The proposed designs, therefore, demonstrate effective subwavelength absorption capability across targeted mid-low and low-frequency ranges.