Design and optimization of a compact double-layer Helmholtz resonator acoustic metamaterial for low-frequency broadband sound absorption

Acoustic metamaterials offer a promising approach for low-frequency sound absorption and have broad applications in noise control and architectural acoustics. In this study, a double-layer Helmholtz resonator acoustic metamaterial, consisting of a slit-neck Helmholtz resonator and an embedded-neck Helmholtz resonator, is proposed. By exploiting the coupling effect of the double-layer structure, the design produces two absorption peaks while maintaining high space utilization and relatively low structural complexity. Theoretical and numerical models were established to investigate the absorption mechanism of the unit cell and the effects of key structural parameters. The results show that thermal–viscous dissipation is mainly concentrated in the neck regions, and the two absorption peaks can be independently tuned by adjusting the structural parameters. As a result, the unit cell achieves absorption coefficients above 0.9 over 285–349 Hz with a thickness of only 1/22 of the corresponding wavelength. A genetic algorithm is then employed to optimize the parallel configuration. With a thickness of 50 mm, two parallel units achieve absorption coefficients above 0.9 over 259–359 Hz, while four parallel units achieve absorption coefficients above 0.8 over 283–501 Hz, with both configurations approaching the minimum thickness dictated by the acoustic causality constraint. Finally, impedance tube experiments validate the proposed design. This work provides a new strategy for the design of acoustic metamaterials capable of efficient broadband sound absorption below 500 Hz.