Low-frequency broadband sound absorption of the Helmholtz resonator-spiral metasurface

This study proposes an acoustic metamaterial featuring a Helmholtz resonator-spiral metasurface (HRSM) to effectively achieve low-frequency broadband sound absorption. A theoretical model is developed to predict the HRSM's sound absorption performance, and it is validated by numerical simulations and acoustic impedance tube experiments. Results indicated that the spiral metasurface of the HRSM extended the depth of the Helmholtz resonator cavity compared to the Helmholtz resonator, causing the first sound absorption peak of the HRSM to shift to a lower frequency. Additionally, the spiral channels of HRSM impeded wave propagation in the second layer, shifting the second sound absorption peak to a lower frequency, as compared to the double layer Helmholtz resonator. Optimal design was explored further in terms of diameters and lengths of the necks in the upper and lower layers of the HRSM, by taking advantage of the additive nature of absorption peaks generated by distinct units. With a structural thickness of 52 mm, the HRSM achieved closed to 90 % continuous absorption within the frequency range of 270-580 Hz, demonstrating excellent broadband absorption capabilities. This study offers an effective feasible methodology for designing sub-wavelength low-frequency broadband absorption structures.