Tunable Low-frequency Sound Absorber via Helmholtz Resonators with Embedded Spiral Tube

Low-frequency sound waves have the characteristics of long wavelengths, minimal attenuation during propagation, and a tendency to resonate with human organs, causing significant health impacts. To address this issue, a novel type of acoustic metamaterial known as Helmholtz Resonator Embedded Spiral (HRES) acoustic metamaterial is proposed. We first establish the theoretical model of HRES and validate it through numerical simulations. The results show excellent agreement between theoretical predictions and finite element analysis, confirming the absorptive capabilities of HRES at low frequency. The proposed HRES structure not only reduces the thickness of the metamaterial but also significantly enhances tunability of system impedance by introducing additional geometric degrees of freedom. The HRES with deep sub-wavelength thickness (λ/50) achieves perfect sound absorption at 124Hz. With the same thickness of an external Helmholtz cavity, perfect absorption can be obtained in certain frequency in the band from 102Hz to 164Hz via changing the parameters of the embedded spiral. Such capability facilitates the design of broadband sound absorption within the cavity using tunable structures. The research presented in this paper holds potential significance for designing low-frequency, ultra-thin, tunable sound-absorbing materials.