A Helmholtz resonator based on spiral neck acoustic metamaterial for noise reduction

Reduction of the noise generated by sound waves emitted from various sources in the audible range is of utmost importance for comfort of the residents. A novel approach to addressing this issue involves the use of acoustic metamaterials to suppress sound transmission. In this study, we focus on Helmholtz resonator-based acoustic metamaterials and propose a compact unit cell that achieves enhanced acoustic performance by shifting the sound transmission loss (STL) peak to lower frequencies without increasing the geometric footprint. By extending the length of the neck of the resonator through a spiral configuration, the resonant frequency is shifted downward, achieving STL at lower frequencies compared to conventional straight-neck designs −approximately reduced from 5000 Hz to 3000 Hz− due to localized resonance phenomena. Band structure analysis further highlights a downward shift in the first band gap for the spiral-neck unit cell design. Numerical simulations were performed to investigate the effect of various geometric parameters on the STL frequency and amplitude. Experimental validation was carried out using impedance tube measurements on 3D-printed samples. The results showed a 6 % deviation in the STL peak for C-shape resonators and a 12 % deviation for spiral-neck designs, attributed to geometric complexity. Scaling up to larger 5 × 5 arrays and adjusting the channel thickness according to fabrication precision (±0.2 mm) minimized discrepancies, emphasizing the importance of precise fabrication for consistent acoustic behavior. Furthermore, incorporating thermoviscous boundary layer effects −particularly in narrow air channels− in the simulations, improved agreement between simulation and experimental results.