Low-frequency broadband sound absorption and optimized design of a spiral-hexagonal sonic black hole–Helmholtz resonator metamaterial

Abstract Urban rail transit systems are facing increasingly severe challenges from low-frequency and broadband noise, which significantly affect passenger comfort and operational environments. To address this issue, a spiral hexagonal sonic black hole–Helmholtz resonator (SBH–HR) composite acoustic metamaterial is proposed in this study to achieve superior sound absorption performance across both low-frequency and broadband ranges. A structural model of the SBH–HR system is established, and its sound absorption coefficient is calculated using the transfer matrix method. The accuracy of the results is validated through finite element method simulations. The key parameters influencing the absorption efficiency are further analyzed, and an elite genetic algorithm is employed to perform multi-objective structural optimization, resulting in an improvement of the average absorption coefficient to above 0.9 within the 0–1600 Hz frequency range. Subsequently, an experimental specimen of the SBH–HR composite was fabricated, and the simulation results were compared with the experimental measurements. The experimentally obtained absorption coefficients exhibited excellent agreement with the simulations, with deviations of less than 9% across the main frequency range. Compared with the conventional SBH structure, the proposed spiral hexagonal SBH–HR demonstrates significant advantages in low-frequency wave trapping and energy concentration, with an approximately 30% increase in effective absorption bandwidth and a 24% enhancement in overall absorption performance, indicating outstanding broadband absorption capability. This study not only provides theoretical insights and practical guidance for the optimization of sound-absorbing acoustic metamaterials but also demonstrates significant application potential in urban rail transit noise reduction, offering a feasible and effective solution for future transportation noise control.