Sound absorption and enhancement mechanism of hierarchical slit-embedded Helmholtz resonators

In this paper, the low frequency sound absorption structure based on Helmholtz resonator (HR) is deeply studied. The influence of hierarchical structure design on the broadband sound absorption is emphatically discussed. Through the introduction of the embedded slits and hierarchical structures, we design a new and efficient hierarchical slit-embedded HR (HSEHR) for sound absorption. This structure not only inherits the advantages of the classic HR, but also realizes the effective sound absorption(α > 0.97) in a lower frequency(225 Hz) range through the interaction of the embedded slit and the hierarchical structure. More interestingly, the thickness of HSEHR is only 1/50 of the corresponding wavelength. In order to verify the sound absorption effect of HSEHR, we have carried out a lot of theoretical analysis and numerical simulation. The results show that HSEHR has excellent sound absorption performance in the low frequency range, and with the introduction of the hierarchical structure, the sound absorption peak moves to a lower frequency, and a higher sound absorption coefficient is obtained. We also found that by adjusting the structural parameters of HSEHR (such as the depth and width of the primary embedded slit.), its resonance frequency can be precisely controlled. So it can better match the target noise frequency and improve the sound absorption efficiency. In addition, genetic algorithm is used to optimize the structural parameters of HSEHR to further improve its sound absorption performance. The optimization results show that HSEHR optimized by genetic algorithm has better sound absorption performance in the broadband low frequency range. It achieves excellent sound absorption at 260–480 Hz. The sound absorption coefficient is up to 0.92, which is infinitely close to perfect sound absorption. It provides an excellent solution to the noise problem.