A hybrid sonic black hole for enhancing low-frequency sound absorption

Abstract Sonic black hole (SBH) is a kind of broadband sound absorber, which features lightweight and high-efficiency. However, achieving low frequency sound absorption at a deep-subwavelength thickness remains a challenge for conventional SBHs. Herein, a hybrid sonic black hole (HSBH) with gradually varying neck-embedded Helmholtz resonators (HRs) and micro-perforated panels (MPPs) is proposed. The working bands are broaden at low frequencies with sub-wavelength and broadband sound absorption in the mid-and-high frequency range. Such HSBH exhibits excellent potential in broadband sound absorption with sub-wavelength. The transfer matrix method (TMM) is used to analyze the low-frequency sound absorption of HSBH, which is validated by the finite element method (FEM). It is shown that the proposed HSBH exhibits high sound absorption with sub-wavelength (the frequency of first peak is 102 Hz, about λ / 30 ). The enhanced low-frequency sound absorption of the proposed HSBH is also validated by the experiment results. Meanwhile, the influences of various structural parameters on the low-frequency properties and broadband characteristics of HSBHs are also revealed. A fast and elitist nondominated sorting genetic algorithm (NSGA-II) is introduced to optimize the HSBH. Finally, a coplanar HSBHs are also designed to achieve a broadband sound absorption from 100 to 3000 Hz.