Low-frequency and broadband sound absorption by nested Helmholtz Resonator based on Sonic Black Holes

Sonic black hole (SBH) absorbers have been proven to be effective and promising for broadband sound absorption. However, achieving effective subwavelength sound absorption solely by relying on the slow-wave effect of SBHs is often challenging, especially at low frequencies. In order to explore the potential of SBHs in the applications requiring effective low-frequency and broadband absorption within a compact thickness, a hybrid SBH absorber (NHR-SBH) incorporating nested Helmholtz resonators (NHRs) is proposed. NHRs enhance low-frequency absorption more effectively than conventional Helmholtz resonators (HRs) due to the increased equivalent acoustic length of their nested cavities. The required structural length of the NHR-SBH decreases as the nested cavities become thinner within a specific range, as determined by analyzing the velocity field, sound pressure, and sound absorption performance at corresponding frequencies. To enhance the broadband property of a single NHR-SBH, coplanar NHR-SBHs (CoNSBHs) arrays are also established. The Genetic Algorithm (GA) is employed to optimize the sound absorption of CoNSBHs in the frequency range of 50 Hz to 1000 Hz. Based on the optimized process that balances the broadband properties above 400 Hz with the deep subwavelength characteristics at low frequencies (below 200 Hz). The underlying physical mechanism of the coplanar SBH structure is elucidated, revealing that the performance of CoNSBHs is governed by the synergy between viscous losses in the NHR necks and resonance within the MPP and NHR cavities. Notably, the relative contribution of viscous losses is found to diminish as frequency increases. Prototypes of NHR-SBH and the coplanar array have been assembled based on commonly accepted machining precision. Besides of the broadband property, experimental results show that excellent absorption can be achieved by these prototypes at low frequencies (100 Hz to 200 Hz) with total lengths of 77.7 mm (λ/50 at 86 Hz), 74.0 mm (λ/40 at 116 Hz), and 79.8 mm (λ/26 at 161 Hz), respectively. Additionally, an average sound absorption coefficient of 0.8604 (from 226 Hz to 1000 Hz) can be achieved by CoNSBHs with a total length of 89.4 mm.