A broadband and low-frequency sound absorber of sonic black holes with multi-layered micro-perforated panels

Sonic black hole (SBH) absorber is a promising strategy in noise reduction. However, low-frequency sound waves are difficult to be captured and dissipate due to the terminal truncation and the discontinuity of the admittance for the actual SBH. To make up the disadvantages, this paper incorporates micro-perforated panels (MPP) into the SBH to improve its low-frequency property. The design is realized by inserting the multi-layered MPPs (MLP) into the rings with decaying inner radius inside a SBH duct, termed the MLPSBH duct. The theoretical and numerical results show that the proposed absorber can achieve effective sound absorption in the frequencies above 225 Hz, in which the thickness of the absorber is only λ/15 with the λ being the corresponding wavelength. Mechanism studies indicate that the specific slow wave effect of the SBH can effectively mitigate the requirements for the absorber length. Closer examination reveals that the high acoustic resistance characteristics of the MLP remarkably compensate for the acoustic resistance mismatch of the SBH in low-frequency range. Thus, attributing to the interplay of the SBH and MLP effects, the proposed absorber achieves broadband and low-frequency sound absorption. Based on the deduction and validation of the transfer matrix method (TMM), the influence of the structural parameters on the sound absorption is investigated. The analysis indicates that the absorption performance of the proposed absorber barely depends on the damping and the number of rings, which allows the MLPSBH duct to maintain the excellent performance without abundant rings and extra absorbent material. The proposed strategy is an inspiration for the application of the SBH-based structures.