Broadband and low frequency sound absorption by Sonic black holes with Micro-perforated boundaries

Acoustic black holes (ABHs) have been so far investigated mainly for flexural wave manipulation in structures. Exploration of ABHs for sound wave manipulation, referred to as Sonic black holes (SBHs), as well as the design of SBH-based noise control devices are scarce. To fill the gap, this paper proposes a SBH sound absorber inside a circular duct in conjunction with the use of Micro-perforated panels (MPPs) to achieve broadband and low-frequency sound absorption. Capitalizing on the ABH-specific wave retarding and trapping phenomena and the energy dissipation ability of the MPP, a compact and ultra-broadband near perfect sound absorbing device with sub-wavelength thickness is realized for noise abatement in a duct. Finite element simulations are performed to assess the achieved sound absorption performance, which is experimentally confirmed by impedance tube tests. Analyses reveal that the physical mechanism underpinning the superior sound absorption is attributed to the combined effects of the ABH-induced wave speed changes, energy trapping and the spatially graded local resonances of the cavity-backed MPP. The proposed solution shows promise for circumventing some existing limitations of traditional noise control devices.