Sound absorption in sonic black holes: Wave retarding effect with broadband cavity resonance

Energy dissipation mechanism of Sonic black hole (SBH) structure with limited number of inner rings inside a duct has not been fully apprehended. On the top are the unclearly revealed dominant energy dissipation region, and inherent relationship among SBH induced wave retarding effect, cavity resonance and the final sound absorption to be achieved. To alleviate these problems, numerical analyses are conducted using finite element (FE) simulations with due consideration of the thermal-viscous losses in a SBH for sound absorption assessment and underlying sound absorption mechanism exploration. The validity of the FE model is confirmed through comparisons with experimental data acquired from impedance tube tests. Analyses first identify the vicinity of the inner ring wall as the predominant energy dissipation region, and the primary source of dissipation comes from the strong friction occurred through cavity resonance-induced high pressure gradient. Moreover, SBH induced wave retarding effect drastically changes the resonance behavior of individual cavity from conventional narrowband resonance with single resonant peak to broadband resonance with multiple resonant peaks, alongside the down-shifting of the resonant frequencies. Altogether, cascaded effects finally lead to the low frequency and broadband sound absorption. Studies presented in this work can provide guidelines for better SBH design in future.