A sonic black hole structure with perforated boundary for slow wave generation

A Sonic Black Hole (SBH) in a retarding duct structure incorporates two indispensable physical processes, i.e., wave energy focalization and dissipation, to entail slow-sound effect and broadband sound absorption. Original SBH design, however, involves a large number of inner rings inside the duct to produce the required impedance changes. In this study, a SBH configuration with perforated boundary (SBH-PB) is examined, in which perforated acoustic boundaries are used to achieve enhanced SBH effects. Upon a dedicated treatment of the perforated boundary (PB) with the backing cavity, the transfer matrix method (TMM) is adopted to analyze the acoustic characteristics of the SBH-PB and to explore the underlying physical mechanisms. The adoption of the PB is shown to bring about threefold benefits: increased accuracy of the TMM modeling owing to the weakened coupling among the backing cavities in the SBH-PB, enhanced sound absorption through micro-perforations, and the realization of SBH effects with a reduced number of inner rings. To visualize the slow-sound effect, the change of sound speed is examined through transient simulations using finite element method by capturing the wavefront propagation inside the duct. Finally, for the first time, an acoustic duct system replicating the transient simulations is developed to experimentally demonstrate the slow-wave phenomenon in the time domain. The proposed structure holds promises for sound wave manipulation and the development of acoustic noise control devices.