Enhancing wave retarding and sound absorption performances in perforation-modulated sonic black hole structures

Sonic black hole (SBH) effects in a retarding duct can be exploited for sound wave manipulation and absorption. The phenomenon relies on two fundamental physical mechanisms: wave speed reduction and energy dissipation. In this study, we demonstrate that these two physical processes can be meticulously balanced through adjusting the perforation parameters in a perforation-modulated SBH (PMSBH). To elucidate the mechanism of slow wave generation and the effect of perforation parameters, an analytic model based on the Wentzel-Kramers-Brillouin (WKB) solutions to the linear acoustic wave equation is established. Alongside transient finite element simulations, the study unveils the roles that major physical parameters play in terms of regulating sound speed and sound absorption. The perforation ratio of the PMSBH is identified as the dominant factor affecting the slow-sound effect, with an optimal range of above 10 % for a PMSBH with densely segmented internal rings. Owing to the inclusion of the perforated boundary, prominent slow wave effects can still be maintained even with a reduced number of rings, provided that the perforation ratio is properly chosen within a reduced variation range. In both cases, the identified perforation ratio largely exceeds the conventional range widely adopted in the micro-perforation community when the slow wave effects are absent. On top of this, tuning the hole size can further enhance air friction for better sound absorption. Theoretical and numerical findings are experimentally validated, and the performance of the PMSBH is demonstrated. While bringing forward the concept of tunable design, this study offers physical insights and guidance for realizing effective sound absorbers embracing slow wave principles and perforation-induced sound absorption.