Acoustic Black hole effects in Thin-walled structures: Realization and mechanisms

Vibrational Acoustic Black Holes (ABHs) on flexural waves are commonly achieved in thick-walled structures whose local thickness should undergo significant variation according to a power-law relationship. The topic has been widely explored despite the fact that the requirement imposed for structural thickness has been constantly challenged by practical applications. Existing open literature does not provide convincing answers to the practically important question of whether sensible ABH effects can still be produced in structures within a thin thickness range through proper design, and if so, what is the underlying mechanism. In this work, through numerical and experimental investigations, we demonstrate that ABH effects can be readily achieved in a compound ABH structure of thin thickness with viscoelastic material filling. The compound structural design makes effective use of both ABH effects and the shear effects of the damping material, mainly along the mid-plane of the structure, to collectively generate a significant enhancement of the structural damping. While the shear-induced damping dominates the lower frequency region, ABH-induced damping becomes dominant once the ABH effect is cut-on. A good compromise between the two effects requires the stiffness of the damping materials and other ABH parameters to be properly chosen in order to ensure broadband benefit in terms of vibration reduction through damping enhancement.