Wave trapping by acoustic black hole: Simultaneous reduction of sound reflection and transmission

Reduction of vibration and sound energy in the form of traveling waves is of vital importance in many applications. Recent development of acoustic metamaterials opens up unusual ways for sound wave manipulation and control. Among acoustic metamaterials, a much newer concept, Acoustic Black Hole (ABH), has been drawing growing attention in recent years, which shows great potential for acoustic energy trapping and dissipation. In a duct ABH with a properly tailored continuous cross-sectional reduction and impedance variation, it is shown that the sound speed can be progressively reduced, which means that sound waves are eventually trapped in the structure. In this paper, such a wave trapping mechanism is further explored in the context of sound transmission problems, in which an exceptional phenomenon - simultaneous reduction of sound reflection and transmission - is realized. The archived trapping mechanism also ensures that little sound waves will be bounced back to the source to jeopardize the overall performance. Transfer matrix method simulations and impedance tube experiments are performed to characterize the behavior of such a structure and to validate the theory. The promising ABH-specific features arising from the proposed design could overcome many existing limitations of traditional noise control devices.