Ultra-Broadband Sound Reduction in Nonlocal Ventilating Structures With Double Mode Degeneracy

Channel-type structures based on Fano resonances show significant potential in simultaneous control of sound isolation and air ventilation. Broad isolation bandwidth is a crucial factor to the Fano-based model but usually relying on multiple element arrangement. In this work, a novel mechanism based on the nonlocal effect via direct mode couplings is proposed to broaden the isolation bandwidth of ventilating structures. The studied model is a ternary structure consisting of two coiling channels coupled to a straight orifice. Two Fano-based transmission valleys, which are independently controlled by two coiling channels, can be merged by designing single-mode degeneracy. Without introducing more elements, the nonlocal mode coupling, achieved by the direct hole connection between channels, is found to produce the double-mode degeneracy behavior such that three transmission valleys can be combined to obtain an ultra-broadband sound insulation. A coupled-mode model is developed to derive the mode degeneracy condition of Fano resonances, and illustrate the working mechanism to manipulate double-mode degeneracy. Both numerical and experimental analyses are conducted to demonstrate the superior performance of the nonlocal effect for a significant increase of isolation bandwidth in Fano-based open sound barriers.