Metasurface absorber for ultra-broadband sound via over-damped modes coupling

Highly efficient absorption of broadband low-frequency sound with a slim subwavelength meta-structure promises extensive applications in acoustic engineering, which remains a major challenge due to the weak coupling of strong scattering resonant modes. Here, we formulate the interaction of resonant modes in different damping states on the basis of a coupled-mode theory and draw the conclusion that couplings between over-damped modes demonstrate superiority to under-damped or critically coupled states in sound absorption. Furthermore, we theoretically propose and experimentally demonstrate an ultra-broadband sound absorber by hybridizing multi-order Helmholtz resonators, which support a series of over-damped modes in a single element with flexible key acoustic parameters, including operating frequencies and loss and leakage factors decorated at will. Due to the intense coupling between these elaborated over-damped modes, the designed absorber demonstrates 81% average absorptance for airborne sound ranging from 100 to 1000 Hz (exceeding three octaves) with a thickness down to 1/18.8 of wavelength at the lower-limit frequency. We envision the design methodology to push forward more versatile functional devices.