Broadening sound absorption bandwidth via multi-order resonances

Acoustic absorbers based on single-order resonant mode of the resonators have been extensively investigated for low-frequency broadband absorption. However, they are usually limited to a certain range of working frequency bands and often do not work effectively in the high frequency range. Here, we present an acoustic metamaterial, perforated panel with tube bundles (PPTB) combined with coiled-up cavity, which effectively expands the operating bandwidth from low to high frequencies by utilizing the multi-order resonances. The PPTB ensures the efficient low-frequency absorption effect though the adjustment of tube diameter and length. The coiled-up prolongs the propagation path of sound waves, thereby facilitating the excitation of higher-order resonance modes at high frequencies. The multi-order resonances mechanism of the metamaterial is revealed thoroughly by theoretical calculations and finite element simulations. The results show that at the first-order peak, the energy dissipation mainly occurs in the tubes, while for the high-order peaks, the energy dissipation of coiled-up cavity gradually increases. Moreover, this work introduces the coupled-mode theory to determine the reasonably structural parameters, which enables to achieve the desired leakage and loss factors simultaneously, maintaining the higher muti-order absorption peaks in a wide frequency band and providing the wider single-order absorption peak in high frequency range. Utilizing multi-order resonances, a broadband metamaterial supporting an average absorption coefficient above 0.93 within 300–3600 Hz and eliminating the tangible absorption dips is obtained, which is mutually verified by theory, simulation and experimentation. Owing to its lightweight, less-complicated structure, terrific acoustic and mechanical performance, this kind of metamaterial may have a broad application prospect in noise control engineering.