Broadband acoustic metamaterial absorber

Constrained by the causality nature, it is a challenge to achieve low-frequency broadband efficient absorption via passive materials. By coupling local resonances as many as possible, broadband metamaterial absorbers can be realized. In this scenario, the coupling effect is crucial for improving the absorption efficiency. Here, we demonstrate a kind of acoustic metamaterial absorber capable of high-efficiency broadband absorption by optimizing the non-local coupling effect. Cascade neck-embedded Helmholtz resonator is designed as the subunit of the metamaterial absorber. By coupling 36 subunits, a metamaterial absorber is constructed with the aid of optimization algorithm. The simulation results suggest that strong non-locality is induced in the near field, which effectively suppresses the impedance oscillations and absorption dips at the antiresonance frequencies. Eventually, the presented metamaterial absorber realizes impedance matching to the air from 320 Hz to 6400 Hz, leading to efficient broadband absorption performance. The results and methodologies in this work reveal the significance of non-local coupling effect in coupled-resonant metamaterials.