Tunable broadband multi-function acoustic metasurface by nested resonant rings

The capability of a metasurface to freely engineer the wavefronts of phase and amplitude shows great prospect in acoustics. However, current designs generally have a fixed function such as wave focusing, wave trapping, cloaking, etc. To break these traditional restrictions, a tunable acoustic metasurface comprising an array of subwavelength unit cells has been proposed to achieve versatile functions. Each sub-unit contains two concentric resonant rings. The reflected phase of the sub-unit can be regulated by dynamically tuning the relative angle of the two nested resonant rings. According to the principle of phase compensation, versatile wavefronts can be realized in a subwavelength scale. Both simulation and experimental results of the sound fields reveal that the proposed metasurface has various functions, including acoustic wave direction control, acoustic illusion and acoustic focusing. Further, the multi-functions can be observed at multiple frequencies, indicating that the design is effective in a broad frequency range. This work presents a novel metasurface for a wide range of applications such as focusing, imaging and concealing.