Anomalous Reflection of Acoustic Waves in Air with Metasurfaces at Low Frequency

An acoustic metasurface made of a composite structure of cavity and membrane is proposed and numerically investigated. The target frequency is in the low frequency regime (570Hz). The unit cells, which provide precise local phase modulation, are rather thin with thickness in the order around 1/5 of the working wavelength. The numerical simulations show that the designed metasurface can steer the reflected waves at will. By taking the advantage of this metasurface, an ultrathin planar acoustic axicon, acoustic lens, and acoustic nondiffracting Airy beam generator are realized. Our design method provides a new approach for the revolution of future acoustic devices.