Broadband convergence of acoustic energy with binary reflected phases on planar surface

We propose to produce efficient three-dimensional sound converging in broadband with binary reflected phases on a planar surface with unit cells consisting of only two kinds of elements. The mechanism is experimentally demonstrated by focusing airborne sound and by forming an acoustic needle, with handmade arrays of commercial test tubes with/ without lids. Both the simulated and measured results show the precise control of converging acoustic energy despite misalignment errors obvious even to naked eyes. Our approach with extreme simplicity yet good robustness may apply in various scenarios that conventionally need complicated elements and continuous variation of parameters for focusing sound. Published by AIP Publishing.