Previously reported acoustic metasurfaces that consist of fixed channels, areuntunable to meet the broadband requirement and alterable functionalities. Toovercome this limitation, we propose screw-and-nut mechanism of tunability anddesign a type of continuously tunable acoustic metasurface with unit componentsof helical cylinders which are screwed into a plate. The spiral channel lengthcan be tuned continuously by the screwed depth; and then a metasurface withcontinuously tunable acoustic phase at independent pixels is attained.Different distributions of the unit components can shape an arbitrarymetasurface profile. We also developed an approximate equivalent medium modelto predict the tunability of the unit component. As an example, we present thedesign details of a circular tunable metasurface for three-dimensional acousticfocusing of different airborne sound sources in a wide frequency region. Asample of the metasurface is manufactured by poly lactic acid (PLA) plasticwith the helical cylinders being 3D-printed. Experiments of sound focusing areperformed. It is shown that the results of the equivalent medium model, thefinite element simulation and the experiments are in a good agreement.
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