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