  The intrinsic limitation of the material nonlinearity inevitably results in
the poor mode purity, conversion efficiency and real-time reconfigurability of
the generated harmonic waves, both in optics and acoustics. Rotational Doppler
effect provides us an intuitive paradigm to shifting the frequency in a linear
system, which needs to be facilitated by a spiraling phase change upon the wave
propagation. Here we numerically and experimentally present a rotating linear
vortex metasurface and achieve close-to-unity mode purity (above 95%) and
conversion efficiency (above 65%) in audible sound frequency as low as 3000 Hz.
The topological charge of the transmitted sound is almost immune from the
rotational speed and transmissivity, demonstrating the mechanical robustness
and stability in adjusting the high-performance frequency conversion in situ.
These features enable us to cascade multiple vortex metasurfaces to further
enlarge and diversify the extent of sound frequency conversion, which are
experimentally verified. Our strategy takes a step further towards the
freewheeling sound manipulation at acoustic frequency domain, and may have
far-researching impacts in various acoustic communications, signal processing,
and contactless detection.
