Ultrathin skin cloaks with metasurfaces for audible sound

We report a design of 2D acoustic skin cloaks by using ultrathin metasurfaces in audible range. The microunit of this metasurface is constructed by a cavity coupled with a membrane. This cloak can completely compensate the wave front discrepancy generated by the scattering of the hidden object because the microunits are capable of arbitrarily modulating the reflected amplitude and phase. The operating frequency ranges from 3.54 kHz to 3.93 kHz. The tolerated maximum incident angle decreases as the height of the hidden object increases. Moreover, the cloak's thickness is only approximately λ /10, so that we can make an object in almost any shape undetectable without obviously increasing the size of the whole system. This intriguing feature forms a sharp contrast to most bulky cloaks on the basis of coordination transformations. (paper)