Extraordinary acoustic transmission through ultrathin acoustic metamaterials by coiling up space

We show that by coiling up space through curled channels, a two-dimensional ultrathin acoustic metamaterial can be constructed to support the extraordinary acoustic transmission. The exotic phenomenon intrinsically stems from Fabry-Pérot resonances while the thickness of the structure is kept deep subwavelength. Broadband unity transmission is observed at a specific incident angle. It is also demonstrated that the coiling factor plays a dominant role in determining the property of transmission peak. Our results should contribute to the understanding of the underlying physics of structures by coiling up space and may potentially benefit their practical application.