Abstract
We systematically propose a thin shell-type acoustic metasurface, which could be used to design a carpet cloak that closely covers an arc-shaped object, therefore providing the necessary support for hiding an object with any arbitrary shape. To facilitate the experimental measurement, however, the work here starts with some rotary spherical shell-type and ellipsoidal shell-type cell structures. The measured and calculated sound transmission loss (STL) results of these structures suggest that the sound insulation performances of the shell-type structure are quite different from those of the plate-type structure, indicating a possible break in the shape of the classical sound insulation curve. Considering also that cylindrical shell structures are more widely used in practice than the rotary shell structures, a number of two-dimensional bilayer cylindrical and elliptic cylindrical shell structures were, therefore, designed in this assay. Due to the asymmetry of the structure, the shell-type cells could exhibit bianisotropic sound absorption, reflection and effective parameters. Furthermore, the stiffness of the thin shell structure changed nonlinearly with the changing of the radius of curvature, with a wing shape tendency. In addition, a bilayer cylindrical shell-type acoustic metasurface and an arc-shaped carpet acoustic cloak were successively designed, wherein the phased compensation of differently shaped cell structures could be adjusted by means of a new engineering iso-phase design method. This work could provide the necessary guidance to extend existing results in the field of membrane- and plate-type acoustic metamaterials for shell-type structures, and the realization of the arc-shaped cloak could provide support for the design of a carpet acoustical cloak for use with arbitrary shapes.