Composite hexagonal pentamode acoustic metamaterials with tailored properties.

Acoustic metamaterials are artificial materials which can manipulate and control acoustic waves in way that may not exist in nature. Pentamode metamaterials, as one kind of metamaterials, have solid structures but behaves like fluid. One application is in building acoustic cloaks. In this paper, composite pentamode metamaterials with hexagonal unit cells are proposed. The phononic band structures of the unit cell show that there are band gaps within which only compressional modes exist. With variation of structures, highly anisotropic properties can be obtained. The influences of geometric dimensions and materials on the effective properties are analyzed. The composite structures introduce more degrees-of-freedom to tailor the effective properties.