Broadband solid cloak for underwater acoustics

The application of transformation theory to underwater acoustics has been a challenging task because highly anisotropic density is unachievable in water. A possible strategy is to exploit the anisotropic modulus rather than density, although it has not been experimentally demonstrated. We present an annular underwater acoustic cloak designed from particular graded solid microstructures. The geometry tailored microstructures mimic metafluid with highly anisotropic modulus through substantially suppressed shear waves. Transient wave experiments are conducted with the cloak in a designed two-dimensional underwater waveguide system and proved excellent cloaking performance for an enclosed target over broadband frequencies 9-15 kHz. This finding paves the way for controlling underwater acoustics using the structured anisotropic modulus metafluid.