Influences of imperfectness and inner constraints on an acoustic cloak with unideal pentamode materials

Pentamode materials are elastic solids with vanishing shear modulus, and can be used to cloak underwater sound with solid state and broadband advantages. However, pentamode materials produced with microstructures have inevitable small shear modulus. This paper systematically studies the effects of the shear rigidity and inner constraints on an acoustic cloak composing unideal pentamode materials. The shear rigidity introduces a new type of resonance in the radial direction, which is different from the traditional whispering-gallery resonance along the circumference. Totally fixed, radially fixed and free constraints on the inner surface are found to show significant differences in the cloaking function. To realize a broadband cloak with a suitable inner constraint in practice, we propose to attach a thin elastic shell on the inner surface to virtually tune the inner constraint. The strategy is validated with microstructure cloak simulation. This study will provide valuable guidance to facilitate practical applications of pentamode acoustic cloaks. (C) 2019 Elsevier Ltd. All rights reserved.