Boundary effect on acoustic cloak with unideal pentamode material

Pentamode materials are elastic solids with vanishing shear modulus, and can be used cloak underwater sound with solid state and broadband merits. However, pentamode materials realized with real microstructure have inevitable small shear modulus. This paper systematically studies the impact of shear rigidity and inner surface constraints on acoustic cloak with unideal pentamode material. The shear rigidity introduces a new kind of resonance in the radial direction, which is different from the traditional whispering-gallery resonance along the circumference. Totally fixed, radially fixed or free boundaries on the cloak inner surface are found to show significant difference on the cloaking function. To realize a broadband cloak with suitable boundary for practice, we propose to attach an elastic thin shell on the inner surface to virtually tune the boundary constraint. The proposed strategy is also validated with microstructure cloak simulation. This study will provide valuable guidance for the practical applications of pentamode acoustic cloak.