The impedance-matched reduced acoustic cloaking with realizable mass and   its layered design
  The authors present an impedance-matched reduced version of acoustic cloaking whose mass is in a reasonable range. A layered cloak design with isotropic material is also proposed for the reduced cloak. Numerical calculations from the transfer matrix methods show that the present layered cloak can reduce the scattering of an air cylinder substantially. 
Huan-Yang Chen, Tao Yang, Xu-Dong Luo, and Hong-Ru Ma