Enhancement of near-cloaking. Part II: the Helmholtz equation
  The aim of this paper is to extend the method of improving cloaking structures in the conductivity to scattering problems. We construct very effective near-cloaking structures for the scattering problem at a fixed frequency. These new structures are, before using the transformation optics, layered structures and are designed so that their first scattering coefficients vanish. Inside the cloaking region, any target has near-zero scattering cross section for a band of frequencies. We analytically show that our new construction significantly enhances the cloaking effect for the Helmholtz equation. 
Habib Ammari, Hyeonbae Kang, Hyundae Lee, and Mikyoung Lim
math.AP