Computational method for acoustic wave focusing
  Scattering properties of a material are changed when the material is injected with small acoustically soft particles. It is shown that its new scattering behavior can be understood as a solution of a potential scattering problem with the potential $q$ explicitly related to the density of the small particles. In this paper we examine the inverse problem of designing a material with the desired focusing properties. An algorithm for such a problem is examined from the theoretical as well as from the numerical perspective. 
A.G. Ramm and S. Gutman