An integral equation technique for scattering problems with mixed   boundary conditions
  This paper presents an integral formulation for Helmholtz problems with mixed boundary conditions. Unlike most integral equation techniques for mixed boundary value problems, the proposed method uses a global boundary charge density. As a result, Calder\'on identities can be utilized to avoid the use of hypersingular integral operators. More importantly, the formulation avoids spurious resonances. Numerical results illustrate the performance of the proposed solution technique. 
Adrianna Gillman
math.NA