The integral equation containing oblique derivative is derived. It is shown by examples, the scattering of spherical wave for rigid and soft boundaries and the propagation of spherical wave by Huygens' principle, that the formulation gives the correct results. Then the uniqueness of the solution is studied heuristically, and showed non uniqueness of this formulation for homogeneous exterios Neumann problem for wavenumber of interior Neumann problem. It is also shown, in order to get the unique solution, coupled equation of BF with this form should be used. The transient from corresponding to this form is shown.
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