-The acoustic ray method rests upon specular reflection, an intuition that gives access only to an approximation of the solution by not taking into account the parts of the field called diffusion and diffraction. In trying to understand rationally the roots of the approximation, it has appeared that the image source could be generalized and also that errors may be partially due to missing generalized sources, already in elementary geometries such as obtuse angles. Indeed, it is shown that the exact integral solution of a 2D acoustic problem, expressed as a series of terms, could be seen as the contribution of the different image sources, via a partial use of the Huygens' Principle. With the correspondence between the terms and the image sources shown, the missing sources would appear and the method would thereby be refined.