We present a geometrical theory of diffraction (GTD) for sound radiation and structural response. Six types of hybrid rays are identified travelling either from structure-tofluid (sound radiation) or from fluid-to-structure (structural response). Three rays correspond to sound radiation by the surface, edge, or corner of a structure while the three others are the reciprocal paths corresponding to sound-tovibration conversion by the surface, edge, or corner. We present the calculation of

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... cal properties of wavefronts (principal directions and curvatures) and their laws of transformation during an interaction process. Furthermore, some simple explicit relationships for diffraction coefficients are given under the light fluid assumption. Finally, two examples are discussed to illustrate the concepts. The first one is a pure radiation problem while the second one involves transmission through walls, structural response and sound radiation.