In the present work we investigate the effect of noncompactncss of aviation noise sources on airframe shielding. Performed experimental results showed what shielding effect is very sensitive to the type of source and geometrical features of the shielding surface. For example, the features of rotor tonal noise and rotor broadband noise shielding are differed. In the present work we seek for the theoretical methods for correct predicting of shielding effect for noncompact sources, Fresnel/kirchhoff methods and Geometry Theory of Diffraction (GTD) are under consideration together with some exact solutions. GTD shows the optimal results as compared with other theories for calculating shielding efficiency in the case of noncompact sources calculation. If GTD is applied to shielding effect for noncompact sources generating rotating azimuthal modes (fan tonal noise), then some interesting features are appeared such as the loss of symmetry, the strong dependence of shielding field on the number of azimuthal mode, existence of directions, in which sound emission is considerably amplified.
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