Maggi-Rubinowicz Diffraction Correction for Ray-Tracing Calculations of Engine Noise Shielding

Abstract

For the assessment of geometrical shielding of engine noise already in the design phase of aircrafts, a ray-tracing code is under development at DLR. Since in the high frequency limit no sound signal reaches the shadow zone, diffraction effects have to be included using special approximations like, e.g., the geometrical theory of diffraction. An alternative approach is presented here which is based on the Maggi-Rubinowicz formulation of the Kirchhoff diffraction theory. Thereby, an approximation of the diffracted field is calculated by a line integral along the shadow boundary on the surface of the body. Results for generic geometries, like a disk, a sphere, a strip and a cylinder will be presented, as well as calculations for a full aircraft geometry. The results will be compared with a boundary element method. The coincidence of the shielding factor for the generic geometries is quite good. Although in case of the aircraft geometry essential features of the shielding are well described by the approximation, more accurate results could be obtained taking into account also the diffraction field of reflected rays.