When calculating sound propagation, substitute sound sources can be used, for example, to model the sound emission from openings in galleries, slots or tunnel mouths. This has already been described in Appendix IV of the NMPB 2008. The energy of a diffuse sound field can be taken into account on the basis of Huygens' principle of wave propagation in the form of an equivalent sound source.The Kirchhoff diffraction at screens can be treated analytically, e.g. with Fresnel integrals. Alternatively, a numerical calculation can also be carried out, in which only the phase-aligned sound propagation of point sound sources must be known. The sound pressure of the sound diffracted at the screen can be calculated from the in-phase addition of the elementary waves arriving at the receiver, which are emitted by substitute sound sources above the screen edge.If the sound propagation is considered energetically, as is the case with the simple engineering sound propagation methods, a directional characteristic of the equivalent sound sources must be assumed when dealing with diffraction phenomena. In addition to pure diffraction, more complicated diffraction and interference phenomena can be treated, such as those that occur with curved noise barriers or noise barriers with attachments.