Calculation of Multiple Diffraction in a Downward Refracting Atmosphere With Ray-Based Engineering Models

With engineering models like Harmonoise, NMPB:2008 or CNOSSOS-EU:2015 favorable propagation conditions are taken into account with curved ray paths. In all these models, the insertion loss caused by a barrier is assumed to be a function of the path length difference between the curved ray over the diffracting edge and the direct curved ray connecting source and receiver in absence of the barrier. With many screening objects – a frequently occurring case in noise-mapping for built-up areas – NMPB:2008 and CNOSSOS-EU:2015 proceed by, in a first step, selecting the relevant diffracting edges with the so-called rubber-band-method and, in a second step, calculating the path length difference by connecting these edges with curved ray segments. Practical experience has shown that this method is flawed and may even produce level-increases if a new barrier is inserted. To empower these engineering methods to properly handle multiple diffraction in downward refracting atmospheric conditions an improved method was developed and tested. The strategy is analogous to the rubber-band-method; it replaces straight segments with arcs and logically extends the concept of a "convex envelope". A step-by-step procedure ready for implementation in software is presented and its performance is demonstrated.