Reflected sound in street canyons – diffuse or specular?

Sound reflections at building facades are generally calculated applying the mirror image method and therefore it is assumed generally that specular reflections occur. But in many cases of structured surfaces with balconies, jutties and other irregularities these facades are obviously not acoustically plain and therefore diffuse reflections may occur. With financial support by the German BAST (Bundesanstalt für Straßenwesen) the differences of specular and diffuse reflections with respect to road traffic noise in street canyons and in other tight built up areas have been investigated. Background was to get a better knowledge about the uncertainties of possible software strategies and to take the level increases caused by high order reflections in street canyons into account in noise calculations. Experimental and theoretical findings with respect to time dependent sound pressure levels and equivalent continuous sound pressure levels are presented and discussed.