On the effects of rough ground and atmospheric absorption on aircraft noise

Abstract The noise received from an aircraft is modified by atmospheric attenuation and reflections from the ground. The interference of direct and reflected waves is simplest for a flat ground, whereas multiple reflections can occur for rough ground or mountainous surroundings. The ground characteristics, like reflection and absorption factors or impedance, also affect the received sound. All these effects have to be considered with respect to the path of the aircraft. Most of the literature about ground effects on aircraft noise considers a point source over a flat ground, using the method of images, that does not extend readily to rough ground. The effect of rough ground on aircraft noise can be modelled by: (i) identification of reflection points (there may be several points); (ii) use of a complex reflection coefficient (with amplitude and phase changes) at each reflection point; (iii) adding all reflected waves within line-of-sight of the receiver, that is not blocked by terrain (there is no blockage for a flat ground).