Excess Attenuation due to Meteorological Influences and Ground Impedance

Gaussian plume models are used to calculate the propagation of air pollution in the atmosphere. The assumption on the physical structure of the atmosphere implied by those models is used to formulate an acoustic ray model which allows to take into consideration the non-linear functional dependence of the sound speed with height above ground. To obtain a simply to handle propagation model, a second order approximation for the ray traces is used. From this the so called ground effect and meteorological excess attenuation are calculated for spherical waves including the effect of turbulence. It is shown that such a model closely reproduces for stable weather conditions the excess attenuation as described in ISO 9613-2. The general method in ISO 9613-2 is reproduced in its general frequency structure for coherent sources or incoherent sources where the angle of observation between the source and the mirror source is small. Applications of the model to calculate the time history of moving sources under specific weather conditions are demonstrated as well as the calculation of the frequency distribution of the levels as they may be observed from one source at a specific site.