Although the selection of the most appropriate ground type will decrease the differences between measured and predicted results, over long distances, ground effects appear to be overestimated in TNM 2.5. That is, for distances greater than about 500 feet, results are being under-predicted for soft ground and over-predicted for hard ground. One possibility for these differences may be that the direct and reflected sound waves become less coherent over distance due to air turbulence and ground (or water) roughness [Chessell 1977] [Plovsing 2001]. Implementing a distance-dependent coherence function for the interacting sound waves is being investigated for possible inclusion in a future version of TNM.
