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Aircraft sound heard by an observer can be influenced by a number of factors. As previously stated, meteorology is one such factor. Another factor is the propagation of sound over the ground and as it is affected by terrain. When an aircraft is directly overhead, the sound experienced by an observer is only affected by meteorology. However, as the aircraft passes by or is at lower elevation angles, the sound heard by an observer is both the sound that travels in a straight line from the aircraft plus the sound reflected off the ground. Although the interaction of sound traveling over the ground is quite complex, recent research has provided new insight. The FHWAâs 1978 report on highway noise prediction model presents one of the best descriptions of how to include noise barrier effects in transportation modeling, and includes detailed methodologies to computer noise barrier effects. Another noise modeling program is NOISEMAP, typically used by the Air Force and other organizations to compute environmental noise around airbases and airports. NOISEMAP presumes the ground is flat, level, and at the same altitude as the airbase and runways, limiting the effect of ground ab- sorption. Although usually applicable, there are situations where these variables should be adjusted to improve model accuracy. Plotkin et al. (1992) present detailed descriptions on how NOISEMAP would have to be modified to accom- modate variations in topography. 22 The NATO/CCMS 1994 Working Group identifies various modeling techniques used in different countries and describes issues associated with topographic effects on modeled aviation noise (NATO/CCMS Working Group Study 1994). The report discusses the effect of topography on slant range distance and the effects of shielding owing to topography. Shielding, which is usually not important when an aircraft is overhead, may be very important when an aircraft is at low-elevation angles. The report presents recommendations for including topographic effects in aviation modeling. New research completed in 2000 (Senzig et al. 2000) ex- amined the applicability of available mathematical models of lateral attenuation. Analysis of the data revealed that lateral attenuation is a function of aircraft geometry. For example, lateral attenuation of aircraft with tail-mounted engines was found to agree with published literature, whereas wing- mounted engine aircraft types were found to be less than predicted in the model, resulting in an under-prediction of sideline noise levels. Similarly, in 2006, the Society of Auto- motive Engineers provided detailed calculation methods to compute lateral attenuation (Method for Predicting . . . 2006). Integrated Noise Model Version 7 includes the methodology specified for such calculation. Shielding calculations are also available in Integrated Noise Model Version 6.2 and later, allowing the Integrated Noise Model to work better near air- ports that have nearby hills or steep valleys. CHAPTER TWELVE EFFECT OF TOPOGRAPHY AND GROUND ABSORPTION ON AVIATION NOISE