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