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21 CHAPTER ELEVEN EFFECT OF METEOROLOGY ON AVIATION NOISE Meteorology plays a very important role in the propagation of adjustment is small. However, at large propagation distances sound. Simply put, air absorbs sound. As sound travels the effects are non-trivial. Even this correction is somewhat through the atmosphere it is attenuated by this absorption. simplistic, in that it is based on a homogeneous atmosphere; Complicating matters is that air absorption varies with tem- that is, constant temperature and humidity and does not attempt perature, humidity, and the frequency of the sound. Sound to correct for temperature gradients. travels downwind better than upwind. Temperature, humidity, and wind speed and direction are not homogeneous in the real One of the consequences of the complex way weather atmosphere, but are changing constantly. Temperature and/or affects sound propagation is that noise models are limited to es- wind gradients cause refraction (bending) of sound waves. timating noise levels for average conditions. Comparing noise model predictions with short-term noise measurements is The Integrated Noise Model (INM), Version 7, includes the meaningless, as atmospheric effects are not adequately effect of meteorology in two ways (INM Users Guide 2007). accounted for in the model. However, long-term measurements First, temperature is used to calculate aircraft performance; that will produce an average noise level in which atmospheric is, an aircraft climbs much better in cool weather than in hot effects will tend to average out making comparison with noise weather. INM 7 now includes an option to match atmospheric model results much more meaningful. sound propagation to aircraft performance. That is the second way INM can use temperature. The INM uses noise data in the Current research includes highly technically complex stud- form of a noise-power-distance (NPD) curve based on stan- ies analyzing how sound levels increase as the refractive dard temperature and humidity. This version includes options curvature goes from negative to positive values and ground to adjust the NPD curves to the user-selected average temper- effects have complex effects on propagation over long dis- ature and humidity. The European Civil Aviation Conference tances. Research indicates large-scale turbulence is a significant provides technical routines to adjust the NPD curves to the cause of acoustic signal fluctuations, particularly in the signal actual temperature and humidity from standard conditions phase. Other more site-specific research evaluates the influence (Report on Standard Method . . . 2005). For temperatures of a pine forest on sound propagation, finding high-frequency near standard conditions or at distances near the airport, this attenuation owing to the forest that increases with distance.