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