Continuous acoustic measurements were taken for two years at two different wind turbine sites in Sweden. The sites were covered by forest; one site is relatively flat while the other has shifting terrain. Meteorological parameters from the ground to above 120 m height were measured in parallel with the acoustics. The long measurement time capture all different sound propagation conditions that occur due to seasonal and daily variations in the weather. Strict criteria were developed to avoid disturbances from ambient noise. The results show that meteorological effects on wind turbine sound propagation increase with distance and start to be important at distances somewhere between 400 – 1000 m. Variations of expected values between 6 - 14 dBA at the immission point are found depending on ground conditions and refraction. The effects are seen for all 1/3-octave bands from 63 Hz – 1000 Hz.
A comparison between the wind turbine sound measurements and the Nord2000, ISO 9613-2 and Swedish Environmental Protection Agency sound propagation models reveals that models underestimates the highest WT sound levels. This shows the importance of including meteorological effects and terrain shape in models, to give accurate predictions.