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A semi-empirical method is employed to calculate the time-average sound level of the wind turbine noise generation and propagation. Both are affected by refraction due to wind speed gradient. Under upwind conditions the partially ensonified zone separates the fully ensonified zone (close to the turbine) and the shadow zone (far away from the turbine). Refraction is described in terms of the wind speed linear profile fitted to the power law profile. The rotating blades are treated as a two-dimensional circular source in the vertical plane. Inside the partially ensonified zone the effective A-weighted sound power decreases when the receiver moves from the turbine toward the shadow zone. It is assumed that no sound energy is diffracted and scattered into the shadow zone.

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