Cylindrical spreading of noise from a wind turbine

•The cylindrical spreading (CS) comes from the low level jet (LLJ).•The wind profile of LLJ is characterized by the jet parameter, the maximal wind speed, and its height.•CS begins at the critical distance, where the superposition of the direct and once reflected rays occurs.

Noise prediction is very important for new wind farms located near to noise sensitive receivers. To minimize uncertainties of noise level calculation, the effect of cylindrical spreading must be taken into account far away from a turbine. Cylindrical spreading of sound from point source (wine turbine) results from downwind refraction, induced by low level jet. An LLJ is characterized by a wind speed maximum of at least 10–20 m/s with peak speeds up to 30 m/s, at a height a few hundred meters above the ground. Two categories of LLJ are discussed: concaved upward and concaved downward. Cylindrical spreading begins at the critical distance, RR, where the superposition of the direct and once reflected rays occurs. The method for calculation of RR, for two LLJ categories, is discussed.