Cylindrical spreading due to downwind refraction.
Downwind propagation is analyzed for a low level jet (LLJ). The LLJ is characterized by a wind speed maximum (at least 10-20 m/s with peak speeds up to 30 m/s) a few hundred meters above the ground. Close to an elevated point source, such as a wind turbine or an aircraft, spherical spreading results in a 6 dB decrease in sound level per doubling of the distance. Wind turbine noise measurements show that at a transition distance, the downwind propagation changes the spherical spreading into a cylindrical spreading with a 3 dB decrease. It is shown how the transition distance and sound intensity depend on the LLJ parameters. The pivotal phenomenon is the non-coherent superposition of ground reflected rays in the turbulent atmosphere.
Institute of Acoustics, Faculty of Physics, A. Mickiewicz University, 61- 614 Poznan, Poland.
Rufin
R
Makarewicz