The US National Renewable Energy Laboratory is about to begin a new study of the turbulent wakes that appear downwind from wind turbines.  Such turbulence can decrease the efficiency of turbines as well as creating physical stresses on the machinery; many researchers also suspect that inconsistent wind speeds across the rotor diameter contributes to increased noise.  It’s certainly easy to imagine that turbines inside the wakes pictured at the left (from an offshore wind farm in Denmark, where moisture reveals the normally invisible patterns) might be under stresses that would increase noise output.

For more on the NREL study, see this feature in Science Daily.