It is well known that as the wind blows harder wind turbines produce more noise, enough so that the setbacks would have to be quite large to keep the noise at the neighbors under typical limits. It is also well known that the wind itself produces more noise as it blows harder. At one time many jurisdictions reasoned that the higher noise from the wind "masked" the higher noise from the turbine, thus the noise limits should be raised during windy times.

Several years ago, Dutch residents who were close to a German project were complaining that the noise was quite loud, much louder than the models and current acoustic theory would have predicted. A Dutch PhD candidate, van den Berg, was curious to find out what caused this discrepency. He discovered that most of the complaints were in the evening and at night. He measured away, and discovered that the noise levels really were higher than predicted, especially during these times. At that time, Germany allowed the higher limits in stronger winds. But van den Berg's measurement discrepencies were during light wind periods, typical of evening conditions. Then he noticed that the turbines seemed to be spinning at full power (the project operator was not being cooperative, so he had to estimate their output) even when the ground wind speed was close to zero. As a consequence, they were producing full-power noise that was not being masked out by ground winds.

It is well known in meterological circles that after the sun goes down, the atmosphere - no longer being heated from below - becomes more stable, no longer mixing vertically. This allows wind speeds to vary with altitude, becoming stratified. It is common for winds at modern turbine heights to be strong while winds on the ground are weak, and there were already formulas to describe this effect. To make matters worse, quite often the winds actually stratify, forming boundaries that also trap noise. Do you remember our discussion of cylindrical vs. hemispherical dispersion, and how it changes the rate that noise decreases? This wind shear effect can be a surprisingly big deal, causing much higher-than-expected noise for recipients, to the point where health effects are a concern, not to mention annoyance and sleep disturbance. Presently (March 2009) only Ontario and New Zealand still permit the use of masking. UPDATE, June, 2009 - it looks like the regulations created pursuant to the GEA in Ontario have eliminated the masking provision.As I find out more I'll post it here.

Amherst Island's own John Harrison has been trying to get the Ontario MOE to join most of the rest of the world, so far without success. I suspect that, under the spell of the developers (and perhaps some pressure from the politicians) they initially allowed the masking and are now afraid of changing their rules and thus admitting to having made a mistake.

• Amherst Island's John Harrison, 0.1mb, letter on wind shear.
• van den Berg Article, 0.6mb, in the Journal of Sound and Vibration.
• van den Berg Paper, 4.8mb, The Sounds of High Winds, technical details about wind shear. The non-technical parts are very readible, and well worth the time.
• Smith Paper, 0.7mb, NREL report on wind shear.

As part of Ontario's review of the noise issues presented by van den Berg, they hired a consultant, Dr. Ramakrishnan, to write a report evaluating wind shear and masking issues, generally opining that Ontario's current rules were sufficient. John Harrison wrote a response to that report.

Final Ramakrishnan Report, 1.2mb, originally from Ontario's web site.
Harrison Response, 0.2mb, directly from him.

Below are references giving more details on all of this. As always, comments, corrections and additions are welcome.

• Archer Paper, 2.8mb, distribution of winds.
• Smith Paper, 0.7mb, NREL, Evaluation of Wind Shear.
• Schwartz Paper, 0.7mb, NREL, Wind Shear Characteristics.