An article from Vermont offers an unusually detailed and complete report on a Public Service Board hearing about noise at a proposed wind farm in Lowell. I highly recommend it.
The article summarizes the testimony of two noise experts, each of whom was proposing what they felt was a proper conservative noise limit; the article presents each approach quite well, and gives a good sense of the judgment calls that regulators are being asked to make about wind farm noise.
Les Blomberg urged a 35dB standard at property lines, which would help keep noise levels low enough for folks to put a chair in the yard and relax. He used an EPA technique to suggest that turbine noise should be regulated to a lower sound level than other noise sources.
Ken Kalinsky proposed a 45dB standard outside the home, which would protect against sleep disruption, and not limit outdoor conversation, though may interfere with quieter outdoor activities such as listening to songbirds. He said that 45dB is more conservative a limit than those used in many other places, notably the World Health Organization standard for protecting health.