Governments generally use some combination of two techniques to control noise at wind farms. The first way is the most direct - establish upper noise limits (in decibels, usually annotated dBA). While this seems to be the most obvious solution, in practice it gets complicated and oft times allows circumvention. The second way is to establish setbacks, and while indirect it is quite difficult to circumvent. The noise regulations of most non-local governments use the first method, usually because the noise regulations pre-dated wind farms.

Local councils, often enacting rules about specific wind farms, may use a combination of both methods, with the first method reflecting the legal mandate and the second used in siting decisions. Even for the same project both methods are used. During the planning phase a computer modeling of the noise levels is generally done, providing the regulators with assurances that the applicable decibel-related noise limits will not be exceeded. But when it comes to actually citing the turbines, most of the discussion and negotiation is about setbacks. And then, once the project is in operation, any complaints or any legal action would probably be judged against the noise level regulations, not the setbacks.

Setbacks are pretty easy to understand, plus there's another section on them, so below I'll concentrate on explaining how decibel related regulations work.

Within decibel-related schemes there are also two techniques: (1)limits above pre-project ambient levels and (2)absolute levels. Often they are used in an and/or combination. Ambient-related rules typically measure or assume an ambient noise level and then the allowance is added to arrive at the upper limit. If measuring, most jurisdictions use L90 as the ambient. Measuring requires professional equipment, set up by a professional, left in several places for several days each, in all sorts of weather. This gets expensive. So more often the area is generalized as i.e. "rural" and assigned an ambient noise level of i.e. 30dB. The allowance typically varies anywhere from 5 to 10 dB. So a typical example would be an assumed ambient of 30 plus the allowance of 10 for a maximum of 40 dB. Absolute levels are just that. They typically range anywhere from 40 to 50 dB. Many jurisdictions add penalties to these limits for certain types of more intrusive noise, i.e. it varies in some sort of pattern (like beats) or has some sort of tone to it.

Although I've used the words "maximum" and "upper limit" I'm not aware of any rules that specify an instantaneous maximum, or even an L10 maximum. Almost without exception they use Leq. Because Leq must be taken over a time period they may specify the mimimum time, i.e. 10 minutes. Of course this allows some amount of noise above the maximum, as long as the average stays below the limit.

Most jurisdictions further define the limit to use one of the standard frequency weightings. I'm not aware of anyone who uses anything but the A weighting. So the final expression of the maximum might be something like LAeq10.

It is well understood that wind turbines produce more noise in certain directions and in certain conditions. So many jurisdictions add language to the rules that specify "worse predictable case" or something to that effect.

So far these rules seem fairly straight-forward. But the devil is always in the details, and I'll have a great deal more to say about the computer modeling and taking measurements in the Regulation Problems section.