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Maine lowers wind farm noise limit to 42dB at night

Human impacts, Wind turbines Add comments

The Maine Board of Environmental Protection has approved new wind farm noise standards that will lower night time noise limits to 42dB at the property line of any nearby homesites, a 3dB reduction from the old standard.  The daytime limit of 55dB remains unchanged.  Community groups, which initiated the review with a formal citizens proposal, had encouraged a standard of 35dB. The new rules will need to be approved by the state legislature in January.

Read new rules in full. (note: first half includes markup edits; second half is final version)

The new rules also include detailed procedures for measuring noise and establishing compliance with the noise limits. In the fine print, there is one provision that appears to add noise protection for neighbors, and some that raise questions.  The additional protections include a 5dB penalty if there is moderate blade swich (amplitude modulation) present; any repetitive pulse of sound of 5dB or more will trigger this penalty, meaning the average sound will then need to be 50dB during the day and 37dB at night.

The criteria for establishing compliance raise some questions. While all recordings made during compliance recording sessions must be submitted, the rules say that compliance will be based on the arithmetic average of at least twelve 10-minute recordings taken during a twelve hour period (7am-7pm for daytime, 7pm to 7am for night). The idea is to collect ten-minute periods in which the turbine noise is clearly dominant, so that the periods used don’t contain passing cars, airplanes, insects, birds, wind in trees, or other nearby loud sounds that cause spikes in sound levels.  The compliance criteria will allow some ten-minute periods to exceed the standard, so long as other quiet times during the day or night bring the average level down (in other words, the rules call for a day-long average of 55dB, and a night-long average of 42dB). Can especially quiet periods be chosen to include in the compliance average? There’s no indication that the loudest turbine-related periods must be included, or that the twelve 10-minute periods should be contiguous.  Can twelve periods be cherry-picked to meet the limit?  This is not clear as the rules are written. But, we should probably assume that picking the quietest 12 periods is not allowed, though the rule isn’t explicit on this.

The goal being to determine a night-long average sound level, these concerns are moot if all un-contaminated 10-minute periods go into the group of “at a minimum” twelve periods to be averaged.  If so, it removes the risk of cherry-picking quiet times, but it increases the chance that an extended loud time (perhaps a couple hours worth) could be “averaged out” over the course of the night if the turbines are not turning for several hours at some point.  Likewise, if there are especially loud times in which leaf sounds equal those of the turbines, these periods are likely to be culled from the periods being averaged, even though the pulse of the turbines may be clearly audible, and disturbing, alongside the steadier and higher-frequency leaf sounds.

It would seem more to the point if the occurrence of one 10-minute period (or perhaps three?) over the limit during a given day or night was designated as an “out of compliance” event. However, these nuts and bolts have been hashed out over the past few months, and it’s unlikely the details of the new rules will be tweaked when they go to the legislature for approval.  For now, Mainers can expect some moderate decrease in the sound they’ll hear at night, which may lead to slightly larger setbacks in order to meet the new nighttime criteria.

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