For the past couple of years the Lake Winds Energy Park in Mason County, Michigan has been embroiled in a contentious dispute about its noise levels (image to left is the “Park” under construction). In April 2013, five months after the 56 turbines began operating, 17 neighbors filed suit, claiming that wind farm noise, vibrations and flickering lights were adversely affecting their health. A few months later, after commissioning an independent sound study, the Mason County Planning Commission formally declared the wind farm out of compliance and demanded a mitigation plan; the developer, Consumers Energy, disputed the findings yet lost two appeals, one at the Zoning Board of Appeals and one in Circuit Court. During that series of challenges, Consumers developed a plan to modify turbine operations for 7 turbines closest to the four sites where they were found to be marginally too loud.
Marginal is indeed the word: the sound study found 4 locations where the sound level peaked at 0.3 to 1.2 decibels over the 45dBA noise limit (it takes 3dB for a difference between two sounds to be audible); when using 10-minute averages, there were no violations. The various explanations by the consultant, Brian Howe, illustrated the fine line that the turbine operations were walking. His report stressed “general compliance with sound level criteria,” and noted that the brief violations “do not represent a statistically significant portion of time and do not indicate a systemic exceedance.” In his initial testimony at an August Planning Commission meeting, he said that there are no recommendations to correct for these times because “there is not a situation where they are predictably going over 45.” Later, in a November letter to the Commission, after learning that the county had previously decided NOT to allow for occasional exceedances, he stressed that “I can assure the County that competent, material and substantive evidence supports the conclusion that the turbines are not in compliance at certain residences on occasion” and elaborated:
Excursions over 45 dBA should have been anticipated since, as outlined in the acoustic study by Tech Environmental prepared in June 2011, the wind energy park was designed with sound levels identically equal to the 45 dBA criteria at some key receptors with no factor of safety to address the fact that the prediction methodology has a stated accuracy worse than +/- 3 dBA. If Tech Environmental was aware that achieving the criteria even 95% of the time was unacceptable to the County, it would have been prudent to incorporate a suitable safety margin to account for the statistical variation in sound levels.
And this is the first half of the central lesson here: it’s essential that enough of a safety factor is built in to the sound models to account for known variability in sound production (how loud the blades are in various unsteady wind conditions) and sound propagation (how far sound travels as it gradually loses power). Regular readers will know that variability is indeed, as Howe mentioned, often more than the simplified 3dB margin of error that was neglected here (see AEI’s 2012 report). The second half the lesson is related: when noise limits—for the sound of the turbines when it reaches nearby homes—are set as high as 45dBA, they will be regularly audible at these homes, and likely well above night-time ambient sound levels. As many acousticians have stressed for years, these situations are very apt to trigger a significant number of complaints, especially if there are dozens of homes in that nearby range. Here, we had the worst of both worlds: turbine siting plans that pushed sound right at the limit into nearby homes, and a limit that was on the high end of tolerability for many neighbors. Indeed, after one such cautionary report was presented to the Mason County Planning Board, it decided to lower the limit to 40dB, but that change was revoked after push-back from Consumers Energy.
With this backdrop, this week the 17 original plaintiffs in the noise nuisance lawsuit agreed to a settlement offer from Consumers;
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