A sneakily fascinating legal response was recently released, in which the State of Vermont Department of Public Service (DPS) responds to request by wind farm neighbor Paul Brouha for relief from noise coming from the Sheffield Wind Farm.  Most of the technical back-and-forth amounts to quibbling between sound experts about 1-3dB differences, caused by slightly different monitoring techniques.  This minutia matters, in that it may determine whether the Sheffield project is just barely in compliance or just barely too loud at times; after all, limits are limits.  However, as usual in such situations, even if slight adjustments in operations were made to bring the sound levels down 1-3dB, such small changes are unlikely to change how loud the sounds seems at the home in question (the human ear generally can’t perceive a difference of less than 3dB).

Still, buried in the data at the end of the submission is some interesting information about how often sound levels reach various thresholds in each season.  The wind farm company, Vermont Wind, had done some on-the-ground sound monitoring at a location slightly closer than Brouha’s home, and the results shed some light on why some wind farm neighbors may be bothered by the noise.

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Paul Brouha has found the noise from Sheffield Wind bothersome inside his home, and commissioned a study to examine the assumptions being used by Vermont Wind regarding sound attenuation (reduction) by windows. The new document is a report by another sound consultant, Acentech, hired by the DPS to respond to both Brouha’s consultant, and to earlier on-the-ground measurements made at a nearby location by a consultant for Vermont Wind.  The report makes for some interesting reading for noise geeks like me, focusing on several key ambiguities in measurement, assessment, and modeling techniques that need to be resolved; Acentech urges regulators to settle on one clear approach to assessing compliance.  It also sharply critiques the thoroughness of Vermont Wind’s consultant, especially an apparent lack of measurements of noise attenuation with windows open.  In this central point, the Acentech report generally agrees that the assumptions about attenuation were greatly lacking, and that Brouha’s home is probably experiencing inside sound levels in some seasons that are very close to the noise standards.  (For the record, Acentech’s preferred technique sets Outdoor-Indoor Level Reductions at 25dBA with windows closed, 6dBA with windows partially open, and 1dBA with windows fully open, the latter two dramatically lower than commonly-used benchmarks of 10-15dB.)  Still, all the ambiguities about how the state wants sound to be measured and assessed leave it unclear whether violations are actually occurring.  Again, this is all deep-in-the-weeds stuff.

UPDATE, 1/4/16: The State PSB has decided to open an official investigation of Brouha’s noise complaints.  It has appointed a hearing officer and charged him with determining whether the noise standards need clarification; Brouha contends that no clarification is needed and is asking for permanent 24-hour sound monitoring at his house.

But coming up for air, at the end of the report (literally on the final page) we find the central piece of data, a chart that illuminates a key question for many wind farm neighbors: how far from the project is turbine sound audible, and how often?  There is a lot of controversy about exactly what noise levels regulations should aim for, with the wind industry generally favoring 50dB, or at most 45dB, and community advocates often pushing for 40dB or even 35dB, hoping to keep the noise from being dominant in their yards.

Sheffield Wind is supposed to hit a maximum of 45dB outside homes, and 30dB inside.  45dB is clearly audible in many situations; this is why many neighbors push for lower limits for outdoor noise levels.  30dB is around the threshold of hearing for most people in their bedrooms (or outside on a quiet night).

What’s interesting about the data shared here is that it quantifies how often the noise reached 45dB, 40dB, and 35dB in each season.  But more than that, these measurements are made at a location that is VERY far from the project, much farther than most regulations require.  The monitoring location is 7180 ft (1.35mi) from the project, and Brouha lives 9290 ft (1.75mi) away.  (Note: These reported distances appear to be from the center of the project; the accompanying map suggests the monitoring location is about 4500 ft from the nearest turbine, while Brouha is about 6500 ft from the nearest, with a total of perhaps 3 within 1.5 miles.)

A 4500-ft. setback is often considered to be an extreme setback; towns that suggest such a thing are usually considered to be precluding wind farm development.  Yet at this distance, the monitoring data suggests that the wind farm noise reaches 38-41dB for 2-7% of the time, varying by season (this is the threshold for keeping inside noise low enough with windows partially open, using two different measurement techniques).  And that it reaches 33-35dB—audible when ground-level winds are nil—for 10-14% of the time (this is the threshold for windows fully open).  In Vermont, summers are relatively less windy, so there are few problems then (1% of the time above 33dB), a relief since this is when windows are most apt to be open.

These proportions do not consider whether the high noise is at night or during the day, just the total hours at or above each level. But the numbers do help give a senses of how often noise levels outside a home will be at levels that some people consider disturbing. And bear in mind that these measurements are being made at a distance that is 3 or 4 times the 1000-1500 foot setbacks that are widely used when planning and building wind farms.

It may seem like 5% or 10% of the time is no big deal.  However, an earlier AEI reality-check shows how noise happening occasionally could be experienced by neighbors as a chronic problem:

But let’s do some math: 4% of the hours in a year could mean 58 days with peak sounds for 6 hours. That’s two months of the year. Or, more days with shorter periods of peak sound; say, 116 days (a third of the year) for 3 hours. It’s likely that the louder times will cluster seasonally, when high winds or shear conditions or turbulence is more common; this would create longer periods in which the loudest times are a regular occurrence.

Looking at the data, we see the roughly 40dB level happening about 6% of the time in winter, 5% of the time in spring, and 4% of the time in fall.  35dB occurs about 10% of the time in every season except summer.

Is this too much noise?  That’s really a personal question; humans differ widely in how sensitive we are to sound.  But seeing these levels happening at this frequency at a location close to a mile from the nearest turbine does cast the questions in a clearer light.

Addendum/context: It should be stressed that much of the time, nearby ambient sounds (leaves rustling, traffic, etc.) will indeed drown out turbine sounds in the 33-41dB range discussed above, so that the percentage of time that these levels will actually be audible is likely less than the time they are occurring.  Levels of 45-50dB are much more apt to be regularly audible above routine ambient sounds (again, note that the measurements above are quite distant from the turbines, suggesting that more problematic noise levels are likely occurring in the typical 1200-1700 foot setback range).  The “worst-case” conditions for neighbors tend to be times when the wind is blowing at turbine hub height but is minimal at ground level; this can commonly occur at night, and is also a natural consequence of wooded landscapes, where wind speeds are notably reduced within the vegetative canopy (even among leaves at the top of trees), as compared to the edges of nearby open areas such as large fields or ponds, and in contrast to the steady flow of winds at a 200-400 feet where the turbines are turning.  Foggy or “socked in” low-cloud conditions also tend to favor sound propagation, with both traffic and turbines being much louder than normal for those at any distance.  For many wind farm neighbors, intrusive sound levels occur sporadically and unpredictably, which can make it all the more difficult to make peace with.  While most neighbors come to terms with the noise as it comes and goes, for some, it becomes an intolerable intrusion on their sense of home and place.  It’s important to get a clear understanding of the audibility of turbines in the surrounding landscape, even at levels as low as 30-35dB; from there, we find ourselves once again facing difficult questions about how much tolerance we should expect from the public—and how much empathy and accommodation should be granted to those affected—when new infrastructure is built near existing homes.