One of the contentious wind turbines in Falmouth, Massachusetts has been ruled out of compliance with state noise rules after an investigation by the state Department of Environmental Protection.  The noise study found that the turbine created noise levels at one residence less than 1500 feet away that exceeded background ambient noise by more than 10dB, which the agency described as “unacceptable to local residents.”  Residents reported a sense of vindication at the finding, and the town has shut down the turbine; the town had already agreed to shut their second turbine down at night while seeking a community-wide consensus on moving forward.  Todd Drummey, 48, a financial planner who lives 3,000 feet from the closest turbine in Falmouth, compared the noise of the turbines to jets and pile drivers, depending on the weather. He said shutting them down, at least temporarily, was a good first step. “But what I would really love to see is that they’re moved,’’ he said, adding he also has trouble sleeping at night.

DEP officials stress that the finding should not be seen as evidence of a more widespread noise issue, noting that turbine designs and topography vary from project to project.  In particular, the offending turbine is an older-style “stall-regulated” design (in which blade speed is controlled by tilting the three blades away from the wind), which is louder in high winds than “pitch-regulated” turbines, in which each blade rotates while remaining in their original shared plane. Falmouth officials suggested that they may do some daytime testing of the offending turbine, to see whether it can join the other unit and operate within 10dB of ambient during the day (the DEP tests focused on the quietest times of the night).

In looking over the DEP’s report, what jumped out at me was that while at only one of the five locations tested by the DEP did the turbine sounds consistently exceed ambient by over 10dB, at every location, the average peak turbine sound was at least 5dB louder, and in 10 of the 12 individual testing sessions, the turbines were more than 7dB louder.  This points to the subtle yet important question of what is used as a standard; many acousticians consider 5dB a difference that is likely to trigger widespread complaints, with 10dB likely to lead to significant problems.   New York State noise law aims to keep noise sources from exceeding a 6dB difference.  Yet the more substantial 10dB difference has been widely adopted as a standard, and could be one factor in seeing more noise complaints than might be expected by project planners. One more subtle but rarely considered factor is that while 3dB is considered a difference that is just audible over a similar ambient background, noise sources with different frequency spectrums than current background levels can be perceived at a few decibels below ambient, as could be the case with quiet night time conditions and a turbine sound spectrum heavy in lower frequencies; this could accentuate the audibility of turbines in some conditions, making a lower “dB over ambient” standard more likely to serve the intended purpose of minimizing audibility, annoyance, and complaints.