State’s turbine study must deal with complex science behind sound 

FAIRHAVEN – Throughout the debate over Fairhaven’s two wind turbines, one sentiment has been shared by opponents and proponents alike: “noise is a funny thing.”

That’s what Fairhaven Wind Developer Sumul Shah said in May when the turbines began operating. He was trying to calm residents’ fears that the turbines would be disruptive.

It’s also what turbine neighbors have been arguing ever since; that people react differently to the turbines, which could be easily ignored by one person and keep others up at night.

“Some people do, other’s don’t,” said Windwise member Peter Goben in May, comparing the turbines to seasickness.

Now, with the Massachusetts Department of Environmental Protection agreeing to do a sound study to determine whether the turbines are in violation of Massachusetts noise regulations, the complexities of turbine sounds will become all the more apparent.

TURBINES AND SOUND WAVES

Sound is a type of energy created by vibrations. When an object vibrates, it bumps into nearby air particles, causing them to vibrate as well. The vibrations, known as sound waves, spread, decreasing in intensity until the particles run out of energy.

The sound produced by a turbine can have different qualities than sounds from other sources due to their size.

The blades of the Fairhaven turbines, for example, have a wingspan of 269 feet. Wind speed is not consistent at different heights, so as the blade rotates, it cuts through faster-moving air at the top of its cycle and slower air at the bottom. Because of this, turbine sounds pulsate, typically every 1.25 seconds, according to DEP Deputy Regional Director Laurel Carlson. That pulsation is often described as a whooshing noise.

James Manwell, the director of the wind energy research program at the University of Massachusetts Amherst, said turbines make a “distinctive sound.”

“You can definitely tell a wind turbine sound from other sounds,” he said.

The human ear can hear sounds that are between zero and 120 decibels. Most sounds in daily life range from 20 to 85 decibels. The rustle of leaves is typically about 10 decibels, while an alarm clock is typically 80 decibels, according to Stephen Wiehe, a project manager at the company that owns and engineers Falmouth’s two turbines.

The volume of noise from turbines people hear on the ground also depends on the characteristics of their surroundings.

If turbines are surrounded by hard surfaces, like concrete, or water, the turbine sound reflects off of them, amplifying it, said Marc Wolman, an environmental engineer with the DEP. Plants tend to absorb sound, making the area quieter.

Manwell said trees can block out the noise of strong ambient winds at ground level, making the turbine sound louder in comparison to background noise, Manwell said.

“It could be windy up high, but the trees block it out, so the turbines won’t be masked by the sound of the wind,” Manwell said.

The biggest factor in turbine volume is how far away you are from them, Manwell said.

Fairhaven’s wind turbines are positioned an average of 1,300 feet from the nearest residents. Their closest neighbors, on Little Bay Road, are 900 feet from the turbines.

Sound waves are measured in two ways. Decibels measure the amplitude, or height, of a sound wave, which relates to the volume of the noise created. Hertz measure the frequency, or pressure, of the sound wave, which relates to the tone the wave creates. Higher noises have higher frequencies, while lower noises have lower frequencies.

Humans cannot detect a sound-volume change of less than three decibels. If a sound becomes twice as loud, it has increased by 10 decibels.

A turbine is in violation of Fairhaven’s zoning laws if it is louder than 30 decibels at the nearest property. By contrast, the Massachusetts sound regulations hinge on the 10-decibel point at which a noise doubles in volume. Therefore, a turbine is in violation of Massachusetts law if it is 10 decibels louder than the background noise of an area.

WHAT IS A SOUND STUDY?

When the DEP did its sound study in Falmouth in September 2011, it chose five locations where there had been complaints about the turbine noise in order to capture the “worst-case” conditions, according to the final report. Testing was done during four nights at each location in order to test the noise level of the turbines in low, medium and high wind speeds. Turbine volume level was recorded every five seconds during a two-to-three hour period at each location.

The DEP then compared the second-quietest measurement of the background noise to the loudest measurement of turbine sound to see if there was a violation.

Some critics of the study say the agency should have compared background noise to the average turbine sound, instead of the maximum sound. But the DEP’s Carlson said that the pulsating nature of turbine sound means there is a distinct difference in volume from when the blades are pointing up than when they are pointing down.

“People react more to the maximum, it’s the maximum that’s going to annoy you because it’s the loud sound,” she said. “Averaging it out isn’t indicative of what people hear. It’s not like your average white noise static sound.”

In Falmouth, the difference between the turbine and the background noise was greater than 10 decibels in low, medium and high winds. One of Falmouth’s turbines was shut off as a result of the testing, and more testing is being done to see if the other turbine, which currently runs from 7 a.m. to 7 p.m. daily, is also in violation during the day.

“That makes you much more confident that you have a condition,” Carlson said. “If you only have one finding that’s in violation, you probably have to do more testing.”

Carlson said the Falmouth tests were the first time the agency has done noise testing on wind turbines. Now, the agency is conducting noise tests during the daytime which include seven sites. The agency added sites to its daytime test because wind direction varies five to 10 degrees at any given time.

“Turbines will effect different locations differently based on wind direction,” Carlson said.

FAIRHAVEN

Carlson said she does not expect to find a noise violation in Fairhaven, because she believes Falmouth is a quieter town.

“In the middle of the night it was 29 decibels (in Falmouth) – we call that national park quiet,” she said. “We’re not expecting to find that level of quiet in Fairhaven or any other community.”

Fairhaven Wind developer Sumul Shah also does not expect that the DEP will find a noise violation in Fairhaven. He said Sinovel, the turbines’ manufacturer, put a “sound guarantee” on the turbines.

Shah said he would not reveal the specific level of the guarantee, but said it was input into a sound model of Fairhaven that calculated the turbines should not be in violation.

“We asked for the guarantee because we want to be in compliance,” he said. “If it’s not in compliance, it’s not performing in accordance to the model.”

Peter Guldberg, a Waltham-based noise engineer, conducted the model used by Shah. He compared the sound guarantee to the wattage of a light bulb.

“It’s just one factor,” he said. “There’s a difference between how bright the light bulb is and how much light is falling on the page of your book. There’s also a difference between how loud the turbine is when you’re standing on top of it and how much you can hear at your house.”

Fairhaven officials said they expect the tests to begin in the next month or so, but do not know when they would be completed. If the turbines are found to be in violation of state law, Shah, the turbine developer, said he would ask the manufacturer to look at the turbines, because his own tests show they should not be in violation.

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Correction: Turbine zoning [July 04, 2012]

A report Sunday incorrectly described Fairhaven’s wind turbine zoning laws. Turbines are in violation if they are louder than 60 decibels at the nearest property.

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