AEI Special Report:
Wind Energy Noise Impacts

Introduction

Wind energy has long been a favorite of many environmental advocates. No carbon emissions, utilizing a free resource without depleting it in the least, even the potential for distributed generation rather than distant centralized power plants: for many of us, wind was the cleanest of green power sources in our dreams of the energy future.

In recent years, as wind turbines have grown from the small backyard kits that the truly committed built in the 70's, the reality has made those dreams become less certain. Modern wind turbines are massive structures, hundreds of feet tall, and often constructed in large wind farms that in effect industrialize rural landscapes, from the rolling grassy hills of California, to the vast rangeland of Texas, to ancient ridgelines in the Appalachians, to the commons in rural England. While the trade-offs may be worth it in some areas, the downsides have become more apparent. Resistance to wind farms is often belittled as NIMBY-ism (Not In My Backyard); but at the same time, proponents often slip into oversimplifeid WARYDU rhetoric (We Are Right; You Don't Understand). If we are to forge a reliable energy future that is respectful of both the environment and the rights of neighbors, we'll need to move past knee-jerk reactions on both sides, and develop best practices that can ensure that the landscape and local residents don't become long-term casualties of today's "Klondike Wind Rush."


Wind farms built and proposed in the UK
From Windbyte.co.uk

This AEI Spotlight Report will focus specifically on the noise impacts of wind turbines and wind farms. However, it is worth mentioning some of the other concerns that rural residents have raised about the sudden appearance of industrial wind farms in their communities. Besides the visual blight, which many residents are, in the end, willing to accept as a tradeoff for producing clean energy, the most common argument against wind energy is that wind farms are notoriously inefficient, rarely achieving even half their rated capacity, due to fluctuating winds. A more important long-term issue, still flying mostly under the radar, is that lease agreements between land owners and power companies can be full of holes, raising the very real specter of ridge lines and rangelands becoming, over time, abandoned junkyards of massive metal hulks, rusting and disintegrating for decades.


How big are modern wind turbines? The ones on the left are 60m and 125m; the one on the right is 95m, with blades sweeping an area the size of a 747.
See full-size images at www.aweo.org/windsize.html and www.mlg.org.au/visual.htm

It is entirely possible that ongoing technical innovation, combined with closer legal scrutiny, can overcome these issues. Similarly, noise impacts are not necessarily deal-killers for wind energy, as long as developers are honest about what is likely to be heard and continue to work diligently to investigate the aspects of wind turbine noise that are still not fully understood. Continually improving turbine design will likely mean that tomorrow's turbines are not as noisy as today's. Local residents should be sure that their fears are not based on others' experiences with older turbines, and wind energy advocates must be diligent in not glossing over potential noise impacts. With continual incorporation of best technology and best practices in siting, wind energy need not be stymied by noise issues. However, with noise impacts gaining more public credence, it is clear that the current boom in wind farm development could well backfire, for both the industry and a clean energy future, if the current generation turbines are sited too close to residences. The Altamont Wind Farm in California, shortsightedly built on a major raptor flyway in the early years of industrial wind development, has rippled through the years as a poster child for the bird-killing power of wind turbines, despite widespread understanding that it was an exceedingly bad siting decision. Similarly, many noise complaints today seem to be coming from people whose homes are on the near edge of fairly loose siting guidelines (within 500 meters in many cases, and often much less). If the thousands of windfarms likely to be built in the coming decade are placed too close to homes, the industry will be faced with an echoing chorus of complaints and resistance for years to come, even if it manages to invent much quieter machines. Better to be conservative, accepting the fact that even occasional atmospheric effects should be factored in to siting decisions today, so as to build a reservoir of good will, rather than a rising tide of complaints.

A 200 ft (65m), 1.5MW wind turbine at the Trent Mesa Wind farm in Texas [PHOTO SOURCE]

A key point is made in a study completed in 2007 by the National Research Council, the research arm of the National Academy of Sciences, which found conflicts are widespread because of a fundamental reality of wind power. The environmental costs -- visual impacts, noise, landscape and wildlife disturbances -- are primarily felt by those near the wind farm. The benefits, however (reduced global warming emissions and other air pollution, less dependence on foreign oil and less mining and drilling) are felt more on the regional or national scale. "Benefits and (costs) don't necessarily affect the same people," said David Policansky, who directed the study. "If you talk to a national representative of an environmental organization, it's quite likely that person will be in favor. Whereas, when you talk to a local representative, it is more likely that person will be opposed to some local project."

This AEI Special Report is designed to provide a layman's introduction to the types of noise produced by wind turbines. It is not our intention to over-emphasize noise complaints, but rather to provide information that can foster informed conversation about any specific wind farm proposal. A recent UK government survey suggests that only about 20% of wind farms tend to generate noise complaints; the question is, what are the factors in those wind farms that may be problematic, and how can we avoid replicating these situations elsewhere?

NOTE: This AEI Special Report will be continually updated, incorporating new research, more recent reports, and suggestions/comments from readers. Several topics will be added over time, including: effects of noise on wildlife and habitat, offshore wind energy, and the health effects of chronic noise exposure.


How Noisy Are Wind Turbines?

The US National Wind Coordinating Collaborative, a multi-stakeholder group that aims to build consensus on best practices to facilitate wind development, summarizes the situation in fairly straightforward terms:

By and large, those affected by the noise generated by wind turbines live within a few miles of a large wind power plant or within several hundred feet of a small plant or individual turbine. Although the noise at these distances is not great -- a 300-kilowatt (kW) turbine typically produces less noise at 400 feet than does light traffic 100 feet away -- it nevertheless is sufficient to be heard indoors and may be especially disturbing in the middle of the night when traffic and household sounds are diminished.

In a similar vein, the American Wind Energy Association's fact sheet on noise notes that "Today, an operating wind farm at a distance of 750 to 1,000 feet is no noisier than a kitchen refrigerator." Which raises a question: how many of us sleep in the kitchen?

The bottom line is that most modern industrial wind turbines are designed to keep noise levels at or below 45dB at 350 meters, which should drop to 35-40dB at 1000m (commercial turbines are quite often built this close to homes). Some are rated at lower sound levels. However, as is noted below, atmospheric conditions can wreak havoc with nice clean sound propagation models, especially at night. And, as turbines get bigger, their noise can be deceptively hard to predict; certainly, they can be quieter at their bases than some distance away, and temperature inversions, wind layers, and other atmospheric effects can lead to surprisingly distant sound impacts.


Two examples of a "Wind Rose"
These diagrams chart the wind patterns at a given turbine location, based on prevailing wind speed from various directions. Most turbines will have an "upwind" side where little sound propogates.

It is important to recognize that night-time ambient noise levels in rural areas are often 35dB or lower; so, it is not that hard for wind farms to become a new and dominant acoustic presence. All too often, wind developers tell local planning boards that the turbines will be inaudible, which is rarely the case. Similarly, some investigations of noise complaints come to the conclusion that anomalously high noise levels occur so infrequently that they are insignificant (a recent UK investigation of Amplitude Modulation found that it was only an issue 5-15% of the time). But if temperature inversions or other atmospheric stability effects that cause excessive noise occur just 10% of the nights, that means that nearby residents may find their sleep disturbed 35 nights a year. Is this insignificant? Such questions need to be considered directly, not shunted aside.

Interestingly, smaller backyard turbines are often noisier than large industrial turbines. This is due to faster blade rotational speeds, and the fact that industry and government money is being devoted to making industrial turbines quieter. And of course, backyard turbines tend to be much closer to residences than industrial machines, making their noise more constant. On the positive side, many new nearly-silent designs are becoming available for small turbines; see the pictures and links at the bottom of the page.

It is sometimes noted that large wind turbines are getting quieter. This is clearly true for the mechanical noise emitted by motors and gears, which have seen noise-reduction design changes including soundproofing of turbine boxes, gears that flex, and sound-dampening to limit vibrations being passed to the tower and ground. While overall noise levels per unit of energy output are dropping, today's turbines are far larger than older ones, so total noise output is not necessarily decreasing, and is now mostly generated by the sound of the turbine arms swinging through huge arcs in the air. Design of turbine blades is of course continually being improved; after all, the noise is a sign of inefficiency (rotational energy sacrificed by aerodynamic turbulence), so newer blades are likely to be quieter.

While in many situations, the sound from turbines is drowned out by nearby wind noise, or is perceived as a gentle whooshing noise that is quite easy to accommodate, in some wind or atmospheric conditions, a pulsing noise can arise, which is much harder to ignore or acclimate to, making it a major source of complaints. Perceptually, the problem is that any pulsed or irregular sound (this rhythmic thumping can wax and wane over the course of a night) will tend to cause more disturbance. It also seems that these sounds travel farther in calm night air; one widely-respected study found that sound levels were 5-15dB louder than predicted in some night-time atmospheric conditions, and noted that residents as far as 1.9km away were disturbed by noise.

In some cases, the pulses are caused as the blades pass the tower, when winds are coming from a particular direction. This has been termed Amplitude Modulation of Aerodynamic Sound, or Aerodynamic Modulation. While the physics of this phenomenon are not fully understood, it appears that only some turbine designs are susceptible to it. Many new turbines are being designed with the rotors on the upwind side of the tower, which should reduce or eliminate this important contributor to turbine noise.


Current Approach to Regulating Wind Farm Noise

While the United States does not have national noise standards, many European countries do. These countries, and many state or county regulations in the US, typically set an absolute sound level that any industrial facility must meet. Commonly, 45dB is used as the night-time limit, and 55dB as the day time limit; higher thresholds are sometimes allowed, but rarely does the night time limit drop below 40dB. The problem comes in rural areas, where night-time ambient noise (wind, distant traffic, etc.) is often 35dB, and sometimes as low as 25dB. Given that 10dB is perceived as twice as loud, the problem is obvious.

It should be noted that the majority of wind farms do not trigger noise complaints. These are likely sited far enough away to work well for nearby residents. A 2007 report from the UK found that roughly 20% of wind farms (27 of 133) had received complaints about noise. While noise modeling (predicting the noise levels around wind turbines) tends to indicate that noise impacts should be insignificant beyond several hundred meters, the French National Academy of Medicine has called for a halt of all large-scale wind development within 1.5 kilometers (roughly 1 mile) of any residence, and the U.K. Noise Association recommends a 1km separation distance. In the US, there is no overall recommendation; siting decisions are made locally, and often are based on a 45dB night-time noise limit, so that turbines are sited no closer than 350m (roughly 1100 feet); 350-700m is often considered a reasonable setback in the US, based on simple sound propagation modeling. Though it is also common for larger set-backs to be used, 1000m or 1500m setbacks are rarely required.


Image: BP.com [IMAGE SOURCE]

The International Standards Organization sets recommendations for all manner of human impacts, including noise. The IMO recommendations are markedly lower than those used in most places, especially in the dead of night (note: ISO 1996-1971 has been replaced by a newer standard, ISO 1996-2:2007; I have yet to find a chart of the new standard. If reader can fill me in, I'll update this chart)

ISO 1996-1971 Recommendations for Community Noise Limits

District Type Daytime Limit Evening Limit
7-11pm
Night Limit
11pm-7am
Rural 35dB 30dB 25dB
Suburban 40dB 35dB 30dB
Urban residential 45dB 40dB 35dB
Urban mixed 50dB 45db 40dB

Low-Frequency Noise

In some cases, low-frequency noise can become an issue with wind turbines. These sounds may be inaudible to the human ear, yet still create physiological responses. Such low-frequency noise could be transmitted through the ground from towers, or be a component of a broadband noise field generated by spinning turbine blades. Low-frequency noise travels greater distances with less loss of intensity than higher-frequency sound.

It is important to measure the noise from turbines using a db(C) scale, which is weighted to accentuate low-frequency components of a broadband sound field. Most noise standards are weighted to the dB(A) scale, which accentuates frequencies heard best by the human ear. It is becoming a standard procedure in dealing with industrial and machine noise to compare db(C) and dB(A) readings; when dB(C) is 20dB more than dB(A), or when dB(C) is 60dB or higher, it is considered an indicator that low-frequency noise is at problematic levels, and the need for special low-frequency mitigation is then generally called for.

Local Regulatory Challenges

Small town governing bodies are generally ill-equipped to address the questions before them when wind energy companies apply for local permits. In many cases, the proposed wind farm is the first outside industrial facility to be proposed in the town; it is almost always the first 24/7 noise source to appear in the local rural landscape and soundscape.

Energy company experts attend town council or selectmen meetings, often submitting comprehensive documentation that is rarely fully comprehensible to the lay members of the town's governing body. While these documents don't generally promise anything quiter than 45dB, the outside experts too often assure local officials that the wind farms will be inaudible--relying on flawed assumptions that high winds will always create enough increase in ambient noise to drown out the turbines. The use of comparisons, such as "a kitchen refrigerator" or "traffic 100 yards away" is likewise a common way of reassuring locals--one such expert went so far as to assure a council that the 45dB drone of turbine noise was "comparable to" bird song on a summer afternoon!

"There are no rules and regulations on windmills," Paul Cheverie, chairman of the Eastern Kings Community Council (Prince Edward Island, Canada) said. "The more we get into it, the more we realize we jumped the gun."

Wisconsin County Approves Stringent Ordinance, Likely to be Sidestepped to State Regulators - Calumet County culminated a two-year process by adopting a wind turbine ordinance that would require all turbines to be no louder than 5dB over background nighttime ambient noise levels, with reductions even from this level when the noise is repetetive or tonal; it also required turbine shut-down whenever there were excessive low frequencies. In practice, this would quite possibly lead to setbacks of a mile or more. The company planning to develop a wind farm in the area expanded the project enough to qualify for state regulation, where they expect to face less stringent requirements. [SEE AEI NEWS DIGEST COVERAGE OF THIS CASE]

Wisconsin towns and counties have been especially proactive in implenting wind farm ordinances. In addition to Calumet County (above), another county adopted a one-mile setback requirement. The state legislature is likely to consider new measures to adopt statewide regulatory standards and preempt these local initiatives. [SOME WISCONSIN NOISE ORDINANCES]

Resources/Advice for landowners signing lease and easement agreements with energy companies

Health Effects

The World Health Organization has found that sound levels during nighttime and late evening hours should be less than 30 dBA during sleeping periods to protect children's health. They noted that a child's autonomous nervous system is 10 to 15 dB more sensitive to noise than adults. Even for adults, health effects are first noted in some studies when the Lmax sound levels exceed 32 dBA, 10-20 dBA lower than the levels needed to cause awakening. The WHO researchers found that sound levels of 50 dBA or more strongly disrupted hormone secretion cycles. For sounds that contain a strong low frequency component, which is typical of wind turbines, WHO says that the limits may need to be even lower than 30 dBA to not put people at risk. This paragraph is from professional comments submitted to the Wisconsin Task Force on Wind Siting Reform, by George Kamperman and Richard James: [READ COMMENTS]

What Some Neighbors Are Hearing

Elmira, Prince Edward Island: 1km from wind farm with ten 120m turbines
Problems began within weeks after the turbines started operating. Downwind from the turbines, when the air was moving just enough to turn them, (12-15 knots from the northeast), the noise was loud. It was a repetitive modulated drone of sound. Dwayne Bailey and his father Kevin both claimed it sometimes was loud enough to rattle the windows of their homes on the family farmstead. The sound was even worse in the field behind their homes. Distances from 1 to 1.5 kilometers were the areas of the most annoying sounds. This spring the winds created constant misery.

"My idea of noise is a horn blowing or a tractor - it disappears," said Sheila Bailey. "This doesn't disappear. Your ears ring. That goes on continuously."

Dwayne developed headaches, popping and ringing ears, and could not sleep. He tried new glasses, prescription sleep aids and earplugs, to no avail. Dwayne’s two year old was sleeping well prior to the wind farm, but began waking up, 5- 6 times a night.

Kevin Bailey stated, “When you are outside working and absorbed in what you are doing, you are OK. If inside, resting or reading, it’s a problem. Forget about sleeping at night. The repetitions would go away, you think that it is gone, and it comes back again.” Kevin tried sound dampening by draping the front walls inside his house, and sleeping in the back, but this did not work. Aspects of their experience (including vibrating appliances) suggest low-frequency noise may have been a key factor here.

Lincolnshire, UK: 900m from wind farm with eight 2MW turbines
Julian and Jane Davis spent 60 nights away from home in the first six months of the wind farm's operation, due to lack of sleep. Mrs. Davis says, "It sounds like a train coming toward you that never arrives, or a toy stuck in a tumble drier." They blame Amplitude Modulation as the issue; the local South Holland District Council recorded the noise, but reported that "the noise does not equate to statutory nuisance at this time."

Juniata Township, Altoona, PA: 600m and 1km from Allegheny Ridge Wind Farm with 40 turbines
A two-year old wind farm was stirring new noise complaints. One noise study put the sound below the township limit of 45dB, but residents claim their readings are higher. Tape on the trailing edge of turbine blades had come lose, and was causing much of the excess noise; a new owner is doing repairs, starting with turbines closest to homes.

Resident Jill Stull (turbines 2000ft/600m from her house) said, ‘‘You know when you're standing outside and you hear a plane coming about 30,000 feet overhead, then it goes off in the distance? It sounds like those planes are 5,000 feet above your house and circling and never land." The Stulls said they could move, but they aren't going to. ‘‘We're not going anywhere. I just want them to be quiet. I'm not going to jump on the ‘I hate windmills' bandwagon because I don't," Jill Stull said. ‘‘I'm just tired of nobody listening. My point is what is your peace of mind worth? I can't play outside with my kids back at the pond in the woods because it gives me a headache." "I know it's going to make some noise, but a lot of times, it sounds like a jet," resident Myrle Baum said.

"On a calm day, you come outside and try to enjoy a nice peaceful day, and all you hear is the noise all the time and you can't get away from it," said Bob Castel, who has two turbines behind his house. "The first time they started them up, I didn't know what it was. I was like man, that's a weird noise. It was that loud." said Castel.

While some neighbors said they aren't exactly bothered by the noise from the windmills, they said there's never peace and quiet either. "You notice them more at night, but you can't really hear them inside," said Charles Holland.

Greenfield Supervisor Ed Helsel, whose property borders the Stulls, said the noise varies. On a morning when Helsel considered it to be loud, he said he shut the window and could barely hear it. Jill Stull, whose property is closer to turbines, said she shuts her windows and stills hears the noise. ‘‘They're loud enough to make me wake up,'' Todd Stull said. ‘‘This is noise pollution.''

Brownsville, Wisconsin: 1500-2000 feet from two turbines, three others sometimes audible within 4000 feet
Noise diary by a family during first two months of operation of the new Blue Sky Green Field wind project [READ DIARY]


Blue Sky Green Field Wind Project
Photo: WE Energies [IMAGE SOURCE]

April 16: From 6PM to 10PM the turbines were loud. I could hear #'s 4, 73, and 74a. Jet sound or like waiting for two trains to crash. We house a 360 (cubic inch) sprint car at our house. Tonight it was run for the first time this year. One of the guys mentioned he could hear the turbine (#4) over the sound of the race car engine while it idled. He was standing between the tire and the engine or right next to the engine. The car has no muffler.

During the day the wind is strong. The weather man says the wind will be 25-35 miles per hour. So strong that I don't hear the turbines very much. Earlier my wife said she worked outside all afternoon in the wind and her ears were fine. When the wind died down and could hear three of the turbines her ears hurt. She also mentioned the whooshes between turbines were out of sync.

April 12: At breakfast asked (my son) if he could hear the turbines in the house. "Yes, and I hear them in my room", he said. "Some nights I can't sleep". His room is upstairs on the SW corner of the house. As I sit below his room (inside the house) I can hear the turbine at this moment.

9:30PM #4 not running, however I can hear the jet sound of #6.

Two farmers I have talked to are really angry about how they were treated. That is, once the contract was signed it meant the contractors could go where ever they wanted to on the farmer's land. One also had issues as to the placement of the tower road, he wanted it on the fence line or to take out the fence line to minimize land loss. He was told it would cost too much to re-survey the land. The farmers said they were not told about the interduct or collection system that would be going through their land. Any complaints made to (the wind developer) was met with "it's in your contract."

Goderich, Ontario: 550m from a 80m turbine
Ernest Marshall moved from his home of thirty years, driven out by the noise of a wind turbine built two years ago, which scares his ponies and purrs incessantly through his bedroom window, keeping the Marshalls from a good night's sleep. Marshall suffered a stroke not long after the new the turbine began operation and, on the advice of his neurologists, has sold his home and is moving. "I'm a nervous wreck," Marshall, 65, said. "We've got to leave ... You can't know how it affects your nerves." Marshall is quick to clarify he's not against what it stands 80 metres high for -- clean energy. "The problem is they're putting them too close to people."

Bothel, West Cumbria, UK: 800m (half-mile) from Wharells Hill Wind Farm
Ron Williams, 73, reports: "The swush, swush, swush as each blade breaks the flow of the wind past the tower, obviously three times per revolution, is extremely debilitating. The affect is worse at nights when ambient noise level from traffic on the A595 is low." Mr. Williams said that wind farm operators claimed that turbine noise levels were within legal limits but he questioned whether the effects of this low frequency noise had been thoroughly investigated.

Mars Hill, ME: 2600+ feet from turbine
Disturbance from noise is undoubtedly subjective: After recently visiting Mars Hill, Richard Jennings of Fayette said he did not see the wind turbines there as ugly or bad. His visit brought him to a home 3,000 feet from seven of the turbines and the noise was no worse than his sleep apnea machine at night. He urged the board to accept a proposed 44-turbine wind farm in Kibby Township because alternative energy is a necessity.

On the other side of the subjective coin, from Mars Hill resident Wendy Todd (house is 2600 feet from the nearest turbine):
Unfortunately for us, the very mountain that has provided the wind facility with a class 3-wind resource often acts like a fence protecting us from the upper level winds that push the turbines. There are many times when winds are high on the ridgeline but are near calm at our homes. The noise and vibrations from the turbines penetrate our homes. At times there is no escape from it. It doesn’t matter which room you go to, there is no escape from the noise. The noise ranges from the sound of a high range jet to a fleet of planes that are approaching but never arrive. When it’s really bad it takes on a repetitive, pulsating, thumping noise that can go on for hours or even days. It has been described as a freight train that never arrives, sneakers in a dryer, a washing machine agitating, a giant heartbeat, a submariner describes it as a large ship passing overhead.

People think that we are crazy. They drive out around the mountain, stop and listen, and wonder why anyone would complain about noise emissions. But, believe me when we are having noise problems you can most assuredly hear the justification of our complaint. We have had people come into our yard get out of their vehicles and have watched their mouth drop. We have had company stop in mid conversation inside our home to ask, “What is that noise?” or say “I can’t believe you can hear those like that inside your house.”


Two views of the Mars Hill wind farm, showing proximity of rural landowners. It is not hard to imagine noise blanketing the fields, especially when the hill is sheltering the lowlands from wind.
Images from National Wind Watch [IMAGE SOURCE]

(Wendy Todd, continued): Visiting a wind facility, or sitting at the end of someone’s driveway once or twice for 2, 3 or even 10 minutes to listen does not make that person an expert on turbine noise. To be an informed witness could take days or weeks for one to know and experience what we are living. Not until an individual has been in a home and has heard turbine noise emissions of 45 decibels or higher does that individual have any right to judge how turbine noise truly affects the lives of people. Even noise experts should be talking to residents who are living next to turbines to ensure they are collecting data that is relevant to the burdensome noise emissions heard by those who live closest to them. Let us tell the sound experts when we are having a noise issue.

(18 families, each with homes less than 3000 feet from the nearest turbine, are experiencing disturbing noise levels; the next closest home is about 5200 feet away, and are only occasionally bothered when inside their homes.)

Nick Archer, our Regional Director with the Maine Department of Environmental Protection thought we were all crazy, too. But he finally made it to our homes and heard what we were talking about. I don’t believe he has ever heard a 50+decibel day but he has heard close to that on more than one occasion and has made statements like these: “This is a problem,” “ We need to figure out what is going on with these things before we go putting anymore of them up,” “I thought you were crazy at first but you are not crazy,” “The quality of life behind the mountain is changed.” Did he say these things just to appease us? I don’t believe so.
[READ WENDY TODD'S FULL STATEMENT TO THE MAINE WIND TASK FORCE]

[READ CONCISE WENDY TODD LETTER SENT TO OTHER MAINE TOWNS WHO REQUESTED INFORMATION]

For an archive of news reports that often quote residents affected by wind turbine noise, and to follow the efforts of residents and local officials to grapple with wind farm permitting and regulation, see the Industrial Wind Action Group's news archives, noise section: [WEB PAGE]


Possible Factors in Noise Complaints

Predicted Noise is Unrealistically Optimized

Manufacturer measurements of the noise emission of their units is often based on optimized laboratory conditions and perfectly new machines; thus, the predicted noise output is likely the lowest that could occur with that particular turbine design. As units age, it is also to be expected that mechanical noise at least will increase (and perhaps aerodynamic noise as well, depending on wear of the blades themselves); a visit to most wind farms will confirm that some towers are louder than others.

In addition, most noise impact studies assume spherical spreading of sound, which is a rather idealized pure-physics approach to estimating how sound energy decreases as it is dispersed over distance, expanding outward in an ever-expanding sphere. While useful in some situations, or as a starting point, the topographic and atmospheric effects mentioned below tend to shatter the simple picture on which these numbers are based.

(Spherical spreading is based on the fact that the total sound energy is spread over an increasingly large sphere as a sound moves outward from its source. Any doubling of distance--whether from 100m to 200m or from 1000m to 2000m--will be marked by a decrease in sound intensity of 6dB, which to our ears, is perceived as cutting the volume almost in half. Higher frequencies are reduced more than this, by atmospheric attenuation, while lower frequencies stay pretty close to this model).

In attempt to account for these and other factors, manufacturers often add 2db to their manufacturer's sound emission estimates when applying for permits.

Note: A 2007 study in the UK, comparing modeled sound levels to measured sound levels, indicates that some models are more reliable predictors of sound transmission. Specifically, the researchers found that when they used a conservative (worst-case) factor for ground hardness (and thus sound transmission) the actual recorded sound was nearly always lower than predicted at close and mid range (100m-500m), with a bit more variation and scattered higher measured sound at longer range (750m) . When using a "mixed cover" factor for ground cover, measured sound was more often louder than predicted, sometimes by as much as 5-7dB (the mixed cover results were only reported at 750m). [Bullmore, Addock, Jiggins, Cand. Wind Farm Noise Predictions: The Risks of Conservatism. Second International Meeting on Wind Turbine Noise, Lyon, France, September 2007)]

Topographical Effects

Sloping landforms can create unusual sound propagation conditions, especially in consort with atmospheric fluctuations. Near the Vancouver Airport, hills rising from a flat plain caused sound levels to be 20dB higher at 5500m than at 4000m, because of the way the increasing ground angles caused sounds to combine, more than nullifying what, in a standard model, would be expected to be a 3dB decrease over that distance.

Atmospheric Effects

Researchers looking more deeply into the readily apparent problems with noise that many residents are experiencing have uncovered a few possible atmospheric factors that deserve further investigation and due consideration from local authorities, as well as inclusion in noise models used to predict likely acoustic impacts of new wind farm developments. Most noise modeling is based on simplified wind speed models, and it is often assumed that higher winds will create higher ambient noise, thus hiding the increased sound of turbines. Among the recent findings:

Night time atmospheric stability

In the daytime, warming air rises, both carrying sound aloft and creating turbulence that scatters turbine noise, as well as creating more ground-based ambient noise that masks turbine sounds. At night, however, when the air stabilizes it appears that noise from wind turbines can carry much farther than expected. This effect can occur with light winds at turbine height and the ground, or, with light winds at turbine height and very little or no wind at ground level.

With light and steady breezes capable of spinning the turbines, but not stirring up much ambient noise, sound levels measured at homes a half mile to nearly two miles away are often 5-15dB higher than models would suggest. Making matters worse, this same atmospheric stability tends to allow multiple turbines to settle into a synchronous rhythm (in more turbulent conditions, small differences in wind between the turbines keeps them out of synch). In this case, the "whish" of the blades as they pass the tower often turns into a more annoying rhythmic "thump;" in quantitative terms, the change in sound level creating the whish or thump can rise from 2dB to 5dB or as high as 9dB, making a clearly audible rhythmic pattern. These rhythmic pulses are likely the strongest factor in annoyance.

Possible solutions: It is hard to escape the implication that setback distances may need to be increased in many places where the prevalence of such night time effects suggest sound will often remain at annoying levels for larger distances. It could also help to monitor for synchronous blade patterns and add random variations at night, to mimic the random variations that atmospheric turbulence causes in the daytime. Finally, noise modeling studies should include calculations based on night time stable atmospheres; G.P. van den Berg, whose 2006 Ph.D. thesis is a comprehensive study of these effects, concludes that "With current knowledge, the effects of stability on the wind profile over flat ground can be modelled satisfactorily." (his measurements indicate that more sophisticated sound models were accurate to within 1.5dB) He goes on to note: "In mountainous areas terrain induced changes on the wind profile influence the stability-related changes and the outcome is less easily predicted: such terrain can weaken as well as amplify the effect of atmospheric stability."

Inversion layers

While the quantitative effect of inversion layers on sound levels has not been systematically studied, many opportunistic measurements suggest the obvious: that when an inversion layer forms above the height of turbines, it can facilitate longer-range sound transmission by reflecting some of the sound back toward the ground, or by forming (with the ground) a channel for sound propagation. In many locations, this will be a relatively rare occurrence, but in areas with frequent inversion layer formation, it should be considered.

Coherence

Wind turbines are often placed in a row; the sound emanating from each one will move outward in concentric circles. There will be places where the sound waves from one turbine interact with the sound waves from another turbine; in these locations, they sound waves may be out of phase, thus reducing the perceived loudness, or in phase, thus increasing the localized sound levels. You might more easily picture this if thinking of pebbles dropped in water; the expanding circles of waves will cross each other, creating a more dynamic pattern of higher and lower waves. Note: the same factors that can disrupt the idealized spherical spreading models, as noted above, also will tend to create a more chaotic pattern of sound waves from each turbine, thus also limiting the impact, and certainly the predictability, of coherence effects.

Amplitude Modulation
(also known as "Aerodynamic Modulation" or "Amplitude Modulation of Aerodynamic Sound")

As noted briefly in the introduction, is appears that a common source of noise complaints by nearby residents is a repeating "thumping" noise, rather than the constant whir of the blades. This thumping is often caused by a slight pressure differential caused by the tower of the turbine: as each blade passes the tower at the bottom of its arc, it passes through this lower-pressure region and creates the thumping sound.

A group of turbines can, at times, turn in synch with each other, causing any Amplitude Modulation to be accentuated, especially at locations equidistant from any two or more turbines.

Many new turbines are being designed with the rotors on the upwind side of the tower, which should reduce or eliminate this important contributor to turbine noise.

Icing

Icing on the leading edge of blades can cause a 3-5dB increase in the source level of noise from the turbine.

At the end of an intensive two-year period of studying the complexities of atmospheric turbulence and stability on wind turbine noise, G.P. van den Berg offered this sage advice:

"There is much more to discover: indeed, it looks like wind turbines have become more fascinating now their sound has proved to be more complex than a simple constant noise from the sky, driven only by wind with a constant profile. This may motivate researchers and consultants to put more effort in better predictions of wind turbine noise, and considering again noise exposure to local residents.

"Several technical possibilities to minimize the noise have been outlined in this book, but we need not just depend on technical solutions. A change in public relations can also make a difference: proponents must accept that wind turbine noise is not (always) ‘benign’, that the noise may affect people, and that people who are complaining are not always just a nuisance. And no, we still do not understand wind turbine noise emission entirely, so proponents should watch their WARYDU (we are right, you don't understand) attitude. "

Likewise, a 2007 report commissioned by the UK government to investigate noise complaints noted that "‘Unrealistic expectations’ was thought to be a factor in at least one case, i.e. the complainant believed that the noise would be less noticeable than it actually proved to be. In this case the local authority considered that the complaint could possibly have been avoided by more accurate information from the developer at the planning stage....Finally, for five of the sites having received complaints, the site met planning conditions. We do not know whether these comments indicate that complaints were unjustified, or alternatively that planning conditions were too lenient."

Indeed, official noise standards can very easily fail to protect nearby residents from disruptive levels of noise. It is crucial that everyone involved (industry, government, residents) resists the easy temptation of relying on "paper" assurances that wind turbines will not create acoustic impacts. Working from a realistic foundation, siting and permitting decisions can proceed in a manner that protects both local residents and the long-term potential for wind energy generation.


From ProtectIllinoisEnvironment.com [IMAGE SOURCE]

How not to do it

The map at left shows both the area around Lee, Illinois, and was originated by FPL Energy, LLC, in the early phases of a proposed wind energy development which was not built.

The black circles represent a 1400 foot radius around each existing houses (1,400 feet being the distance wind turbines must be kept away from a home or residence).

The brown circles represent the proposed site of a wind turbine, each with its 1,400 foot radius.

It is remarkable to see the degree to which the 1400 foot setback was pushed to its limits in this preliminary map; throughout the entire wind farm, we see turbines sited at the very edge of the 1400-foot house circles, with the 1400-foot turbine circles extending deep into the house zones, nearly to the center (the house itself).

This is exactly the wrong response to working with setbacks, and we can be glad this wind farm was not built to such specifications.


Detailed Documents Of Note

The above sections draw on several detailed reports by others. Those wishing to learn more, or to inform themselves so as to discuss these issues in depth with regulatory authorities, company representatives, acousticians, or neighbors, will benefit from reading the source material below.


Emerging Technologies

While there are legitimate noise concerns associated with current wind energy development, this is a vibrant and ever-growing field, and several emerging technologies offer promise in making significant reductions in the acoustic impact of turbine construction and/or operation.

Blue H Floating Offshore Platform - This platform, based on designs used by the oil and gas industry, is being pitched as a way to place wind turbines far enough offshore to overcome resistance to seeing wind farms from shore or placing wind farms in productive near-shore fisheries. But perhaps more significant, this approach avoids the construction noise of pile-driving, which is necessary for traditional permanent bottom-mounted turbines. [BLUE H WEBSITE] [BLUE H WEB PAGE WITH VIDEO OF PROTOTYPE]
News, 3/15/08:
Blue H Leases MMS site far offshore, proposes alternative Martha's Vinyard project with Cape Wind opponents on board [READ ARTICLE]

Sway Deepwater Turbine - Another emerging approach to siting turbines further offshore, from a Norwegian company. Their approach uses a long floating tower underwater to provide stability, with the whole thing tethered to the sea floor; they claim to be able to withstand severe waves with ease. [SWAY WEBSITE] [SWAY PAGE WITH ANIMATION OF CONCEPT]

Selsam Superturbine - An innovative new approach, still in the conceptual stage, which would emply many small turbines on a single tower. Benefits include a much smaller visual impact. [SELSAM WEBSITE]

FloDesign Turbine - This new approach is based on principles from jet engines, and while still in the early phases of development (first prototype is yet to be produced), could lead to important breakthroughs. The small size and less disruption of air currents would allow wind farms to be built much more densely (and so use less land), and would likely be much quieter, especially in low frequencies. [FLODESIGN WEBSITE] [FLODESIGN PAGE WITH VIDEO OF PHYSICS]


Vertical Axis Turbines - This new approach looks somewhat like a corkscrew, and promises much lower noise impacts. Several companies are developing prototypes and doing initial commercial installations. So far, these designs are sized for home, office building, or apartment building applications. None have been scaled for use in wind-farm settings. While some of these companies brag of higher output thanks to operating at lower wind speeds, others claim that such "drag" designs are inherently more inefficient than traditional "lift" designs. Bear in mind, as well, that while noise on the tips of traditional spinning blades can be an issue, often the mechanical noise of a turbine's "insides" is a large contributor to the noise signature of smaller turbines located close to homes.

Windstor - Installation of its second unit in Ishpeming, Michigan, was derailed in 2006 by lack of capital; in Dec 2007 MTI Energy announced it had paid the project's debt and was moving forward with plans to complete the installation. [WINDSTOR WEBSITE] [MTI WEBSITE]
Windhaus - tight closed design [WEBSITE]
Windside - Lightweight, silent turbines modeled on principles of sails; they manufacture several variations, meant for differing conditions. [WEBSITE]

Aerotecture - Their "aeroturbines" are designed for turbulent urban conditions. Several designs, some which stand and some which lie on the roof. [WEBSITE]
Turby - A Dutch company, also touting effectiveness in urban settings. They claim to combine the best features of horizontal axis (efficiency) with the best of vertical. [WEBSITE]
Carbon Concepts - Another approach that attempts to combine the benefits of horizontal and vertical designs. [WEBSITE]
TMA - Unique wind-concentrating design especially useful at high speeds. Aiming for large applications, up to 500kW, though the design is scalable to 1MW. [WEBSITE]
Stormblade - new design, prototype stage, initial interest from manufacturers. Concentrates wind inside housing; quiet, efficient. [WEBSITE]


Trade Organizations

Danish Wind Industry Association - [WEBSITE]
Site includes quite a bit of discussion of noise [SEE PAGES] and includes a noise calculator grid in which you can place one or more turbines and set acceptible noise levels to determine set-backs [CALCULATOR PAGE]

German Wind Energy Institute (DEWI, Deutsches Windenergie Institut) - [WEBSITE]

British Wind Energy Association - [WEBSITE] [UK WIND PROJECTS DATABASE]

American Wind Energy Association - [WEBSITE] [US WIND PROJECTS MAP]
AWEA Siting Handbook [WEBPAGE]

World Wind Energy Association - A association of national and regional wind energy associations. [WEBSITE]
WWEA Wind Energy Technology Website
- Overview of the engineering that goes into wind turbines. [WEBSITE]

Governmental Entities

World Health Organization - Guidelines for Community Noise [WEB PAGE]

US Dept of Energy Wind Powering America Program - Through its state Wind Working Groups, programs at the National Renewable Energy Laboratory, and partnerships, this initiative will establish new sources of income for American farmers, Native Americans, and other rural landowners and meet the growing demand for clean sources of electricity. [WEBSITE]

US Bureau of Land Management - In 2006, the BLM released a Programmatic Environmental Impact Statement governing wind energy development on BLM lands in 9 western states. [WEBSITE] [DOWNLOAD EIS] [DOWNLOAD BIOLOGICAL OPINION(pdf)]

US National Academies of Science - The NAS's National Research Council released a 2007 report, Environmental Impacts of Wind Energy Development, focusing on the mid-Atlantic mountains. The report is available for purchase at the NAS store, in whole or by chapter. [WEBSITE] [4-PG REPORT IN BRIEF(pdf)] [EXECUTIVE SUMMARY(free pdf)]

US Forest Service - In late 2007, the US Forest Service announced plans to revise its Wind Energy Directive. They have not set up a dedicated web page to track the process, but the Federal Register Notice contains the text of the proposed directive. [FED REG NOTICE]

US Fish and Wildlife Service Wind Turbine Guidelines Advisory Committee - [ADVISORY COMMITTEE WEBSITE] [FWS WIND ENERGY WEBSITE]

Landowner Support

Resources/Advice for landowners signing lease and easement agreements with energy companies

Iowa State University Center for Agricultural Law and Taxation - Legal Issues and Related Concerns for Landowners [WEB PAGE]

Ontario Federation of Agriculture -30 Suggestions on Wind Power Leases for Farmers [DOWNLOAD PDF]

Wind Advocacy Organizations

National Wind Coordinating Collaborative - A U.S. consensus-based collaborative formed in 1994, NWCC members include representatives from electric utilities and support organizations, state legislatures, state utility commissions, consumer advocacy offices, wind equipment suppliers and developers, green power marketers, environmental organizations, agriculture and economic development organizations, and state and federal agencies. They've issued many Issues Briefs, some of which address noise, but none of which focus on it. [WEBSITE]

Wind Works - Longtime wind energy advocate Paul Gipe's website, replete with many of his articles and information on his books. [WEBSITE]

Wind Turbine Noise Activists

National Wind Watch - Collects news reports, new research, and most interesting, several videos documenting community response to wind farm projects in the United States. [WEBSITE] [VIDEO PAGE]

Nina Pierpont - An MD who has been investigating "Wind Turbine Syndrome" for several years. She has compiled two concise, annotated pages of papers and letters. [HER PAPERS] [PAPERS BY OTHERS]

Rock County (WI) Better Plan - Regional activist group with wide-ranging online resources and a rambunctious tone [WEBSITE]

Windbyte - This UK group uses maps to track the "Klondike Wind Rush" sweeping northern England and southern Scotland, where wind farm applications, proposals, and scoping studies far outnumber existing wind farms, prompting fears of a "wind energy landscape". [WEBSITE]

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