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6/4/09 PART TWO: When the Minnesota Department of Health took a closer look at wind turbine noise and shadow flicker, what did they find? 

In March of 2009, the Minnesota Department of Health began an evaluation of health impacts from wind turbine noise and low frequency vibration, which resulted in the May 22, 2009 report called

"Public Health Impacts of Wind Turbines"
[Download the entire report by clicking here]

Here's PART TWO of our summary of the report:

(Scroll down to the previous post to read PART ONE)

What are some of the impacts of Wind Turbine Noise?

Page 15-17: "Potential Adverse Reaction to Sound

Human sensitivity to sound, especially to low frequency sound, is variable.

Individuals have different ranges of frequency sensitivity to audible sound; different thresholds for each frequency of audible sound; different vestibular sensitivities and reactions to vestibular activation; and different sensitivity to vibration.

Further, sounds, such as repetitive but low intensity noise, can evoke different responses from individuals.

People will exhibit variable levels of annoyance and tolerance for different frequencies. Some people can dismiss and ignore the signal, while for others, the signal will grow and become more apparent and unpleasant over time (Moreira and Bryan, 1972; Bryan and Tempest, 1973).

These reactions may have little relationship to will or intent, and more to do with previous exposure history and personality.

Stress and annoyance from noise often do not correlate with loudness. This may suggest, in some circumstances, other factors impact an individual’s reaction to noise.

A number of reports, cited in Staples (1997), suggest that individuals with an interest in a project and individuals who have some control over an environmental noise are less likely to find a noise annoying or stressful.

Berglund et al. (1996) reviewed reported health effects from low frequency noise.

Loud noise from any source can interfere with verbal communication and possibly with the development of language skills. Noise may also impact mental health.

However, there are no studies that have looked specifically at the impact of low frequency noise on communication, development of language skills and mental health.

Cardiovascular and endocrine effects have been demonstrated in studies that have looked at exposures to airplane and highway noise.

In addition, possible effects of noise on performance and cognition have also been investigated, but these health studies have not generally looked at impacts specifically from low frequency noise.

Noise has also been shown to impact sleep and sleep patterns, and one study demonstrated impacts from low frequency noise in the range of 72 to 85 dB(A) on chronic insomnia (Nagai et al., 1989 as reported in Berglund et al., 1996).

Case studies have suggested that health can be impacted by relatively low levels of low
frequency noise. But it is difficult to draw general conclusions from case studies. Feldmann and Pitten (2004)) describe a family exposed during the winter to low frequency noise from a nearby heating plant.

Reported health impacts were: “indisposition, decrease in performance, sleep disturbance, headache, ear pressure, crawl paràˆsthesy [crawling, tingling or numbness sensation on the skin] or shortness of breath.”

Annoyance, unpleasant sounds, and complaints:

Reported health effects from low frequency stimulation are closely associated with annoyance from audible noise.

“There is no reliable evidence that infrasounds below the hearing threshold produce physiological or psychological effects” (WHO, 1999). It has not been shown whether annoyance is a symptom or an accessory in the causation of health impacts from low frequency noise. Studies have been conducted on some aspects of low frequency noise that can cause annoyance. Noise complaints are usually a reasonable measure of annoyance with low frequency environmental noise. Leventhall (2004) has reviewed noise complaints and offers the following conclusions:

“ The problems arose in quiet rural or suburban environments
The noise was often close to inaudibility and heard by a minority of people
The noise was typically audible indoors and not outdoors
The noise was more audible at night than day
The noise had a throb or rumble characteristic
The main complaints came from the 55-70 years age group
The complainants had normal hearing.
Medical examination excluded tinnitus.

These are now recognised as classic descriptors of low frequency noise problems.”

These observations are consistent with what we know about the propagation of low intensity, low frequency noise.

Some people are more sensitive to low frequency noise. The difference, in dB, between soft (acceptable) and loud (annoying) noise is much less at low frequency (see Figure 4 audible range compression). Furthermore, during the daytime, and especially outdoors, annoying low frequency noise can be masked by high frequency noise.

The observation that “the noise was typically audible indoors and not outdoors” is not particularly intuitive. However, as noted in a previous section, low frequencies are not well attenuated when they pass through walls and windows.

Higher frequencies (especially above 1000 Hz) can be efficiently attenuated by walls and windows.

In addition, low frequency sounds may be amplified by resonance within rooms and halls of
a building.

Resonance is often characterized by a throbbing or a rumbling, which has also been associated with many low frequency noise complaints.

Low frequency noise, unlike higher frequency noise, can also be accompanied by shaking, vibration and rattling.

In addition, throbbing and rumbling may be apparent in some low frequency noise.

While these noise features may not be easily characterized, numerous studies have shown that their presence dramatically lowers tolerance for low frequency noise (Berglund et al., 1996).

As reviewed in Leventhall (2003), a study of industrial exposure to low frequency noise found that fluctuations in total noise averaged over 0.5, 1.0 and 2.0 seconds correlated with annoyance (Holmberg et al., 1997).

This association was noted elsewhere and led (Broner and Leventhall, 1983) to propose a 3dB “penalty” be added to evaluations of annoyance in cases where low frequency noise fluctuated.

In another laboratory study with test subjects controlling loudness, 0.5 – 4 Hz modulation of low frequency noise was found to be more annoying than non-modulated low frequency noise. On average test subjects found modulated noise to be similarly annoying as a constant tone 12.9 dB louder (Bradley, 1994).

Page 17-19: B. Studies of Wind Turbine Noise Impacts on People

1. Swedish Studies

Two studies in Sweden collected information by questionnaires from 341 and 754 individuals (representing response rates of 68% and 58%, respectively), and correlated responses to calculated exposure to noise from wind farms (Pedersen and Waye, 2004; Pedersen, 2007; Pedersen and Persson, 2007).

Both studies showed that the number of respondents perceiving the noise from the wind turbines increased as the calculated noise levels at their homes increased from less than 32.5 dB(A) to greater than 40 dB(A).

Annoyance appeared to correlate or trend with calculated noise levels.

Combining the data from the two studies, when noise measurements were greater than 40 dB(A), about 50% of the people surveyed (22 of 45 people) reported annoyance.

When noise measurements were between 35 and 40 dB(A) about 24% reported annoyance (67 of 276 people).

Noise annoyance was more likely in areas that were rated as quiet and in areas where turbines were visible.

In one of the studies, 64% respondents who reported noise annoyance also reported sleep disturbance; 15% of respondents reported sleep disturbance without annoyance.

2. United Kingdom Study

Moorhouse et al. (UK Department for Business Enterprise and Regulatory Reform, 2007) evaluated complaints about wind farms. They found that 27 of 133 operating wind farms in the UK received formal complaints between 1991 and 2007. There were a total of 53 complainants for 16 of the sites for which good records were available.

The authors of the report considered that many complaints in the early years were for generator and gearbox noise. However, subjective analyses of reports about noise, (“like a train that never gets there”, “distant helicopter”, “thumping”, “thudding”, “pulsating”, “thumping”, “rhythmical beating”, and “beating”) suggested that aerodynamic modulation was the likely cause of complaints at 4 wind farms.

The complaints from 8 other wind farms may have had “marginal” association with aerodynamic modulation noise.

Four wind farms that generated complaints possibly associated with aerodynamic modulation were evaluated further.These wind farms were commissioned between 1999 and 2002.

Wind direction, speed and times of complaints were associated for 2 of the sites and suggested that aerodynamic modulation noise may be a problem between 7% and 25% of the time.

Complaints at 2 of the farms have stopped and at one farm steps to mitigate aerodynamic modulation (operational shutdown under certain meteorological conditions) have been instituted.

3. Netherlands Study

F. van den Berg et al. (2008) conducted a postal survey of a group selected from all residents in the Netherlands within 2.5 kilometers (km) of a wind turbine.In all, 725 residents responded (37%).

Respondents were exposed to sound between 24 and 54 dB(A). The percentage of respondents annoyed by sound increased from 2% at levels of 30 dB(A) or less, up to 25% at between 40 and 45 dB. Annoyance decreased above 45 dB. Most residents exposed above 45 dB(A) reported economic benefits from theturbines. However, at greater than 45 dB(A) more respondents reported sleep interruption. Respondents tended to report more annoyance when they also noted a negative effect on landscape, and ability to see the turbines was strongly related to the probability of annoyance.

4. Case Reports

A number of un-reviewed reports have catalogued complaints of annoyance and some more severe health impacts associated with wind farms.

These reports do not contain measurements of noise levels, and do not represent random samples of people living near wind turbines, so they cannot assess prevalence of complaints.

They do generally show that in the people surveyed, complaints are more likely the closer people are to the turbines.

The most common complaint is decreased quality of life, followed by sleep loss and headache.

Complaints seem to be either from individuals with homes quite close to turbines, or individuals who live in areas subject to aerodynamic modulation and, possibly, enhanced sound propagation which can occur in hilly or mountainous terrain.

In some of the cases described, people with noise complaints also mention aesthetic issues, concern for ecological effects, and shadow flicker concerns. Not all complaints are primarily about health.

Harry (2007) describes a meeting with a couple in Cornwall, U.K. who live 400 meters from a wind turbine, and complained of poor sleep, headaches, stress and anxiety.

Harry subsequently investigated 42 people in various locations in the U.K. living between 300 meters and 2 kilometers (1000 feet to 1.2 miles) from the nearest wind turbine. The most frequent complaint (39 of 42 people) was that their quality of life was affected.

Headaches were reported by 27 people and sleep disturbance by 28 people.

Some people complained of palpitations, migraines, tinnitus, anxiety and depression. She also mentions correspondence and complaints from people in New Zealand, Australia, France, Germany, Netherlands and the U.S.

Phipps (2007) discusses a survey of 619 households living up to 10 kilometers (km; 6
miles) from wind farms in mountainous areas of New Zealand. Most respondents lived
between 2 and 2.5 km from the turbines (over 350 households).

Most respondents (519) said they could see the turbines from their homes, and 80% of these considered the turbines intrusive, and 73% considered them unattractive.

Nine percent said they were affected by flicker.

Over 50% of households located between 2 and 2.5 km and between 5 and 9.5 km reported being able to hear the turbines.

In contrast, fewer people living between 3 and 4.5 km away could hear the turbines.

Ninety-two households said that their quality of life was affected by turbine noise.

Sixty-eight households reported sleep disturbances: 42 of the households reported occasional sleep disturbances, 21 reported frequent sleep disturbances and 5 reported sleep disturbances most of the time.

The Large Wind Turbine Citizens Committee for the Town of Union (2008) documents complaints from people living near wind turbines in Wisconsin communities and other places in the U.S. and U.K.

Contained in this report is an older report prepared by the Wisconsin Public Service Corporation in 2001 in response to complaints in Lincoln County, Wisconsin.[click here to download] The report found essentially no exceedances of the 50 dB(A) requirement in the conditional use permit.

The report did measure spectral data accumulated over very short intervals (1 minute) in 1/3 octave bands at several sites while the wind turbines were functioning, and it is of interest that at these sites the sound pressure level at the lower frequencies (below 125 Hz) were at or near 50 dB(A).

Pierpont (2009) postulates wind turbine syndrome, consisting of a constellation of symptoms including headache, tinnitus, ear pressure, vertigo, nausea, visual blurring, tachycardia, irritability, cognitive problems and panic episodes associated with sensations of internal pulsation.

She studied 38 people in 10 families living between 1000 feet and slightly under 1 mile from newer wind turbines.

She proposes that the mechanism for these effects is disturbance of balance due to “discordant” stimulation of the vestibular system, along with visceral sensations, sensations of vibration in the chest and other locations in the body, and stimulation of the visual system by moving shadows.

Pierpont does report that her study subjects maintain that their problems are caused by noise and vibration, and the most common symptoms reported are sleep disturbances and headache.

However, 16 of the people she studied report symptoms consistent with (but not necessarily caused by) disturbance of equilibrium.

Page 19: Noise Assesment and Regulation

Concise description of noise regulations in Minnesota

Page 19: Low frequency noise assessment and regulation

Concise and technical explanation of how low frequency noise is measured and regulated, why people are more annoyed by wind turbine noise at lower levels than by noise generated by airplanes, traffic and trains, and why that matters.

Two exerpts from this section from the World Health Organization:

WHO says “[i]t should be noted that a large proportion of low-frequency components in noise may increase considerably the adverse effects on health.” (WHO, 1999)

"In their noise guidance, the WHO (1999) recommends 30 dB(A) as a limit for “a good night’s sleep”. However, they also suggest that guidance for noise with predominating low frequencies be less than 30 dB(A)."

Page 22-24: Wind Turbine Sound Measurements

Concise and technical explanation of how wind turbine sound is measured, why low frequency turbine noise is more likely to penetrate through walls and windows than high frequency noise, and why, as one moves away from a wind turbine, the low frequency sound becomes more pronounced.


Wind turbines generate a broad spectrum of low-intensity noise. At typical setback distances higher frequencies are attenuated.

In addition, walls and windows of homes attenuate high frequencies, but their effect on low frequencies is limited.

Low frequency noise is primarily a problem that may affect some people in their homes, especially at night. It is not generally a problem for businesses, public buildings, or for people outdoors.

The most common complaint in various studies of wind turbine effects on people is annoyance or an impact on quality of life.

Sleeplessness and headache are the most common health complaints and are highly correlated (but not perfectly correlated) with annoyance complaints.

Complaints are more likely when turbines are visible or when shadow flicker occurs.

Most available evidence suggests that reported health effects are related to audible low frequency noise.

Complaints appear to rise with increasing outside noise levels above 35 dB(A).

It has been hypothesized that direct activation of the vestibular and autonomic nervous system may be responsible for less common complaints, but evidence is scant.

The Minnesota nighttime standard of 50 dB(A) not to be exceeded more than 50% of the time in a given hour, appears to underweight penetration of low frequency noise into dwellings.

Different schemes for evaluating low frequency noise, and/or lower noise standards, have been developed in a number of countries.

For some projects, wind velocity for a wind turbine project is measured at 10 m and then modeled to the height of the rotor.

These models may under-predict wind speed that will be encountered when the turbine is erected. Higher wind speed will result in noise exceeding model predictions.

Low frequency noise from a wind turbine is generally not easily perceived beyond 1⁄2 mile.

However, if a turbine is subject to aerodynamic modulation because of shear caused by terrain (mountains, trees, buildings) or different wind conditions through the rotor plane, turbine noise may be heard at greater distances.

Unlike low frequency noise, shadow flicker can affect individuals outdoors as well as indoors, and may be noticeable inside any building. Flicker can be eliminated by placement of wind turbines outside of the path of the sun as viewed from areas of concern, or by appropriate setbacks.

Prediction of complaint likelihood during project planning depends on:

1) good noise modeling including characterization of potential sources of aerodynamic modulation noise and characterization of nighttime wind conditions and noise;

2) shadow flicker modeling;

3) visibility of the wind turbines; and

4) interests of nearby residents and community.

P.26 Recommendations

To assure informed decisions: Wind turbine noise estimates should include cumulative impacts (40-50 dB(A) isopleths) of all wind turbines.

Isopleths for dB(C) - dB(A) greater than 10 dB should also be determined to evaluate the low frequency noise component.

Potential impacts from shadow flicker and turbine visibility should be evaluated.

Any noise criteria beyond current state standards used for placement of wind turbines should reflect priorities and attitudes of the community.

Preparers of the Report:

Carl Herbrandson, Ph.D.

Rita B. Messing, Ph.D.
Supervisor, Site Assessment and Consultation

[References are included at the end of this report which can be downloaded by clicking here]

Click on the image below to hear what wind turbines sound like and watch video recorded by a Wisconsin wind farm resident last winter.

Posted on Thursday, June 4, 2009 at 07:59PM by Registered CommenterThe BPRC Research Nerd | Comments Off

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