Sound
Modern turbines are quieter than their predecessors. In addition, the background noise of the wind often masks the sound generated by the turbine. Nonetheless, wind turbines produce audible sound, which must be considered in selecting their locations. Well-sited wind farms employ appropriate setbacks between the turbine and nearby residences to reduce or eliminate discernible sound from the turbines. Depending on the specific site, proximity to nearby residences, and the permitting regulations, wind farms are typically required to address potential sound issues in the permitting process and need to demonstrate that the project will comply with the applicable sound-level regulations. In most cases, acoustic modeling is performed before and after a wind project is constructed to ensure that sound to residents in proximity of a wind turbine is below the appropriate thresholds.
Sound Perception
Sound perception at a receptor can be measured on a logarithmic scale of sound pressure level in units of decibels (dB). This means that a perceived doubling of sound is represented by a difference of 10 dB. Background sound level is usually between 30 and 45 dB. Below is a diagram showing examples of the decibel levels of several common sounds.
Source: American Wind Energy Association
The majority of wind turbine sounds are masked by background noises, including the sound of the blowing wind. However, all rotating wind turbines will make some type of sound. From less than a quarter of a mile away, the sound from wind turbines is not expected to exceed 55 dB, which is about as loud as an average home or office. The specific topography of each wind farm location will affect the extent to which sound travels. A preconstruction sound study can be conducted to estimate the distance wind turbine sound is likely to travel at a specific site.
While sound can be measured objectively, studies have been done on sound perception and annoyance levels as a function of both increasing sound levels (higher decibels) and attitudes toward other aspects of wind power. A 2005 study found that residents in a rural setting were increasingly annoyed by turbine sound when they perceived the sound to be louder. In some instances, the sound had actually gotten louder, but in other instances, the sound level was held constant. The study found a correlation between increased annoyance and negative attitudes toward turbines, negative attitudes toward turbine impacts on landscapes, a lack of control over the wind project, or a sense of being subjected to injustice from the wind project (Pederson and Waye 2005).
Types of Wind Turbine Sound
Wind turbines make different types of sound, including broadband, infrasonic, impulsive, and tonal sound. The presence of wind turbine sound is dependent on atmospheric conditions, including air flow patterns and turbulence, and the ability to perceive wind turbine sound varies based on the presence of other nearby sources of sound, manmade or otherwise, and site-specific topography.
Broadband Sound
Broadband sound is made up of a combination of sound waves with different frequencies. Broadband sound has no distinct pitch and could be described as a humming, whooshing, or swishing sound. Broadband sound does not start or end abruptly. Broadband sound has frequencies higher than 100Hz and is most typically caused by the interaction of the turbine blades and atmospheric turbulence. Low-frequency sound (between 20 to 100Hz) usually only occurs when the wind turbine blades are located on the downwind side of the turbine tower. The turbine blades experience airflow deficiencies because airflow is partially blocked by the tower. Low-frequency noise can often be felt before it's heard clearly. An example of low-frequency broadband sound is the rumbling of a train heard from far away.
Infrasonic Sound
Infrasonic sound is low-frequency sound of less than 20Hz. Infrasonic sound is always present in the environment. Depending on the locations, examples include the sound of flowing water, waves, or air turbulence. Infrasonic sound can propagate further than higher, more audible frequencies, but it has higher levels of dissipation and blends in with ambient noise. Though infrasonic noise is barely audible, it can cause structural vibration, such as windows rattling. In the majority of cases, window vibration caused by infrasonic sound from a wind turbine will be inaudible.
Impulsive Sound
Impulsive sound can be generated when disturbed airflow interacts with turbine blades or when multiple turbines making swishing noises synchronize in stable winds. Impulsive sounds are characterized by thumping sounds that can vary in amplitude over time. As with low-frequency sound, impulsive sound from a single turbine tends to occur in downwind turbines as a result of air flowing around the tower to reach the blades. Examples of impulsive sounds include a door slamming, a person stomping, or a clap of thunder.
Tonal Sound
Tonal sound can be caused by the rotation of shafts, generators, and gears operating at natural frequency; unstable airflow over holes or slits; or non-aerodynamic instabilities interacting with the blade surface. Tonal sounds can have a distinct pitch, such as a music note, and do not start or end abruptly. Because tonal sound can be problematic, wind turbine manufactures are paying particular attention to addressing tonal sound produced by the operation of their turbines.
Turbine and tower design can also affect sound output. Modern commercial turbines are designed so that the turbine is upwind of the tower, which mitigates low frequency and impulsive sound. As previously referenced, the hub, rotor, and tower components of a turbine sometimes emit mechanical sound. Further research is being done to develop sound mitigation techniques.
More Information
Some of the following documents are available as Adobe Acrobat PDFs.
Download Adobe Reader.
Primer on Issues of Wind Turbine Noise and Perceptions (PDF 2.3 MB)
Tony Rogers of the University of Massachusetts Renewable Energy Research Laboratory presented a primer on issues of wind turbine noise and perceptions at a Massachusetts Wind Working Group meeting in January 2006. The presentation was based on the report, "Wind Turbine Acoustic Noise (PDF 776 KB)." Fact Sheet on Wind Turbine Sound (PDF 127 KB)
The American Wind Energy Association has created a fact sheet on wind turbine sound. Study on Wind Turbines and Low Frequency Sound (PDF 919 KB)
"Low Frequency Noise and Vibration and its Control: Do wind turbines produce significant low frequency sound levels?" was presented at the 11th International Meeting in Maastricht, The Netherlands. August 30-September 1, 2004. -
Provided by the Danish Wind Industry Association.
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Information on the second international conference on wind turbine noise.
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From the Department of Trade and Industry of the United Kingdom.
Sound Regulations
Some states have regulations that set limits for the acceptable sound impact of a wind generator on its neighbors. In addition, many towns have their own noise ordinances.
Massachusetts
The Massachusetts Department of Environmental Protection (DEP) prohibits wind turbines from adding more than 10 dB to the ambient, or background, sound level. This criteria is measured at both the property line and the nearest inhabited residence.
Maine
Maine's sound standards begin on page 11 of this document (MS Word 169 KB) Connecticut
Connecticut's regulations on noise control (PDF 74 KB) Environmental Protection Agency
Since 1981, noise regulation has been handled at the state and local government level. Nonetheless, the U.S. Environmental Protection Agency's (EPA's) noise guidance documents are still on the books. The EPA has identified noise levels affecting human health and welfare. They specify levels below 55 dB for outdoor sound as preventing activity interference and annoyance in its sound guidelines.
Sound Mitigation and Management Tools
Tools that are available to manage or mitigate the potential impact of wind turbine sound include:
- Maximizing the distance of wind turbines from the nearest property line.
- Conducting studies to ensure that sound at property lines (turbine sounds + background noise) do not surpass the Environmental Protection Agency sound guideline of 55 dB for outdoor sound.
- Ensuring setbacks from nearest abutting property lines of at least the height of one turbine.
- When setbacks alone cannot fully mitigate all potential sound issues, other approaches may be used.
- "Sound easements" on adjacent property that gives the developer the right to generate sound carrying over onto that property.
- Sharing of some portion of landowner royalty payments with affected neighboring landowners.
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