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(Reprinted with permission of the American Wind Energy Association)

What is Noise?
“Noise”, when one is talking about wind energy projects, basically means “any unwanted sound.”

Whether a noise is objectionable will vary depending on its type (tonal, broadband, low-frequency, impulsive, etc.) and the circumstances and sensitivity of the individual who hears it (often referred to as the “receptor”).

As with beauty, often said to be “in the eye of the beholder”, the degree to which a noise is bothersome or annoying is largely in the ear of the hearer. What may be a soothing and relaxing rhythmic swishing sound to one person may be quite troublesome to another.

Because of this, there is no completely satisfactory and impartial way to measure how upsetting a noise may be to any given person. Still, it is possible to objectively measure how loud a noise is. Here is a table showing the loudness (“sound pressure level”) of some common noises.

Comparison of common Sound Pressure Level

Source: Canadian Centre for Occupational Health and Safety
(see www.ccohs.ca/oshanswers/phys_agents/noise_basic.html)

What kinds of noise do wind turbines produce?
Wind turbines most commonly produce some broadband noise as their revolving rotor blades encounter turbulence in the passing air. Broadband noise is usually described as a “swishing” or “whooshing” sound.

Some wind turbines (usually older ones) can also produce tonal sounds (a “hum” or “whine” at a steady pitch). This can be caused by mechanical components or, less commonly, by unusual wind currents interacting with turbine parts. This problem has been nearly eliminated in modern turbine design.

How noisy are wind farms?
Good question and a difficult one.
Wind plants are very, very quiet compared to other types of industrial facilities such as manufacturing plants, but most industrial plants are not located in rural or low-density residential areas. In those types of areas, background noise tends to be lower than in urban areas.

On the other hand, wind plants are always located where the wind speed is higher than average, and the “background” noise of the wind tends to “mask” any sounds that might be produced by operating wind turbines – especially because the turbines only run when the wind is blowing. The only occasional exception to this general rule occurs when a wind plant is sited in hilly terrain where nearby residences are in dips or hollows downwind that are sheltered from the wind – in such a case, turbine noise may carry further than on flat terrain.

Virtually everything with moving parts will make some sound, and wind turbines are no exception. However, well-designed wind turbines are generally quiet in operation, and compared to the noise of traffic jams, trains, aircraft, and construction activities, to name but a few, the noise from wind turbines is very low.

Noise used to be a very serious problem for the wind energy industry. Some early, primitive types of turbines built in the early 1980s, were extremely noisy, to the point that it was annoying to hear them from as much as a mile away. The industry quickly realized that this problem needed to be dealt with, however (particularly in Europe, where turbines are often located in or near residential areas), and manufacturers went to work on making their machines quieter.

Today, an operating wind farm at a distance of 750 to 1,000 feet is no noisier than a kitchen refrigerator or a moderately quiet room.

Source: the Scottish Office, Environmental Department, Planning Advice Note, PAN 45, Annex A: Wind Power, A.27. Renewable Energy Technologies, August, 1994. Cited in “Noise from Wind Turbines,” British Wind Energy Association, http://www.britishwindenergy.com.uk/ref/noise/html

The best test is to simply experience the noise from a turbine for yourself. You will find that you can stand directly beneath a modern wind turbine and have a normal conversation without raising your voice.

What have manufacturers done to reduce wind turbine noise?
Most rotor are upwind: a wind turbine can be either “upwind” (that is, where the rotor faces into the wind) or “downwind” (where the rotor faces away from the wind). A downwind design offers some engineering advantages, but when a rotor blade passes the “wind shadow” of the tower as the rotor revolves, it tends to produce an “impulsive” or thumping sound that can be annoying. Today, almost all of the commercial wind machines on the market are upwind designs, and the few that are downwind have incorporated design features aimed at reducing impulsive noise (for example, positioning the rotor so that it is further away from the tower).

Towner and nacelles are streamlined: Streamlining (rounding or giving an aerodynamic shape to any protruding features and to the nacelle itself) reduces any noise that is created by the wind passing the turbine. Turbines also incorporate design features to reduce vibration and any associated noise.

Soundproofing in nacelles has been increased: The generator, gears, and other moving parts located in the turbine nacelle produce mechanical noise. Soundproofing and mounting equipment on sound-dampening buffer pads helps to deal with this issue.

Wind turbine blades have become more efficient: As the wind energy industry and wind engineers gain more experience with wind turbine operations, turbine blades are constantly being redesigned to make them more efficient. The more efficient they are, the more the wind’s energy is converted into rotational energy and the less aerodynamic noise is created.

Gearboxes are specially-designed for quiet operation: Wind turbines use special gearboxes, in which the gear wheels are designed to flex slightly and reduce mechanical noise. In addition, special sound-dampening buffer pads separate the gearboxes from the nacelle frame to minimize the possibility that any vibrations could become sound.

What other noises are associated with large wind projects?
Wind turbines are large pieces of industrial equipment and installing them is, in essence, a major construction project. The construction phase of a project lasts only a few months, but during that period, noises will be produced that are typical of heavy construction including:
Truck traffic: A modern wind turbine is larger than a Boeing 747, with rotor blades that weight thousands of pounds each and must be trucked to the site along with tower sections and other large components. The sound level is that caused by a highway truck moving at slow speed.
Heavy Equipment: A large construction crane is usually needed to install the nacelle and rotor atop the turbine tower. Cement mixing is necessary for turbine foundations. The sound levels of this equipment are comparable to highway trucks moving at a slow speed.
Foundation blasting: May occasionally be required if the wind plant is being installed in bedrock close to the surface and it cannot be broken up by other any means. More frequently, Foundation holes are excavated using backhoes, sometimes with pneumatic hammers to break up subsoil rocks.

Obviously, it is desirable for construction activities that are likely to produce noise to be scheduled during normal working hours.

What can be done to reduce the likelihood of a noise problem from a wind project?
A noise analysis can be done based on the operating characteristics of the specific wind turbine that will be used, the type of terrain in which the project is located, and the distance to nearby residences. Particular attention will need to be paid if residences are sheltered from the wind.

Also pre-construction noise surveys can be conducted to find out what the normally-occurring background noise level are at the site, and to determine later on what, if anything, the wind project has added to those levels.

The most common method for dealing with a potential noise issue as indicated above is to simply require a setback or minimum distance between any of the wind turbines on the project and the nearest residence. That is sufficient to reduce the sound level to a regulatory threshold.

Some permitting agencies have set up noise complaint resolution processes. In such a process, typically, a telephone number through which the agency can be notified of any noise concern is made public, and agency staff work with the project owner and concerned citizens to resolve the issue. The process should include a technical assessment of the noise complaint to ensure its legitimacy.

In general, wind plants are not noisy, and wind is a good neighbor. Complaints about noise from wind projects are rare, and can usually be satisfactorily resolved.

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