Wind Turbines and Noise

Sagrillo Noise Article   |  Common Noise Levels   |  Whisper 4500 Noise
Whisper 1000 Noise  |  Links  |  Simple Sound Measurements

A very common complaint against wind turbines is that they are noisy.  The fact that they make sounds cannot be disputed.  Almost every man-made device makes noise.   The real question is whether the wind turbine will make enough sound to be irritating to those near it.  Irritation is like beauty; it is in the eye of the beholder.  We find the soft whoa-whoa-whoa of the blades as soothing as wind through the leaves of a tree.  Others may find the sounds like the water drips from a leaky faucet.

Mick Sagrillo, a wind turbine guru, has written an article for Windletter magazine which presents the noise issue in a sensible and understandable fashion. The article is reproduced here.  Also included is a table of noise levels for common machines/situations, noise measurements for both a Whisper 4500 and 1000, an email response from an acoustical engineer on taking sound measurements, and links to a few useful web pages.  Some people feel taking wind measurements is inaccurate.  The Danish web page has a formula and on-line calculator which provides an estimate of the dB level at a given distance from a turbine with a known dB level.   In our situation, we do not know the dB rating for the Whisper 4500 right next to the turbine so the calculator had limited usefulness.  This did not stop a neighbor from attempting to use the formula against us.

Small Wind Generators and Noise
by Mick Sagrillo, Windletter Feb & Mar 1997

Frequently, a prospective wind generator owner needs approval for his or her project from a local zoning board. Occasionally, permission for the wind turbine is held up by someone with concerns about the noise that the wind system may produce. Many times, the installation in question is the first that the zoning board has dealt with. Problems are compounded because there is not another residential wind system nearby to serve as a basis from which the zoning board can make first-hand observations and, subsequently the proper decisions.

Noise is a very subjective topic with people. Sounds that are soothing to one person may make another crazy. One person's playful puppy may be a yapping mutt to a neighbor. Fortunately. noise is measurable in relation to the background sounds in our environment. As a result, zoning boards can choose to deal with the noise issue objectively.

Sounds detectable by the human ear are measured in decibels, or dB(A), with a device known as a decibel meter. The average background noise in a house is about 50 dB(A), while a car driving down a street will be measured at 60 dB(A) at a distance of 300 feet. A "quiet" vacuum cleaner will emanate sound at 70 dB(A), about the same noise level that is attributed to an expressway when standing 100 feet away from it. Trees on a windy day will measure about 55 dB(A) on a decibel meter.

Two additional facts are key to this discussion. First, the human ear can distinguish sound differences of about3 dB(A). This means that a sound source must be 3 dB(A) louder than ambient sound before there is a noticeable difference between a sound and other background noises. Second, sound decreases with distance from the source. Therefore, any sound emitted from a wind turbine will blend into the background noise as one moves further from the tower.

Sound from a wind generator comes from two sources: the drive train and the blades. Utility-scale equipment used in wind farms has a drive train that includes a transmission, high-speed generator, couplings between the two, brake, mainframe, and nacelle. Much of the noise generated by utility-scale turbines comes from the high-speed transmission and generator. In contrast, most residential-sized wind generators are direct-drive devices and, therefore, do not have such transmissions. Direct-drive generators are quite slow-speed when compared to wind farm equipment. The one system on the market that does have a gearbox utilizes a quiet, low-speed transmission.

Most of the sound that comes from a residential sized wind turbine is aerodynamic noise caused by the blades passing through the air. These systems are variable speed devices, which means that as wind speed increases, so does the speed of the blades. Therefore, the aerodynamic noise that the blades generate increases with increasing wind speed.

Aerodynamic noise is also a function of the tip speed of the blades. Tip speed ratio (TSR) is a term that refers to the speed of the tip of a wind generator blade in relation to wind speed. For example, a wind system that operates with a TSR of 10 means that when the wind speed is 25 mph, the tips of the blades are moving at 250 mph. Increasing tip speed results in more noise. Slow-speed wind generators operating with 7 TSRs of about seven emit noise that is barely discernible from ambient noise.

The sound that is emitted by any source is compared to existing background noise. In the case of a wind generator, that background noise might include traffic, farm machinery, barking dogs, children playing, lawn mowers, and even the environment itself. Trees, shrubs, and cornfields rustle and buildings may tattle as they interact with the wind. All of these sounds increase with wind speed until a howl is created.

Residential-sized wind turbines generate aerodynamic noise rather than drive-train noise. Since these turbines are variable speed devices, they create more sound as wind speed increases. In addition, the wind itself creates sound and ambient background noise increases with increasing wind speed, thereby effectively masking much of a small turbine's aerodynamic sound.

With this backdrop of ambient noise, aerodynamic wind generator noise may be discernible from background noise, but barely. People can detect a change in sounds if there is a 3 dB(A) difference between those sounds. Karl Bergey of Bergy Windpower Company points out that even though the intensity of the sound coming from a wind generator may be the same as the ambient noise, the frequency may be different. Therefore, wind turbine sounds may be distinguishable from ambient noise even though they, are not louder. In fact, if you listened carefully, you would be able to identify all of the other components of ambient noise as well, including things like barking dogs, traffic, kids, tractors, and even trees.

Bergey disclosed the results of a sound test carried out on his company’s 10 kW BWC Excel wind system. At a distance of 300 feet and in 25 mph winds, the BWC Excel generated sound with a 54 dB(A) to 55 dB(A) rating, making the wind generator barely audible over the 52.5 dB(A) rating of the surrounding environment's background noise. At about 500 feet, the BWC Excel sound rating was 53 dB(A), making it just another part of the background sound.

The Clinton (Iowa) Detective Bureau responded to a noise complaint against a IO-kW Jacobs wind system belonging to dealer Dan Whitehead by sending a trained person out to Whitehead's site to monitor the wind System with a decibel meter. In wind speeds between 16 mph and 36 mph and at a position only 50 feet from the wind generator, the decibel meter registered the sound of the wind generator between 55 dB(A) and 59 dB(A). The detective noted that, "at this location, the sound output front the generator was observed to be partially masked by the sounds from the rustling of leaves in the trees". When the decibel meter was pointed at the trees (which were 300 feet away), the meter registered the tree sounds at 60 dB(A) to 62 dB(A). The conclusion was that the wind generator sounds were "inconsequential in total noise emission."

Unfortunately, there is currently no sound rating system in the United States for small wind generators. Bob Sherwin of Atlantic Orient Corporation served as the chairman of the AWEA Standards Coordinating Committee from 1985 to 1995, and is currently a technical advisor on the Acoustical Working Group for the International Electrical Technical Commission in Geneva, Switzerland, which developed international standards .for all types of electrical generating equipment, including wind generators. According to Sherwin, "noise hasn’t been a significant issue with small wind turbines".

All of the small turbine manufacturers currently doing business in the United States agreed. Even so, anyone complaining about noise at zoning hearing can, and sometimes does, effectively stop the installation of a residential-sized wind system. Usually, the antagonist has no experience with the sound emissions of a small wind turbine, and has formed an opinion based on hearsay, speculation, or something they have seen on television.

In a presentation delivered at the WINDPOWER ’95 conference, Emil Moroz of the University of Texas-El said that noise problems "are compounded by the different subjective responses of individuals, which often depend more on personal attitude than actual noise level." Also, in Wind Power Comes of Age, Paul Gipe wrote "wind turbines will always remain audible to those who choose to hear them".

However, since sound is measurable and quantifiable, zoning boards should be able to take wind generator sound out of the realm of the subjective and make it more objective. Hearsay and speculation are no basis for decision making.

Mick Sagrillo

Typical Sound Pressure Levels (SPL) in dB(A)
From Paul Gipe's Wind Power for Home and Business

Sources: Handbook of Noise Measurement, General Radio, European Wind Energy Assoc., and Bergey Windpower Co.

Whisper 4500 Sound Measurements

Site: Mountain Spirit Temple Tehachapi, California 7/1/98, Owner Eric Berall
Machine: 4.5 kW Whisper - 48 Vdc
Tower: Rhon 45 G, 60 feet
S.P.L. Device: Realistic -C weighted
Conditions: Variable wind 8-18 mph, clear, 80 degrees

NOTE:
Start Time: 15:00 Hrs
Stop Time: 17:30 Hrs.
Machine output varied between 300 and 1,500 watts.

Whisper 1000 Sound Measurements

Site: Shrewsbury, VT, Owner: Arthur Krueger, P.E.
Machine: Whisper 1000
Tower: 64 foot, 4 inch guyed pipe tower
S.P.L. Device: Realistic -C weighted
Conditions: Wind Advisory, 25-35 mph, turbine going in and out of tipped position

Links to web sites that address noise

Danish Wind Turbine Manufacturer's Web Site
This web site is very useful.  The provided link takes you directly to the section dealing with wind turbine noise.  It even includes an on-line dB calculator.  If you know the dB output of your wind turbine, you can calculate the dB level at any distance from the turbine.  Please note that most of the wind turbines the Danish site deals with are industrial sized (e.g. 40 meter diameter rotor).  Therefore, the baseline 100 dB sound in their calculator may be high for a modern small residential wind turbine.

National Wind Technology Center - Environmental Issues
This web site has many references to useful studies.  The link will take you to the section dealing with sound measurements and wind turbines.

American Wind Energy Association - Low Frequency Noise
The entire AWEA site is useful.  This link takes you directly to a brief description of low frequency noise.  Some industrial-sized down-wind (no tail) wind turbines can create a vibrational noise that may be damaging.  Low frequency noise is generally not a problem for up-wind residential-sized wind turbines. 

Simple Sound Measurements of Windmills

Brief email message I received from an Acoustical engineer.

The measurement taken when facing into the wind, indicates that the self noise of the wind shield and microphone has resulted in an even higher noise level. To check whether the noise you measured was the turbine or wind, try turning the turbine off during the measurement and see if there is a corresponding drop in the noise level measured. The way to get over this measurement problem is to undertake measurements on a ground board (minimum diameter 1 metre) with the microphone diaphragm placed as close to the surface of the board as possible. This takes the mic out of the wind, well almost, but you have to allow for the effect of pressure doubling due to the very close proximity of the board surface. This effect is equivalent to an increase of 6 dB over the free field measurement, i.e. the one you made holding the sound level meter at chest height.
Malcolm D Hayes
Consultants in Acoustics
Hayes McKenzie Partnership
Lodge Park
Tre'r-ddol
Machynlleth
Powys SY20 8PL
UK