Acoustics Information
Intro | Sound and Decibels | Outdoor Sound Propagation | | Noise Impact and Regulation
Wind Turbine Sound
The sources of sound from operation of wind turbines are mechanical noise from gears, motors and cooling equipment in the turbine nacelle and the aerodynamic effects of the rotor blades traveling through the air. When operating at or near full sound output, the primary noise source from a wind turbine is rotation of the rotor blades with more sound energy generated from the outer sections of the blade and blade tip.
An international standard has been developed as IEC 61400-11 Wind turbine generator systems – Part 11: Acoustic noise measurement techniques that provides specific and detailed procedures for determining the sound power level from wind turbines. The IEC standard was develop by industry and acoustic experts to establish a consistent and repeatable method with full documentation for determining the sound output of any type of vertical blade wind turbine. Manufacturers of utility-scale wind turbines follow this method to determine the sound output and uncertainty of their turbines for purposes of estimating community sound levels and providing performance guarantees to owners and operators of wind energy facilities.
There has been much advancement in the technology of wind turbines over the last 10 to 20 years. The first generation of utility wind turbines consisted of downwind rotors that were capable of generating significant levels of low frequency sound. Turbines with upwind rotors have replaced the early designs and drastically reduced low frequency sound emissions. Modern wind turbines are known to generate a “whoosh” type sound under certain operating and weather conditions that results from the passage of each blade. A short-term increase in sound levels often occurs on the down-stroke motion of the blade that is referred to as “amplitude modulation” and generally results in sound level fluctuations of 2 to 5 dBA for utility-scale wind turbines with occasional excursions above 6 dBA.1 Amplitude modulation occurs at a mixture of audible frequencies and should not be confused with low frequency and infrasound.
Sound from wind turbines has been the subject of extensive research, conferences and publications over the past 10 to 15 years. There is considerable technical and other information available that addresses the characteristics, control and impact of sound from wind turbines. There is an abundance of well-researched and informative studies and reports from reputable institutions and individuals.
It is a common assertion that wind turbines generate significant and perhaps harmful levels of infrasound and low frequency sound. In relation to the modern generation of upwind turbines, there is little basis for this claim that can be found in any well-researched and impartial technical studies and literature. In fact, the consensus of the independent research community is that annoyance from wind turbine sound is primarily in the most audible mid to high frequencies and not from infrasound or low frequency sound.2
The noise impact that results from wind turbines depends on several factors, notably the change or increase in ambient or background sound levels that will result from turbine operation. For rural areas where hill or ridge top wind turbines are located, the ambient sound level at lower elevations and community locations varies by time of day and to some degree by season. Sound levels from wind turbines vary based on the wind speed and turbulence at the turbine hub and can range from no sound output during calm winds to full sound output when winds at the turbine hub reach approximately 20 miles per hour. Wind turbines will be most noticeable during stable atmospheric conditions when surface winds are light and the winds aloft (at the turbine hub) remain high enough for full turbine sound output. At other times, when surface winds increase or when wind turbine output diminishes, the sound from operating wind turbines will be less noticeable.
During the planning stages of a wind energy project, considerable effort is made to accurately map land uses and the topography of the entire area potentially impacted by sound from wind turbine operation. Along with wind turbine sound level performance data, this information is used to develop a sound level prediction model for the project. The model inputs and settings are typically adjusted to produce conservative sound level estimates for wind turbine operation. These results are compared to various noise regulations and guidelines to assess the impact of the proposed wind energy project.
1. Observations and analysis of sound level measurements for Mars Hill Wind Farm and Stetson Wind Project, R.S. Bodwell, P.E. and Van den Berg, G. P.,The Sounds of High Winds, the effect of atmospheric stability on wind turbine sound and microphone noise, University of Groningen, 2006.
2. Van den Berg, G. P., The Sounds of High Winds and Danish Electronics, Light and Acoustics, Low Frequency Noise from Large Wind Turbines, 2008.
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