Documents Home
View PDF, DOC, PPT, and XLS files on line

Add NWW documents to your site (click here)

Sign up for daily updates

Keep Wind Watch online and independent!

Donate $10

Donate $5

News Watch

Selected Documents

Research Links


Press Releases


Publications & Products

Photos & Graphics


Allied Groups

Effect of Atmospheric Stability on Low Frequency Modulated Sound of Wind Turbines  

Author:  | Noise

Abstract: Sound from wind turbines involves a number of sound production mechanisms related to different interactions between the turbine blades and the air. An important contribution to the low frequency part of the sound spectrum is due to the sudden variation in air flow which the blade encounters when it passes the tower: the angle of attack of the incoming air suddenly deviates from the angle that is optimized for the mean flow. Hitherto, low-frequency sound from wind turbines has not been shown to be a major factor contributing to annoyance. This seems reasonable as the blade passing frequency is of the order of one hertz where the human auditory system is relatively insensitive. This argument, however, obscures a very relevant effect: the blade passing frequency modulates well audible, higher-frequency sounds and thus creates periodic sound: blade swish. This effect is stronger at night because in a stable atmosphere there is a greater difference between rotor averaged and near-tower wind speed. Measurements have shown that additional turbines can interact to further amplify this effect. Theoretically the resulting fluctuations in sound level will be clearly perceptible to human hearing. This is confirmed by residents near wind turbines with the same common observation: often late in the afternoon or in the evening the turbine sound acquires a distinct ‘beating’ character, the rhythm of which is in agreement with the blade passing frequency. It is clear from the observations that this is associated to a change toward a higher atmospheric stability. The effect of stronger fluctuations on annoyance has not been investigated as such, although it is highly relevant because a) the effect is stronger for modern (that is: tall) wind turbines, and b) more people in Europe will be living close to these wind turbines as a result of the growth of wind energy projects.

Noise Notes, Volume 4, Number 4, October 2005, pp. 15-40

Also published in Journal of Low Frequency Noise, Vibration and Active Control, Volume 24, Number 1 / March 2005, pp. 1-24

Download original document: “The Beat is Getting Stronger: The Effect of Atmospheric Stability on Low Frequency Modulated Sound of Wind Turbines

This material is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

The copyright of this material resides with the author(s). As part of its noncommercial effort to present the environmental, social, scientific, and economic issues of large-scale wind power development to a global audience seeking such information, National Wind Watch endeavors to observe “fair use” as provided for in section 107 of U.S. Copyright Law and similar “fair dealing” provisions of the copyright laws of other nations. Queries e-mail.

Wind Watch relies entirely
on User Funding
Donate $5 PayPal Donate


Get the Facts Follow Wind Watch on Twitter

Wind Watch on Facebook


© National Wind Watch, Inc.
Use of copyrighted material adheres to Fair Use.
"Wind Watch" is a registered trademark.