Potential impacts from operational noise produced by wind turbines is a major issue during the project planning and permitting process, particularly for projects east of the Mississippi River in fairly populous areas. While still an issue farther west, more buffer space and lower population densities sometimes make noise less of a factor. In general, however, noise may be the principal obstacle, from an environmental impact standpoint, to the more rapid growth of this renewable energy source in the United States. Proposed projects are frequently opposed on noise concerns, if not outright fear, usually aroused by the highly biased misinformation found on numerous anti-wind websites. While significant noise problems have certainly been experienced at some newly operational projects, they are usually attributable to poor design (siting units too close to houses without any real awareness of the likely impact) or to unexpected mechanical noises, such as chattering yaw brakes or noisy ventilation fans. A common theme at sites with legitimate complaints is that no one-not the developer, their consultants or the regulatory authority-really understood the import and meaning of the sound levels predicted at adjacent homes in project environmental impact statement (EIS) noise modeling. This paper seeks to address this lack of knowledge with suggested design goals and regulatory limits for new wind projects based on experience with the design of nearly 60 large wind projects and field testing at a number of completed installations where the apparent reaction of the community can be compared to model predictions and measurements at complainant's homes.

Despite the use of windscreens, the measurement of ambient sound levels or noise emissions in quiet environments can be adversely affected by wind blowing over the microphone. This is especially true when environmental impact assessments are being carried out for proposed wind turbine power projects where the objective is to determine the level of background masking noise available as a function of wind speed, since any potential noise impact from the project will only occur under moderately windy conditions. Under calm conditions the project will produce no noise at all. A number of windscreen products are commercially available for short and long-term sound level monitoring in adverse weather conditions. Generally, these windscreens vary by physical size and the method of preventing water from reaching the microphone. High frequency attenuation effects are usually available from the product suppliers but, in general, low frequency turbulence effects are not available. Consequently, a controlled laboratory test program was carried out in a state-of-the-art wind tunnel at the Fraunhofer Institut fur Bauphysik in Stuttgart, Germany to quantify the level of low frequency interference (down to 6.3 Hz) associated with a number of different foam windscreens and an aerodynamic microphone nose cone. A total of nine configurations were tested with "quiet" airflow only, artificial noise only and noise plus airflow to evaluate both low frequency wind induced noise and high frequency attenuation effects. The test program demonstrated that the largest size foam-based windscreens provided the most protection from flow induced noise due to wind. Flow induced noise by air flow alone was estimated from the study results and compared to community noise measurements at a typical wind turbine site. It was determined that flow induced wind noise does not have a significant or detrimental effect on the measurement of A-weighted sound levels under wind conditions of concern as long as the suggested measurement techniques described herein are followed

Noise limits or criteria in many residential communities are based on an allowable increase in level above the prevailing ambient sound level, usually on the order of 3 to 5 dBA. The theory is that such a small increase results in masking of the new or existing intruding facility noise by the environmental ambient sound. However, many facilities are planned for installation or already exist in remote and very quiet ambient environments with residual levels as low as 25 dBA during the day or nighttime. This paper summarizes a study conducted to determine appropriate criteria in such remote and quiet areas where little or no noise masking will occur from ambient sources. The paper discusses both audible noise measured by the A-weighted level, low frequency noise characterized by the C-weighted level and tonal noise.

Hessler and Hessler collected a two week database of wind turbine 10-min L-90 A-weighted levels in three orthogonal directions from an essentially solitary wind turbine (unit located at end of north-south row). These data have been analyzed statistically and this analysis shows that the wind turbine emissions during the day and at night form a clearly normal distribution with a mean level of 32 dB and standard deviation of 2.4 dB for daytime, 7 Am to 10 PM and a mean level of 36 dB and standard deviation of 2 dB, for nighttime, 11 PM to 5 AM. The nighttime hours were selected as the louder hours of the night when, presumably, an inversion was present. The statistical plots clearly show the data collected for wind turbine non-operation, the transition region between non-operation and full operation, the region of full turbine power, and the data that represent discreet noisier events. This result from a comprehensive, single survey suggests that it is necessary to analyze the noisier nighttime hours separately from daytime or the entire 24-h day, if one is to correctly predict or measure the noise during these critical nighttime hours.