Challenges facing wind turbine noise propagation modeling and the input data requirements are well understood by ATCO Acoustical Consultants' acoustical engineers.  As the largest and most experienced acoustical engineering & design company in North America, we can guide you from the start to finish of your project.  Some of the measurement and modeling challenges we have addressed are:

Wind Speed – wind turbines typically operate in windy conditions, normally above the wind speed limits called up in international standards for outdoor noise measurement.  Wind can have a huge contribution in the low frequency bands and therefore mask the low frequency noise produced by the turbine.  Regulatory criteria (e.g.,  Ldn, Leq) needs to be qualified by a wind speed parameter in order to be meaningful – 8-10m/s is a common design/measurement range – where the balance of wind and turbine noise produces the highest combined sound pressure level.  Beyond this speed, the wind noise itself starts to mask the turbine noise.  Historical hourly wind data (speed and direction) for both the proposed turbine site and the noise measurement position is useful information for determining the likelihood of complaints.  Wind speed variation with height is also critical – recognizing this data needs to be collected over a relatively long period of time to be useful.

Ambient (Background) Noise – can do a good job of masking the wind turbine noise.  Nevertheless, the ambient noise level depends on how exposed the receiver is to the wind.  It is important to measure the background sound pressure levels at the receiver position in the wind conditions at which the wind turbine will be operating.

Topographical Conditions – receivers that are positioned within deep valleys or are sheltered from the wind in other ways may be exposed to low levels of background noise, even though the wind is strong at the position of the turbine.  These receiver positions relative to the turbines are therefore more likely to be a problem in terms of meeting the noise criteria.  Topographical data is a must for analysis.

Temperature Inversions – are more critical than usual because of the elevation of the noise source above ground.  Weather balloons may be required to obtain the required meteorological data.

Tip Speed – sound power generated by the turbine blades is very sensitive to their tip speed (a fifth order power relationship), and therefore this data is required for analysis.  The blade pitch setting and blade shape are also significant noise prediction factors.  Are the blades fixed or variable pitch design?  Is the rotor upwind or downwind?  While the turbine manufacturer may be able to offer Lw data, it is prudent to also calculate Lw to verify accuracy.

• Gearbox Noise – most turbines are fitted with mechanical noise suppression as standard – emissions are therefore dominated by broadband aerodynamic noise.  However, the mechanical noise should still be included in the acoustical model.
• Measurements – are made in accordance with IEC 61400-11:2006 “Wind Turbine Generator Systems – Acoustic Noise Measurement Techniques.” In accordance with the IEC standard, wind speed at 10m elevation should be measured.

For assistance in reviewing and interpreting the regulatory guidelines that apply to your wind farm project, develop a noise level prediction model, prepare your noise impact assessment for submission to the appropriate authority, please contact ATCO Acoustical Consultants.