With the increasing number of operational wind farms/turbines, the requirement for noise measurements required to demonstrate compliance with planning conditions is increasing as well. The British ETSU-R-97 noise limits are often set relative to measured or standardised 10 m height wind speeds and therefore the assessment of noise from wind turbines requires simultaneous noise and wind speed/direction measurements. For financial reasons, smaller and single turbine sites are often not equipped with a meteorological mast. If no independent hub height wind measurements are available, wind speed is either taken from nacelle anemometers or derived from power measurements combined with the power curve for the respective wind turbine type. Noise measurements referenced to nacelle anemometer data will be compared with the same measurements but correlated with derived power curve wind speed, and measured wind data from separate met mast or other remote sensing devices. The influence of incorrect filtering of wind data for shadow effects (mast and/or nearby wind turbines) on the noise assessment may be presented, depending on how much time is available. The advantages and disadvantages of the various methods will be discussed.
Input data for wind farm noise assessments are provided by the turbine manufacturers in the form of sound power level values for ‘standardised’ 10 m height wind speeds. The format, in which sound power levels are provided, together with possible uncertainties, differs from manufacturer to manufacturer. Some specifications recommend the use of an additional safety margin to allow for measurement uncertainties. Others state measurement uncertainties from test reports, standard deviation from averaging several test reports, K-values depending on the extent of the confidence level (according to IEC/TS 61400-14:2005) or no information about uncertainties at all. An overview of different methods of publishing wind turbine sound power levels and the result of comparing different wind turbine types for the same development site is given.