This Nordtest method specifies the way in which sound pressure levels are to be measured under prescribed conditions and the way in which prescribed positions around a wind turbine and the way in which the source strength and other acoustical parameters are to be calculated from the measured sound pressure levels.
The source strength is useful for calculations of sound pressure levels in areas adjacent to the wind turbines and for comparing noise emission from different wind turbines.
For the purpose of this Nordtest method the source strength is a measure of the part of the source sound power level which is relevant for the calculations of sound pressure levels at some distance from the wind turbine(s). The measuring positions are chosen so that sound radiated at an angle of 56º below horizontal is measured. This angle is within the range where the maximum emission below horizontal occurs. In general the source strength includes information on source-directional characteristics in the horizontal plane.
The method prescribes 4 measuring positions, but for most purposes the reference position will be sufficient.