Combining Statistical Noise Levels and Application to Wind Farm Guidelines

It is stated in wind farm standards that logarithmic addition and subtraction of L_AF90,T sound pressure levels is “not strictly mathematically correct”. An analytical and experimental study reported in Tonin 2024 (a related article) examines the underlying accuracy of combining statistical noise levels as a general proposition, particularly the L_AF90,T and the L_AF10,T. The objective of that study was to explore the accuracy of combining statistical noise levels and what might influence that accuracy. The objective of this study, as foreshadowed in Tonin 2024, is to apply the results to wind farms. It was concluded in Tonin 2024 that values of D₉₀ (being the difference between the logarithmic sum and actual values of L_AF90,T) are negative in the range 0 to − 3 dB (for the cases in the study), meaning that the logarithmic sum of L_AF90,T for the ambient and source sound pressure level distributions is less than the actual value of L_AF90,T for the combined distribution. As a result, in deriving the wind farm noise level (as a contribution), the actual value of L_AF90,T will be less than that determined by logarithmic subtraction of the individual components. In respect of the question of the underlying accuracy of combining statistical noise levels for wind farms, it is concluded that the difference between the logarithmic addition of the L_AF90,T and the true value is less than 1 dB (for the cases in the study). The results are applied herein to a typical wind farm concluding that the simple energy subtraction method adopted in wind farm guidelines is conservative even allowing for the hypothesis that the fluctuation strength of wind farm noise is not invariant but increases with distance. It is also concluded that if wind farm guidelines were to assess wind farm noise on the basis of L_Aeq,T rather than L_AF90,T then adding a value of 2.5 dB to the derived wind farm noise level L_AF90,T as currently specified in the guidelines (i.e., with D₉₀ = 0 dB) would be conservative even allowing for the hypothesis that the fluctuation strength of wind farm noise is not invariant but increases with distance.