# Cumulative Impact of Wind Farm Noise
## 3 Materials and Methods
The following mechanisms for reducing the noise level were taken into account:  
* Reducing the noise level caused by geometric deviation--reducing the level of noise with the increase in distance from a particular wind turbine;
* Reducing the noise level caused by atmospheric absorption--further reduction of noise in contact with air;
* Reducing the noise level caused by the land effect--further reduction of noise as it travels through the soil between a wind turbine and a receptor. The said calculation method is used in conditions when the sound travels in the direction of the wind. The level of noise is lower when the sound travels in the opposite direction from the direction of the wind. Therefore, the said method is a conservative scenario since it implies that receptors are always positioned in the wind's direction. Separate models were made and noise calculations performed for each of the 49 turbines in the future farm and the 57 turbines that are already in operation. The total noise produced by all the wind turbines was modeled in the zone of the identified sensitive receptor. The paper points out the use of the most conservative method--the noise traveling in the direction of the wind (from the wind turbine towards the receptor), in which case Cmet (meteorological correction factor) was not taken into account in calculations, meaning the case of a wind blowing in the opposite direction was not taken into account, which would produce lower values. In the analysis, the guidelines of A Good Practice Guide to the Application of ETSU-R-97 for the Assessment and Rating of Wind Turbine Noise have also been used [57]. This document provides a framework for measuring wind farm noise and presents indicative noise levels that call for a reasonable level of protection without imposing unreasonable limitations on wind farm development. The working group that dealt with the noise impact in this document recommends the fixed nighttime limit be derived from the criteria for healing [58; 59; 60; 61; 62] and sleep disturbance of 35 dB(A), which served as a basis for noise level projections in this paper. When it comes to analyzing the cumulative effect of several wind farms, the conditions or criteria on the selected receptors that have to be met remain unchanged as compared with the analysis of a single farm's noise production. The allowed values of operating noise are established in line with the recommendations of IFC PSI, i.e., Environmental, Health, and Safety (EHS) Guidelines--General EHS Guidelines: Environmental Noise Management, for wind energy (International Finance Corporation Performance Standards 1--World Bank Group). Allowable wind turbine noise levels recommended by the World Bank (Table 1) are examined against the allowable values envisaged by local legislation in the Republic of Serbia (Table 2) and/or Directives on the noise indicators, limit values, methods for the assessment of noise indicators, disturbance, and negative environmental effects of noise [51; 56]. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**LAeq (dBA)**} \\ \hline Receiver—receptor & Day & Night \\  & 07.00–22.00 & 22.00–07.00 \\ \hline Residential; institutional; educational & 55 & 45 \\ \hline Industrial, commercial & 70 & 70 \\ \hline \hline \end{tabular}  
* The data are for the noise registered outside the buildings [49].  
\end{table} 
Table 1: Recommended noise levels (World Bank) *. Calculating the cumulative effect of noise produced by several wind farms can be a complex process depending on multiple factors, including the size and number of wind turbines, the distance between the farms, and the different characteristics of the surrounding area. The computer-generated model can be used for the simulation of the noise level at different points around the wind farm, including the nearby residential facilities. The results can be used for the prediction of potential wind farm impacts on local levels of noise and for the identification of areas requiring the implementation of noise reduction measures.