## 4 Results
The presence of standing waves was detected by a 3D Camera operated by 48 microphones arranged in a ring shape, whose technique and applications are discussed elsewhere [16]. In order to assess the stability of the noise generated by the transformers and by the masking system, Figure 9 presents spectrograms of the sound pressure level at measurement point #6. Finally, Figure 10 presents the sound pressure levels \(L_{Aeq,1s}\) at measurement point #6, in the absence and presence of sound masking. Figure 9: Spectrograms of the sound pressure levels measured at measurement point #6, in the absence and presence of sound masking. The top image refers to spectra measured with the masking system turned off, while the bottom image refers to spectra measured under the effect of the masking system. Once the tonal component of the acoustic noise was identified based on the regulation criteria defined in Table 1--which incurs a penalty of + 5 dB in the measured values--measurements were taken of the occupational and environmental noise descriptors, characterized by mean values to over a period of time (\(T_{e}\) = 5 min), expressed by instantaneous values that result from the weighting based on Curve A, expressed by Equation (2). To represent the physical characteristics of the measurement of a quantity that varies in the fields of frequency and time, the CESVA SC 420 sound meter also performs independent measurements of the acoustic descriptors \(L_{10}\) and \(L_{95}\) previously described. Data in Table 3 summarizes measurement results of the characteristic statistical descriptors performed at the same three measurement points #2, #3, and #6, at 11:30 p.m.; i.e., near the changeover from day to night periods (the night period starts at 10:00 p.m. and ends at 10:00 a.m.). In this table, \(LA_{eq}\) denotes the value corresponding to the value \(L_{eq}\) shown in Table 2, but weighted in Curve A. The value \(L_{R}\) is defined by the value of \(L_{95}\), penalized by +5 dB in cases where the tonal component in noise is present, as explained in Section 2.3. The results shown in Table 3 confirm, for all three measurement points investigated, that, when properly implemented, the masking sound (continuous noise) is able to flatten the tonal characteristics associated with the background noise (i.e., compliance of the descriptor \(L_{R}\) with the regulation). Except for the value 50.9 dB (measured at measurement point #2), all other values comply with the regulation imposed for the night period (<50 dB). This small discrepancy in the threshold value should not be a matter of concern as this measurement point is very close to the heavy steel access gate of the substation, whose  
Figure 10: Sound pressure levels measured at measurement point #6, in the absence and presence of sound masking. The top image refers to the levels measured with the masking system turned off, while the bottom image refers to the levels measured under the effect of the masking system. The blue shaded area indicates a region that has been edited to remove the sound of a dog barking. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Measurements with the Masking System Disabled**} & \multicolumn{3}{c}{**Measurements with the Masking System Activated**} \\ \cline{2-9}
**Measuring Point \#** & \(LA_{\text{eq}}\) & \(L_{\text{10}}\) & \(L_{\text{95}}\) & \(L_{R}\) & \(LA_{\text{eq}}\) & \(L_{\text{10}}\) & \(L_{\text{95}}\) & \(L_{R}\) \\ \cline{2-9}  & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** \\ \hline Meas.Point \#2 & 50.3 & 50.2 & 50.2 & 55.2 & 53.1 & 55.7 & 50.9 & **50.9** \\ Meas.Point \#3 & 49.9 & 50.4 & 48.8 & 53.8 & 50.4 & 51.1 & 48.8 & 48.8 \\ Meas.Point \#6 & 44.7 & 45.5 & 43.3 & 48.3 & 46.3 & 46.8 & 45.7 & 45.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of measurements of characteristic statistical descriptors. The red color indicates values that exceed the legal limit for the night period (50 dB).