## 4 Results  
After being validated in the laboratory, the performance of the masking system was evaluated in situ, in the neighborhood of the high-voltage substation studied. The amplitude and spectrum of the sound pressure generated by the transformers in operation were monitored externally to the substation with the masking system, installed, and activated within the internal environment of the substation. The reference microphone was installed in positions of measurement points #2, #3, and #6 (shown in Figure 3). All acoustic pressure measurements were performed with the calibrated sound meter CESVA SC 420, for the two acoustic conditions studied, i.e., with the masking system activated and deactivated, thus allowing the comparison of the acoustic noise spectra obtained for both studied situations. To establish compliance, the measurement results were also compared with the limits of the sound pressure level recommended by the Brazilian standard ABNT NBR 10151:2019 [8], as defined in Table 1.  
Figure 8 shows the respective processed frequency spectra for the three selected measurement points positioned in the vicinity of the substation. The images on the left document the respective spectra in the absence of the masking effect (disabled masking system) while the images on the right document the respective frequency spectra under the effect of the masking sound superimposed on the sound induced by the operation of substation, i.e., with the masking system activated. Figure 8: Sound pressure frequency spectra in the absence and presence of sound masking. The images on the left refer to spectra measured with the masking system turned off, while those on the right refer to spectra measured under the effect of the masking system. The three lines correspond to measurement points #2, #3, and #6, respectively. The frequency spectra shown in the images on the left of Figure 8 clearly indicate the existence of potential tonal components in the acoustic signal (characterized by red bars) for all three sound pressure measurement campaigns carried out at measurement points #2, #3, and #6. This is, however, only a visual finding made explicit by visual inspection of the frequency spectrum graphs. As shown in Table 2, which documents the processed results of the sound pressure measurement level (SPL), it is necessary to be aware of the existence of false positives (failure to meet the criteria defined in Table 1) in this process of identifying possible tonal components. This is the case of the results associated with measurement point #2 which, although presenting three peaks (red bars) at the frequencies of 125 Hz, 250 Hz, and 400 Hz, in fact, only one of them (at the frequency of 125 Hz) can be characterized as tonal when the regulation criteria are applied. The peaks at frequencies of 250 Hz and 400 Hz simply reflect the presence of harmonics and should be disregarded as they do not meet the criteria defined in Table 1.  
As shown by the arithmetic differences between the values of the acoustic pressure measured in the central band and in the respective adjacent bands, for all three measurement points (#2, #3, and #6), the results shown in red confirm the presence of tonal for the 125 Hz frequency, that is, \(\Delta P\) (dB) \(\geq\) 15 dB. It is important to note here that the criterion needs to be met for both adjacent frequencies. Just to elucidate this framing criterion, it is worth noticing that for the results of measuring point #3, at the frequency of 250 Hz, the regulation criteria of Table 1 is only satisfied for one of the adjacent frequencies. For this case, the applicable filter criterion considered in Table 2 is \(\Delta P\) (dB) \(\geq\) 8 dB.