## 2 Theoretical Background  
### Basics of Acoustics  
Acoustic quantities are commonly expressed in decibels (dB), a logarithmic ratio defined by the magnitude of the acoustic power in relation to a reference power level (normally \(P_{0}\) = 20 uPa). However, due to the different sensitivity of the ear to the different frequencies, the raw values obtained are not directly related to the loudness of the sound in question. In order to make the measurements more representative of the loudness associated with any sound, it is often desirable to weigh the values for each frequency in accordance with the way in which the human ear would respond to the sound.  
The use of weighting in acoustic measurement is equivalent to making use of filters, allowing frequencies to be given different weights during the measurement process. The reason for assigning different weights to different frequencies is due to the hearing ability of the human ear, which perceives midrange frequencies better than high and low frequencies. In this context, the A-weighting curve allows simulating the response curve of the human ear during measurements. To meet different purposes, the majority of the sound pressure measurements (or noise levels) are expressed based on the A-weighting curve, considering that there is, usually, an acceptable correlation between the measured results and the subjective evaluations. In this sense, curve A fulfills the purpose of producing a single-value representation of a complete sound spectrum. The A-weighted sound pressure level is then defined as  
\[L_{A}=10 log_{10}\bigg{(}\frac{P_{A}}{P_{0}}\bigg{)}\ dB \tag{1}\]  
The A-weighted equivalent continuous noise level is used as a descriptor of both occupational and environmental noise and, for an average over a period of time \(T_{e}\), it may be written in terms of the instantaneous A-weighted sound pressure level as  
\[L_{A_{\text{eq}},T_{e}}=10 log_{10}\bigg{[}\frac{1}{T_{e}}\int_{0}^{T_{\text{e }}}10^{\frac{L_{A}(t)}{10}}dt\bigg{]} \tag{2}\]  
In this work, the period \(T_{\text{e}}\) = 5 min was used, leading to the simplified notation \(L_{A_{\text{eq}}}\) for this descriptor.  
Two other descriptors, \(L_{10}\) and \(L_{95}\), provide statistical insight into sound pressure levels that fluctuate significantly over time. Considering the \(L_{A}\) values, \(L_{95}\) is the level that has been exceeded 95% of the time and is often used as an approximation of the background noise level or as a descriptor for continuous noise sources. On the other hand, \(L_{10}\) is the level that has been exceeded 10% of the time and can be used to assess noise from vehicle traffic and other mobile sources.  
In practical situations, a specific sound might be of interest, and another specific descriptor \(L_{A_{\text{eq}},sp}\) would be defined depending on the situation. In the case of a continuous noise polluted by intrusive noises, \(L_{95}\) is generally a good descriptor to assess the sound pressure level generated by the functioning of high-voltage transformers, except in cases where the background noise is so high that it masks the specific noise of interest or makes it impossible to be identified.