# On the Control of Low-Frequency Audible Noise from Electrical Substations: A Case Study
## 3 Results and Discussion  
### Before Situation  
In this section, RPS substation described in Section 2.1 is considered. Both numerical simulations and experimental results will be shown. After setting up the ray-tracing model as described in Section 2.3, and using the source characteristics as described in Section 2.2, it is possible to compute the sound pressure level distribution in the simulation area on a plane located 4 m above the ground, i.e., the height of the receiver. The result is shown in Figure 10.  
The sound pressure level around the sources is about 75 dB(A). Due to the close vicinity of a building, the sound pressure level distribution is not uniform around the source. The presence of  
Figure 10: Overall A-weighted SPL distribution in the investigated area as obtained from simulation 4 m above the ground. Figure 9: Transformer TR4 surrounded by a noise barrier in the fully validated case. other buildings causes reflections and shadow zones clearly visible in the map. The SPL at the receiver, located in the north-western part of the investigated area (Figure 1), is between 40 dB(A) and 45 dB(A) and it is caused by both direct and reflected sound. The Italian legislation (Law 26/10/1995 n. 447 [33]) defines the limit value of emission as the maximum noise that can be emitted by a noise source, measured close to the source itself, and the limit value of immission as the maximum noise that can be introduced by one or more noise sources in the environment, measured close to the receivers. The D.P.C.M. 14/11/1997 [34] sets the limit values of emission and immission for six classes defined according to the intended use. Class I includes particularly sensitive areas where a quiet environment is deemed essential; predominantly residential areas with low population density are zoned in Class II, while mixed-use areas with medium population density, local and connection traffic, commercial activities and offices but no industries belong to Class III. Class IV is characterized by intense human activity and high connection traffic from major roads, railways or harbors. Class V defines mainly industrial areas with low residential population density, whereas Class VI is devoted to exclusively industrial areas with no dwelling. It is worth noting that the legislation requests that a buffer zone is set around roads and railways, and that contiguous areas must belong to contiguous classes (for example, an area zoned in Class III can only be surrounded by Class II and/or Class IV areas). In the presented case, the receiver has been zoned in Class III due to the presence of the railway nearby (zoned in Class IV). This means that the daytime and nighttime immission limits on the measured equivalent sound pressure levels are 60 dB(A) and 50 dB(A), respectively. The power transformers are working continuously during both time periods. For this reason, it can be assumed that the most critical situation is reached during the night period, when the class limit is lower. The measurement of the background noise during the nighttime, at the balcony located on the facade of the house where the receiver has been positioned (4 m above the ground), gave an equivalent sound pressure level of 46 dB(A). Figure 11 shows the spectrum of the minimum values measured for each band, which is the reference spectrum type for the detection of tones according to Italian legislation [35]. It can be observed that there are no tonal components in the background noise spectrum and the background noise perception is mainly related to the traffic noise contribution from a close free-way, represented by a broadband noise around the 630 Hz-1 kHz frequency range. The measurements carried out on the balcony of the receiver's house with the two transformers turned on gave an equivalent sound pressure level of 47.4 dB(A). This value also includes the background noise, while the model only provides the effect of the sources.