# Experimental Light Rail Traffic Noise Assessment in a Metropolitan Area
## 3 Effects of Different Parameters on Sound Pressure Level
### Tram Type Analysis  
As mentioned, any variation in traffic conditions may lead to a change in the noise levels generated by the circulating vehicle. In this section, we perform a comparative analysis of the sound pressure levels generated by the two types of vehicles circulating in the metropolitan network of Porto, which were presented in Section 2. For that purpose, and to have similar conditions for each vehicle, we will consider two different types of tracks, Type II (ballast) and Type III (slab). In each case, similar circulation speeds will be used. In Figure 6, the records are compared for a speed of 58 km/h. As can be seen, the sound pressure levels in both receivers and for each train type are very similar. The _Eurotram_ vehicle records the maximum sound pressure levels for 1000 Hz, whereas from 4000 Hz onwards, there is a slight increase in the sound pressure levels of the _Flexity Swift_ vehicle compared to the _Eurotram_ vehicle. The \(L_{Aeq,T}\) value for the _Flexity Swift_ vehicle  
Figure 5: Schematic diagram showing the position of the microphones in relation to the track.  
is 87 dBA and 87 dBA for the _Eurotram_ vehicle, corroborating what had already been concluded, that the behavior of both vehicles is very similar.  
In Figure 7, the sound pressure levels are illustrated for a speed of 30 km/h. The conclusions that can be drawn when comparing the two types of vehicles are very similar to those recorded for the previous case (speed of 58 km/h), although a considerable change in the noise spectra is registered at both microphones. Indeed, it seems that the peak registered at 1000 Hz is no longer evident, with the spectrum plots showing an almost flat sound level between 200 Hz and 1000 Hz. This is an interesting change from the previous results, clearly showing the influence of vehicle speed on the emitted spectrum. In general, the sound pressure levels for the same speed and in the case of a straight track are very similar for both vehicles. It thus seems that, in the case of a straight track, and with vehicles and tracks in good condition, the speed of circulation is the variable that most influences the noise levels. Once again, the spectra are very similar, and so are the values of the \(L_{Aeq,T}\) indicator, giving a value of 82 dBA for the _Flexity Swift_ vehicle and 84 dBA for the _Eurotram_ vehicle.  
Finally, Figure 8 presents the comparison for the two vehicles traveling at about 40 km/h on a Type III track. Unlike the previous analyses, the sound pressure levels generated by the passage of the two vehicles seem to be quite distinct. In this case, the _Eurotram_ vehicle has higher sound pressure levels than the _Flexity Swift_ vehicle (up to 10 dBA), namely for frequencies above 2000 Hz. The explanation for this evidence is not entirely clear, but different reasons could cause it, from vehicle load distribution to geometrical or mechanical characteristics. For Type III, the values of the sound pressure levels are slightly higher and, as such, are reflected in the value of the \(L_{Aeq,T}\) indicator, obtaining a value of 79 dBA for the _Flexity Swift_ vehicle and a value of 87 dBA for the _Eurotram_ vehicle.  
Figure 6: Comparison of sound pressure levels (dBA) for _Flexity Swift_ and _Eurotram_ vehicles traveling at 58 km/h on a Type II track: (**a**) Microphone 1; (**b**) Microphone 2.  
Figure 7: _Flexity Swift_ and _Eurotram_ vehicles traveling at 30 km/h on a Type II track: (**a**) Microphone 1; (**b**) Microphone 2.