# Experimental Light Rail Traffic Noise Assessment in a Metropolitan Area
## 3 Effects of Different Parameters on Sound Pressure Level
### Comparative Analysis between Rolling Noise and Bridge Noise  
In this section, it is intended to characterize the noise levels generated by the crossing of a superstructure. In this case, the passing of the vehicles over a metallic bridge crossing the Douro river which is part of the metro network was analyzed. The noise due to traffic on bridges is one of the particular types of noise associated with rail traffic, and according to the literature [22; 27], the reasons for the development of this type of noise are related to two mechanisms. Firstly the vibrations that are transmitted to the elevated structure when vehicles pass over it. Since they are usually large structures, these vibrations are then radiated by the structure in the form of noise, which can reach quite high sound levels on several occasions. Secondly, how the rail is fastened to the superstructure plays a fundamental role since it influences the vibration of the rail which, according to the same authors, may have higher vibration levels than a current track. In this example, for a vehicle of the _Eurotram_ type, at a speed of approximately 20 km/h, the records are compared with  
Figure 19: Spectrograms of a _Eurotram_ vehicle passing the measuring section at acceleration and at null acceleration: (**a**) Microphone 4—accelerating; (**b**) Microphone 4—null acceleration.  
Figure 17: Comparison of sound pressure levels for the _Eurotram_ vehicle traveling on a Type II track, under acceleration and at null acceleration, for a speed of 58 km/h: (**a**) Microphone 1; (**b**) Microphone 2.  
Figure 18: Spectrograms of a _Flexity Swift_ vehicle passing the measuring section at acceleration and null acceleration: (**a**) Microphone 4—accelerating; (**b**) Microphone 4—null acceleration.  
other types of track, considered to be conventional, in this case, Type I, Type III and Type IV. In Figure 20, it can firstly be observed that two tracks of similar construction, namely Type I and Type IV, present very different levels of sound pressure, with the absorption capacity and the restriction of movement of the rail having a dominant influence on the noise levels. However, the most pertinent observation is the fact that the noise levels at the bridge crossing are substantially higher than the noise levels recorded for the other track types under analysis. It can be concluded that the records presented corroborate what had been previously identified, with the crossing over the metal bridge generating noise levels that are up to 10 dBA higher than those recorded on conventional tracks. It should be noted that the maximum levels of sound pressure are around 70 dBA, although all the records correspond to low-speed crossings, around 20 km/h. Finally, it should be noted that there is a large difference, about 20 dBA between the records of the grass track and the records of the metal bridge crossing. The differences registered with the analysis of the frequency spectrums are also evident in the values of the \(L_{Aeq,T}\) indicator. For the Type I track the value is 71 dBA, for the Type III track the value of the indicator is 75 dBA, for Type IV track a value of 76 dBA is recorded and finally for the metallic bridge, a value of 85 dBA was obtained. As mentioned, the crossing of the metal bridge originates noise values much higher than those recorded for the other tracks for vehicles traveling at the same velocity. This discrepancy is evident in the values of the indicator \(L_{Aeq,T}\), which are about 10 dBA higher compared to the other examples.