# Experimental Light Rail Traffic Noise Assessment in a Metropolitan Area
## 1 Introduction  
One of the primary objectives of modern society is to move towards a more environmentally and economically sustainable type of mobility. Rail transport appears to be the most efficient way of making this transition due to its low pollutant emissions and its capacity to transport a large number of people and goods. Thus, the growth of societies necessarily involves the use of this mode of transport; however, noise induced by rail traffic appears as a major problem that needs to be analyzed and solved. Several studies point to an increase in the number of people exposed to noise [1, 2], and the problems that can result from this exposure have been the subject of study in several places, where there is a strong presence of urban railway traffic [3, 4, 5, 6, 7, 8]. For the facts listed above, the experimental characterization of noise induced by rail traffic presents itself as an essential tool to characterize rail noise in all its dimensions to create measures to mitigate the natural effects of noise. The complete study of the noise problem implies the analysis of several aspects, namely, generation, propagation and reception. The characterization through in situ noise measurement in several types of scenarios, with different speeds, types of vehicles, types of tracks and different urban networks allows a clearer view of the sound pressure levelsinvolved and, especially, the variations associated with the different circulation conditions mentioned above. Taking these factors into account, several studies have addressed the issue to characterize and mitigate the noise levels existing in various locations on the European continent [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. The studies mentioned above reveal a clear connection between noise levels and frequency content, depending on specific traffic conditions. Factors like vehicle speed, vehicle characteristics, track solution, and other relevant aspects contribute to significant variations in the recorded noise levels. Based on the conclusions of this study, it is evident that the specific conditions of each location have a noticeable influence on noise levels. This paper presents the results obtained in an experimental campaign developed with the following objectives:  
* To characterize the noise coming from the interaction between the track and the vehicle and the vehicle components;
* To identify its main frequency content;
* To assess the influence of different traffic conditions and types of tracks on noise levels. Railway noise has several sources. It may originate, on the one hand, from the interaction of the track elements with the vehicle, or, on the other hand, it may originate from the components constituting the vehicle. Notwithstanding the various railway noise components, the noise generated by the wheel-rail interaction plays the most important role in noise generation. The variable that most affects the sound pressure levels and the origin of the noise is the velocity of circulation of the vehicles, as illustrated in Figure 1, where the main noise sources are shown as a function of circulation speed. The experimental characterization was carried out entirely on the railway tracks that are part of the surface metro network constitutes the Metro do Porto network, where the speed range is between 0 km/h and 80 km/h. Thus, as mentioned, the noise levels recorded are strongly influenced by rolling noise, as highlighted in Figure 1.  
The aim of this experimental campaign and consequently of this article is to identify the noise sources within the speed range allowed on the surface metro network. As the noise depends on the speed of movement, the data collected concerns mainly the noise coming from rolling noise, as mentioned before. Rolling noise is associated with the interaction between track and vehicle, and results either from track or wheel irregularities or from the vibration of individual parts [21; 22; 23; 24].