# Performance of Low-Height Railway Noise Barriers with Porous Materials
## 1 Introduction  
Railway transport is the most sustainable mode of transport, with the lowest energy consumption and carbon footprint compared to any other mode of transport. However, a report by the European Environment Agency (EEA) [1] from 2019 on this subject states that at the European level, rail noise is the second most dominant source, with an estimated 22 million people exposed to at least 55 dB during the day and night periods. On the same subject, however, with a different time horizon, the report by the European Environment Agency [2], whose aim is to project scenarios for the decade 2020 to 2030, states that the situation of the population's exposure to environmental noise level in Europe will worsen in practically all areas responsible for current exposure levels. The projections suggest that in 10 years more than one million people will be exposed to excessive rail noise, both in urban centres and outside these agglomerations. In light of the above, a report of the World Health Organisation (WHO) Regional Office for Europe [3] is presented. This document highlights the effects of noise and incorporates a number of indications for certain policies that must be implemented in order to ensure health and well-being for people living with the most diverse forms of environmental noise. The WHO working group statethat, for the daytime period, noise levels should be below 54 dB, and for the right-time period, they should not exceed 44 dB. Finally, it stresses that interventions to reduce noise levels and to comply with the limits indicated should focus on interventions at the track level, the improvement of rolling stock and the implementation of small noise barriers. Rail noise mitigation measures can be applied in three different locations and according to the environment and the level of noise pressure reduction that one wants to achieve [4]. Usually, the most widely deployed solutions are those that act at the level of the propagation path or at the level of the receivers. In places where housing density is high, the solutions that act on the path of propagation are more advantageous in economic terms [4]. Acoustic barriers are usually artificial and solid elements made of different types of material and placed in different positions, depending on the place to be protected. This noise mitigation solution has been widely adopted in the context of road noise mitigation, and there are several methods for designing these solutions. Acoustic barriers can have different operating principles depending on the material they are made of; i.e., they can work by reflecting acoustic waves and/or absorbing them. In general, the barriers are vertical elements between 3 m and 4 m high positioned along the road or railway. However, despite the inherent benefits of reducing noise and improving the quality of life of the population living nearby, this type of solution faces the reluctance of the populations living near the railway infrastructure. This situation is related to the size of the barrier, affecting the field of vision, causing a sense of imprisonment, loss of natural light or affecting air circulation. From another perspective, for those who travel on trains, complaints are also registered for similar reasons [5; 6; 7]. In order to tackle some of the negative points identified for the higher acoustic barriers, namely, being obstacles to one's field of vision, the natural evolution of thinking has led to the creation of solutions whose working principle is similar to the one intended to be applied in the work presented in this document. The inherent advantage of low-height solutions is related to the positioning of this element. As the mechanisms of noise generation are mostly at the level of the rail [8], the placement of the barrier in a position close to the track allows the propagation of sound waves to be interrupted close to the source. The reduced height of these elements thus allows this positioning close to the source without constituting an obstacle to the field of vision of passengers and passers-by.