# Performance of Low-Height Railway Noise Barriers with Porous Materials
## 1 Introduction
Bearing this in mind, several authors have worked on this issue in order to develop a solution to mitigate the noise levels associated with rail traffic. The studies from the literature present solutions for the design of the barriers and for a more effective numerical modelling. [PERSON] [9] studied, both numerically and experimentally, the use of gabion walls as a form of mitigation. The results indicate that this solution can achieve up to 8 dB(A) of insertion loss. [PERSON] [10] presents a full-scale model of an L-shaped barrier built with wooden panels and inside with absorbent fibrous material. Tests carried out by the experimental author revealed an attenuation of 10 dB(A). [PERSON] [11] showed that the Dutch calculation scheme for conventional barriers is reasonably applicable for low-height solutions. Finally, [PERSON] [12] proposes corrective functions that allow one to efficiently calculate complex geometries using BEM, in order to apply more complex geometries in noise mapping programs. To control the reflections between the car body and the noise barrier, an absorptive treatment is required. Fibers and foams are commonly used in passive noise control; however, for external applications, these materials require protection against environmental agents and structural reinforcement. Because of these requirements, the interest in sound absorptive solutions, such as porous concrete, made using consolidated lightweight and sustainable granular materials have increased over recent decades [13; 14; 15; 16; 17; 18; 19; 20; 21]. The investigation of the fluid-equivalent representation of porous concrete made with expanded clay has been shown to be relevant in the scientific community. [PERSON] et al. [22] studied perforated concrete and highlighted the higher durability and the excellent strength-to-weight ratio of this solution. [PERSON] et al. [23] studied the influence of the water-cement ratio, the expanded clay grain size, and the sample thickness in the sound absorption behaviour, while [PERSON] [24] studied the fluid-equivalent representation of porous concrete using different aggregates. The modelling of the railway scenario has, for the reasons given, an important role to play in forecasting and creating measures to mitigate rail traffic-induced noise. The Boundary Element Method (BEM) is widely used to solve acoustic problems [7; 25], and can be an excellent option to model the effect of mitigation measures. The versatility of this numerical method allows the creation of simply reflective acoustic barriers and/or the inclusion of porous material which acts as a sound absorption element, allowing the improvement of the performance on the barrier, mitigating the energy reflected between the vehicle and the barrier. In this article, the BEM will be used to solve an external acoustic problem, essentially to test the geometry of the barriers. In addition, a BEM formulation considering multiple material regions is implemented to allow the modelling of the effect of possible absorptive materials coupled to the noise barrier. Using this model, it becomes possible to model the absorptive materials using equivalent fluid theories, leading to a realistic representation of such media. To assess the capacity of the barrier, the insertion loss was used, i.e., the difference between the scenario with and without the barrier. Using the insertion loss allows the barrier to be assessed as a noise control measure placed in a location with specific characteristics. In this way, the IL calculation presents the actual losses over a wide range of frequencies, for any set of receivers, regardless of the scenario to be evaluated and mainly regardless of the type of material and geometry of the prototype. This paper's structure is as follows: Section 2 shows the experimental railway noise characterisation. Section 3 presents the experimental procedure used to characterise porous concrete samples, allowing the fluid-equivalent theory representation.