# Performance of Low-Height Railway Noise Barriers with Porous Materials
## 7 Conclusions  
This paper presents the development of a low-height acoustic barrier to be used close to the noise source in a railway environment. The development of the solution is composed by two distinct phases, namely the optimisation of the barrier geometry and the integration of a porous material in order to increase the acoustic performance of the solution. Taking advantage of sound pressure level records acquired in the railway environment in the metropolitan area of Porto, it was possible to define the most important frequency content and thus design a solution whose performance was superior in that frequency range. The numerical modelling and study of the various solutions were carried out by applying a BEM formulation with multiple regions, simulating the porous material as an equivalent fluid and thus incorporating its acoustic absorption properties. The parametric study presents the methodology for sizing the curved barrier. Through the simulation of a sound wave, the inner face of the barrier was constructed so that it coincides with the shape of the incident wave front coming from the source. Thus, the reflection normal to the propagation direction is favoured and, as such, more energy is sent in the direction of the noise source and the railway. In this way, an integrated solution was built taking  
Figure 21: Insertion loss maps for the curved noise barrier with absorptive layer for frequencies 1250 Hz, 1600 Hz, 2000 Hz, 2500 Hz, 3150 Hz and 4000 Hz (**a–f**), with the presence of the receivers (black dots) used to calculate mean IL (Figure 18). Figure 20: Insertion loss maps for the curved noise barrier with absorptive layer for frequencies 315 Hz, 400 Hz, 500 Hz, 630 Hz, 800 Hz and 1000 Hz (**a–f**), with the presence of the receivers (black dots) used to calculate mean IL (Figure 18). advantage of the acoustic absorption capacity of the track to absorb the energy sent back. In a complementary manner, a porous concrete layer was added, which on the one hand has a good acoustic absorption capacity and on the other hand guarantees the durability required for solutions used outdoors. The main purpose of the porous material is to absorb part of the energy arising from the reflections between the barrier and the vehicle, ensuring that the energy that is not sent back to the track can be sent in the direction of the external receivers. The results presented show the clear improvement achieved by using porous material as a means of absorbing part of the energy in detriment of purely reflective solutions. The curved solution with porous material presents an IL in the defined receivers higher than 10 dB in all the calculated frequency range; for some frequencies the IL value is even higher than 15 dB, with the maximum registered in the frequency 3150 Hz where the IL value is higher than 25 dB. In this way, the presented solution appears to be an useful element for the reduction in train-induced noise, guaranteeing an effective mitigation. In addition, due to its low-height, this solution does not represent a visual obstacle, as is usual for noise barriers, but it still effectively reduces noise levels at the receivers of interest. Conceptualisation, J.L., M.P., P.A.C. and L.G. ; methodology, J.L., M.P., P.A.C. and L.G. ; software, J.L., M.P. and L.G. ; validation, J.L., M.P., P.A.C. and L.G. ; formal analysis, J.L., M.P., P.A.C. and L.G. ; investigation, J.L. and M.P. ; resources, J.L., M.P., P.A.C. and L.G. ; data curation, J.L. and M.P. ; writing--original draft preparation, J.L. and M.P. ; writing--review and editing, P.A.C. and L.G. ; visualisation, J.L., M.P., P.A.C. and L.G. ; supervision, P.A.C. and L.G. ; project administration, P.A.C. and L.G. ; funding acquisition, J.L., P.A.C. and L.G. All authors have read and agreed to the published version of the manuscript.