# Performance of Low-Height Railway Noise Barriers with Porous Materials
## 6 Results  
Initially, the acoustic performance of a purely reflective vertical barrier 1.20 m high and 0.15 m thick was evaluated. In Figure 15, we present the sound pressure levels for the scenarios with and without a barrier (see Figure 15a-f), and the respective Insertion Loss (see Figure 15g-i, for three different frequencies: 500 Hz, 2000 Hz and 4000 Hz. From the analysis of the figure, it was found that there are relevant differences between the scenario without a barrier (see Figure 15a-c) and the scenario with a barrier (see Figure 15d-f), meaning that the barrier prevents part of the energy from propagating. This phenomenon is even more remarkable when analysing the insertion loss (see Figure 15g-i), where it was observed that the reduction (warmer colours of the figures) is higher than 15 dB. The second phase of the parametric study is to analyse the proposed curved geometry. The proposed shape was analysed using the BEM model, and its performance was compared with that of the vertical barrier. In Figure 16, the results computed for the new barrier shape are illustrated. This figure is divided into three parts, illustrating the propagation in the case without a barrier in Figure 16a-c, the scenario with the presence of a barrier in Figure 16d-f, and finally the insertion loss in Figure 16g-i. In the case of the curved barrier, the attenuation effect is very clear, with reductions of over 15 dB. Compared to the vertical barrier (see Figure 15) the insertion loss presented by the curved barrier (see Figure 16g-i), is higher. The phenomenon is once again observed in the warmer colours which make up the insertion loss colour map of the barrier under analysis. The use of the track as a means of mitigation plays an important role in this case, since part of the performance improvement is due to the characteristics of the ballast that absorbs energy sent by the reflection of waves on the inner surface of the curved barrier. Figure 14: Representation of the receiver mesh used in the simulations. Figure 13: (**a**) Measured absorption coefficient in the diffuse field (adapted from [38]); (**b**) corresponding surface impedance used in the calculation model. From the analysis of the geometries of the barriers, it is concluded that the curved barrier is visibly better in reducing the levels of sound pressure in the proximity of the noise source compared to the vertical barrier. However, secondary reflections such as those that occur between the barrier and the vehicle do not suffer significant attenuation and part of the energy is sent back to the external receivers. Thus, an absorptive treatment in the internal face of the barrier is required to reduce the SPL between the train body, the track and the acoustic barrier. As mentioned above, the placement of material with sound absorption characteristics in the barriers guarantees an improvement in acoustic performance, in particular concerning the reflection of sound waves between the body of the vehicle and the body of the barrier. For this purpose, porous concrete was chosen because of the higher durability and excellent properties for external application, without the requirement of protection against environmental agents and structural reinforcement. Figure 17 shows the geometry of the curved barrier filled by porous material, with an irregular geometry, to increase the absorptive  
Figure 16: Comparison of the sound pressure levels in the cases without (**a–c**) and with the barrier (**d–f**) and the respective insertion loss, first, second, and third rows (**g–i**). Figure 15: Sound pressure level calculated without (**a–c**) and with (**d–f**) the noise barrier and the insertion loss (**g-i**). surface. The geometry of the porous material presented is composed of two parts, namely a regular layer with a thickness of 0.08 m and an irregular layer. The irregular layer is composed of elements 0.06 m thick and 0.10 m wide with equal spacing between them.