# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
## 3 Results and Discussion
### Noise Barrier Simulation  
The material analyzed above has been incorporated into different numerical models, as described in Section 2. Initially, a first set of simulations was performed using CadnaA prediction software, in order to estimate the change in the noise levels on the source side of the barrier using the CNOSSSOS-EU methodology. Receivers were placed 0.25 m from the barrier's surface so as to correctly assess the changes in the noise levels introduced by its presence. The simulated scenario was also described in Section 2 and considers a passing train, with the noise emission spectrum calculated from CNOSSSOS-EU. Flat barriers were used in this first comparison, considering both a reflective noise barrier (designated in the plots as \(\alpha\) 0.01) and three variants of the barrier with an absorbing layer, identified as ISO 354, London 1, and London 2. The sound absorption curves of the absorbing layers were obtained, one experimentally (ISO 354) and the other two through the London equations (London 1 and London 2), as depicted in Figure 13. It can be observed that the theoretical curves obtained with both London's equations are very close to the ones computed from experimental sound absorption values under diffuse conditions. On the other hand, as  
Figure 13: Theoretical and experimental sound absorption curves in diffuse field conditions, in 1/3 octave bands, for both London models. expected, it appears that next to the absorbing layer of the noise barrier, there are lower sound pressure levels compared to a reflective barrier. Clearly, the approximation given by London's equations is sufficient to allow their use in environmental noise prediction models without significant loss of accuracy. The influence of the absorbing material is particularly noticeable in the mid-high and high frequencies (see Figure 14), where the higher sound absorption values are registered. The same type of behaviour is registered both for the 4 m and 1.5 m tall barriers, although with different SPL levels values due to the different distances from the source. It should be noted that on the source side of the barrier, it is always expected to observe an increase in the sound pressure levels with respect to a reference situation without any noise barrier (not shown), due to the constructive combination of the incident wave with the first reflection from the surface. One of the main objectives of this paper was to compare, for different scenarios, the results obtained with the CNOSOS-EU methodology with those given by a detailed numerical code solving the frequency domain acoustic wave equation with the BEM. The physical principles that form the basis of each calculation method are quite different, with the first one being energy-based, without accounting for phase change during propagation, while for the second the complete physical phenomenon is being taken into account in the modelling process. The chosen parameter for comparison purposes of the acoustic performance was the Insertion Loss (IL). Flat (\(\alpha\) 0.01, ISO 354, London 1, London 2, and CadnaA) and L-shaped (\(\alpha\) 0.01 L, ISO 354 L, London 1 L, London 2 L, and CadnaA L) noise barriers were used in this evaluation. The studied configurations have been schematically described in Figure 9, and include reflective barriers (\(\alpha\) 0.01) and barriers with an absorbing layer on the exposed side to the noise source (ISO 354, London 1 and London 2). The sound-absorption curves of the absorbing layers were obtained experimentally (ISO 354) and through the London equations (London 1 and London 2). In Figure 15a,b, results calculated by both methodologies are shown considering a rigid reflective barrier. Both L-shaped and vertical noise barriers were analysed, both for the low-height and tall barriers. It is clear that the curves of IL obtained with CNOSOS-EU exhibit lower IL values, in particular at lower frequencies, and they tend to have a more regular behaviour throughout the analysed frequency domain.