# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
## 3 Results and Discussion
### Noise Barrier Simulation
The fact that this method is mostly based on following the path and energy of sound waves, without accounting for the oscillatory character of the wave propagation may help in justifying this result. The BEM  
Figure 14: Sound pressure levels calculated using the CadnaA CNOSOS-EU methodology, with the sound absorption obtained experimentally (ISO 354), through the London equations (London 1 and London 2) and reflective reference surface (\(\alpha\) 0.01), at 0.25 m from the noise barrier: (**a**) for the 1.5 m height barrier; and (**b**) for the 4 m height barrier. results exhibit a less regular behaviour, due to the oscillatory character that is accounted for in this numerical methodology. The two methods seem to predict more similar IL values at higher frequencies. As expected, the L-shaped cantilevered barrier returns higher insertion loss values than the flat barrier since the top of the barrier limits the diffractions of the waves that pass to the other side of the barrier. However, for the BEM calculation and for the low-height barrier, this difference is quite small. In Figure 15(a),b, the influence of the porous material on the insertion loss curve is observed. This influence is visible mainly for the mid-high frequencies, for which the studied material exhibits higher sound absorption values. In that range, noise barriers with an absorbing layer have somewhat better acoustic performance (higher IL values) than reflective barriers. However, the change registered in the vertical barrier results, due to the introduction of sound absorption, is very small and never exceeds 1 dB. In that case, since the sound absorption is located on the source side it affects mostly the reflected field, and very little changes occur behind the noise barrier. For an L-shaped barrier with sound absorption on the inner part, no changes are visible, and thus this case is not represented. In this respect, it should be noted that all the curves in this plot have been obtained with BEM. Indeed, when the sound absorption is introduced in the CNOSOS-EU model of CadnaA, no differences were registered either in the L-shaped or in the vertical barriers with respect to the case of the reflective barrier; for those cases, since the sound absorption is located only on the source side of the barrier, only the sound levels on that side seem to be affected and no changes are registered behind the noise barrier. The described behavior changes considerably if the sound-absorbing material is also introduced on the top of the L-shaped cantilevered noise barrier, whose numerical result calculated with the BEM is also depicted in Figure 16. Clearly, the presence of the sound-absorbing porous material seems to have a strong influence on the diffraction phenomenon that occurs in this barrier, providing an additional attenuation at high frequencies. This increase can be of up to 5 dB at the higher frequencies, which is quite a remarkable improvement but one that still requires experimental validation. At this point, it is worth mentioning that both the low-height and the more traditional tall vertical barrier provide significant noise protection, which can go up to 25 dB, for the first, and 32 dB, for the second types of noise mitigation devices. Indeed, this result also indicates that the smaller noise barriers placed closer to the source can be a viable alternative to the conventional taller ones whenever space restrictions, aesthetics or visual impact effects are relevant. Figure 15: Insertion loss curves calculated with Cadna A CNOSOS-EU and BEM methodologies. Comparisons for reflective barriers, with heights of: (**a**) 1.5 m and (**b**) 4 m.  
Finally, Figure 17 compares the results obtained with the BEM considering the sound absorption coefficients derived from the experimental test (ISO 354) and from the application of London 1 and 2 equations (Equations (15) and (16)).