# Experimental Analysis and Simulation of a Porous Absorbing Layer for Noise Barriers
## 3 Results and Discussion
### Material Parameters and Acoustic Behavior  
Table 2 shows the results obtained for the experimental and corrected macroscopic parameters for each Aggregate/Cement sample shown in Table 1, with a thickness of 8 cm. These parameters were determined as the average of the experimental test values of three samples for each Aggregate/Cement ratio. Observing, first, the experimental values measured in the laboratory, it can be seen that the measured values of the tortuosity, porosity, and airflow resistivity are relatively similar for the three sets of samples analyzed, with no significant variations when the Aggregate/Cement ratio is changed. This result indicates that there is no advantage, concerning acoustical absorption, in using higher cement quantities in the production of this type of material, and using the minimum that ensures sufficient mechanical stability and durability is an adequate choice. Although this was expected, it is also interesting to note that, if improved mechanical properties are required, for a specific application, the moderate increase in the cement quantity does not degrade the acoustical absorption properties. The adjustment procedure proposed above has then been used to fine-tune the macroscopic parameters, in order to obtain a better match of the sound absorption estimated using the Horoshenkhov-Swift model with the one measured using the impedance tube. One should note that only small variations were allowed, with a maximum of 10%. Observing the adjusted values of the macroscopic parameters, it is possible to note that the larger changes occur in the tortuosity values, which reached almost 10% in two of the sets. It should be mentioned that the test method used, based on the electrical conductivity of the sample, can pose some challenges in laboratory implementation and can present variations from specimen to specimen. As for the porosity and airflow resistivity, only the sample type AE 5.18 required more significant adjustments, while the remaining only required minor adjustments. The same occurred for the average pore size standard deviation, although in this case, the initial value was just fixed based on the literature. The theoretical and experimental curves for the sound absorption coefficient with incident plane waves are displayed in Figure 10. The experimental outcomes were attained in compliance with the ISO 10534-2 [17]. The theoretical curves were achieved with the model proposed by [PERSON] and [PERSON], where experimental macroscopic parameters (PMe) and corrected macroscopic parameters (PMc) were used. The curves exhibit the typical behavior of a porous concrete material, with a peak-valley structure, in which a significant peak occurs around 650 Hz. This position of the sound absorption peak is mainly related to the thickness of the sample, as other tests with different thicknesses have revealed (presented later in this section). It can be seen that the experimental results were well-fitted by both theoretical curves, even before the adjustments were performed. However, it can also be seen that the adjustment procedure allowed for a better final agreement between the experimental and theoretically derived curves, as expected. This is particularly evident in the case of the AE 5.18 sample, which, even after the adjustment, still revealed some discrepancies. Despite having distinct Aggregate/Cement ratios, the samples exhibit relatively similar behaviors, since the differences between them are negligible. Between 400 and 1000 Hz, significant sound absorption can be observed for all cases. It can also be seen that the sound absorption curve shifts very slightly to lower frequencies when higher cement contents are considered. As the cement composition increases, the sample's density rises as well, resulting in narrower pores and channels, hence altering several macroscopic factors that affect the samples' acoustic behavior.