## Materials and Methods
### Influence of Sloping Ground on Global Acoustic Insertion Losses  
As can be seen in Table 2, when the ground was angled to a certain degree (in this instance \(5^{\circ}\) or \(10^{\circ}\)) and a Y-shaped double cap was attached to the top of a conventional barrier (Figure 29), the global SPL at the receiving point decreased for person-like receivers ('Car-Person' and 'Ambulance-Person' configurations) as compared with a flat road scenario. However, there is no clear improvement ('Ambulance-Balcony') or the result is even slightly worse ('Car-Balcony') for \"balcony\" receivers when compared with the SPL obtained in the scenario involving a flat road and Y-shaped double caps. This could be explained by the fact that the height of the barrier increased with the ground positive slope (\(\Delta H_{\text{bar}}=d_{1}\cdot\text{tan}(\alpha)\)), thereby leading to 'Person' locations to be more deeply immersed in the shadow region (despite also being elevated due to the sloping ground by \(\Delta H=(d_{1}+d_{2})\cdot\text{tan}(\alpha)\)).  
Figure 29: All configurations with a road slope at angle \(\alpha\).  
The simultaneous use of both techniques (sloping and absorbing grounds), together with double Y caps on the top of barriers, led to the most promising results for the 'Person' listener configuration, with an additional global SPL mitigation of about 6-7 dB when compared with scenarios with flat and reflecting grounds and double Y caps. Figure 30 shows the SPL comparison for the 'Car-Person' configuration, where the improvement was confirmed. It is also important to note that the performances for the configurations with 'Balcony' listeners in the presence of sloping or absorbent grounds did not worsen, but were similar to the results obtained in the absence of both strategies.  
Influence of Increasing the Number of Diffracting Devices (More than Two) while Maintaining the Same Envelope Size  
As can be seen in Table 3, this study also verified that, when the number of diffracting elements is progressively increased (from two caps) while the maximum envelope size is maintained (Figure 31), _IL_ does not rise accordingly in all cases as seen when Y-shaped double caps are used. Rather, and contrary to what might be expected, SPLs for Y-shaped triple caps performed less effectively than Y-shaped double caps (except in the 'Car-Balcony' configuration). This finding is consistent with the fact that, below a certain size, the diffracting object can start to lose its effectiveness over certain frequency bands.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{‘**Car’ (Road Traffic)**} & \multicolumn{2}{c}{‘**Ambulance’ (Ambulance Siren)**} \\  & \multicolumn{2}{c}{Lw (dBA) = 110.2} & \multicolumn{2}{c}{Lw (dBA) = 122.6} \\  & \multicolumn{2}{c}{Lp(dBA) = 99.2} & \multicolumn{2}{c}{Lp(dBA) = 111.6} \\ \cline{2-5}
**Scenario** & **LdBArx** & **LdBArx** & **LdBArx** & **LdBArx** \\  & \multicolumn{2}{c}{‘**Person’**} & \multicolumn{2}{c}{‘**Balcony’**} & \multicolumn{2}{c}{‘**Person’**} & \multicolumn{2}{c}{‘**Balcony’**} \\  & \multicolumn{2}{c}{**Conf 1**} & \multicolumn{2}{c}{**Conf 2**} & \multicolumn{2}{c}{**Conf 3**} & \multicolumn{2}{c}{**Conf 4**} \\ \hline Y-shaped double cap with ground slope angle of 5\({}^{\circ}\) & 45.7 & 61.1 & 59.8 & 87.8 \\ Y-shaped double cap with ground slope angle of 5\({}^{\circ}\) and w/o ground reflection & 42.7 & 59.7 & 58.1 & 87.8 \\ Y-shaped double cap with ground slope angle of 10\({}^{\circ}\) & 46.3 & 61.4 & 60.7 & 87.4 \\ Y-shaped double cap with ground slope angle of 10\({}^{\circ}\) and w/o ground reflection & 43.1 & 59.9 & 58.8 & 87.4 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Global SPL (dBA) at the receiver positions for Y-shaped double cap and sloping ground.  
Figure 30: Influence of sloping and reflecting grounds on the SPL spectrum for the ‘Car–Person’ configuration and barrier with optimum double Y caps.