## Materials and Methods
### Theoretical Method
#### 2.3.3 Global Sound Pressure Level Estimation  
Once the 124 simulations were conducted with PARDOS and an optimum barrier cap (in terms of maximum IL) was identified for the four configurations considered, the global SPL in dBA at the receiver locations was calculated. This was achieved by considering the noise spectra of road traffic and typical siren sources (depicted in Figures 1 and 2, respectively) along with the optimum noise barrier previously identified. This calculation aims to assess whether the expected global values are below those recommended by the World Health Organization (WHO) [2] for the situations considered. To carry out this assessment, the following steps were applied:  
* Transformation of the A-weighted power level \(Lw\) (\(dBA\)) into the A-weighted SPL \(Lp\) (\(dBA\)) at 1 m: \[L_{p}(\text{dB}(\text{A}))=L_{w}(\text{dB}(\text{A}))-\left|10\text{-}\log_{10 }\left(\frac{Q}{4\pi r^{2}}\right)\right|,\] (26) where \(Q\) is the directivity factor and \(r\) is the distance to the source. The distance \(r\) to the sound source for which the sound pressure levels were calculated was 1 m for both types of sources ('car' and 'ambulance'). Spherical propagation (isotropic propagation) was also considered, for which Q must be equal to 1. Under these two assumptions, the sound pressure level (\(L_{p}\)) of a point source was 11 dB less than its sound power level (\(L_{w}\)) at any frequency. * Normalization of the pressure level from one-third octave band spectra to 1 Hz frequency resolution with a linear distribution. A simple loop runs for all the center frequencies \(f_{0,i}\) of each 1/3 rd octave frequency band (\(\Delta fi\)). The sound pressure levels at the 1 Hz band resolution was obtained by normalizing each \(L_{p}(\text{dB}(\text{A}))\) value by their correspondent frequency band \(\Delta fi\), as shown in Equations (27) to (30): \[\Delta fi=f_{2,i}-f_{1,i}=\sqrt[3]{2}f_{1,i}-f_{1,i}\ \,\] (27) \[f_{0,i}=\sqrt{f_{1,i}\text{-}f_{2,i}}\ \,  
Figure 14: _IL_ in Configuration 3 ‘Ambulance–Person’ for single caps and a single screen barrier as a reference. Figure 12: _IL_ in Configuration 1 ‘Car–Person’ for single caps and a single screen barrier as a reference. Figure 13: _IL_ in Configuration 2 ‘Car–Balony’ for single caps and a single screen barrier as a reference. As can be observed, the _IL_ results for single caps revealed similar behavior as a function of frequency in all cases, with maximum _IL_ values sitting at approximately 40 dB in 'Car-Balcony' and 'Ambulance-Person' configurations and up to 60 dB in the 'Car-Person' configuration. However, the Y-shaped cap showed slightly better performance (higher _IL_) in 'Car-Person' and 'Ambulance-Person' configurations and the cylindrical single cap showed better performance in the 'Car-Balcony' configuration. However, as expected, significantly lower performance for all cap types was recorded in the 'Ambulance-Balcony' configuration. This is because the transmitter and the receiver were less immersed in the shadow zone in this configuration (that is, a LoS between the source and the receiver exists.). The presence of clear repetitive patterns can be instantly discerned from Figures 12-15 with insertion losses periodically falling even below 0 dB. Specifically, for Figure 12, the insertion loss drop occurred every 2054 hertz, while in Figure 13, every 775 hertz, or every 420 and about 155 hertz in Figures 14 and 15, respectively. This was because the direct and ground reflected path signals in the 'free space' had an opposite-phase at specific frequencies and interfered destructively and almost canceled each other out. As a consequence, \(\phi_{fs}\) reached a minimum and _IL_ also decreased to the lowest value following Equation (25). Furthermore, the _IL_ achieved by the 25 possible double-cap barriers in the four configurations was compared. For the sake of clarity and space, only a selected few simulation results are presented in this manuscript.