## 4 Conclusions  
In this work, the performance of acoustic noise barriers when additional structures (cap elements) of different types were attached to the top of conventional screen barriers was analyzed for a broad frequency band (from 100 Hz up to 10 kHz). Various configurations were tested, with two transmitters (a \"Car\" source at a height of 0.5 m and an \"Ambulance\" source at a height of 2.5 m), two receivers (a \"Person\" location at a height of 1.7 m and a \"Balcony\" listener at a height of 5 m), and five different cap shapes (\"Y-shaped,\" \"T-shaped,\" \"rectangle,\" \"cylinder\", and \"trapezoid,\" along with double combinations). Caps were attached to the top of 3 m high conventional, plain barriers. Exhaustive analyses revealed that caps improved noise mitigation when compared with conventional plain barriers of the same height not only for traffic road noise, but also for vehicle siren sounds. Specifically, the results revealed that optimal performance was achieved when Y-shaped double caps were used, with an additional noise abatement of about 14 dB being achieved in the \"Car-Person\" and \"Ambulance-Person\" configurations and slightly more than 6 dB in the \"Car-Balcony\" configuration as compared with the use of conventional screens. Furthermore, the analysis of the ground reflection on the source side revealed that its removal by absorbing materials could also provide a meaningful additional improvement in terms of _IL_, and SPLs can accordingly be diminished by about 2.5 dB in the \"Car-Person\" and \"Car-Balcony\" configurations and 1 dB in the \"Ambulance-Person\" configuration, as shown in Table 1. The simultaneous use of both techniques (sloping and absorbing grounds), together with double Y caps on 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-shaped caps. This study also verified that it is not worthwhile to indefinitely increase the number of diffraction elements (e.g., the use of Y-shaped triple caps) while maintaining the global envelope size of the barrier since the SPL is higher in these instances than in other, simpler settings (e.g., the use of Y-shaped double caps). In contrast, the consideration of the slope of the road can lead to an increase in noise isolation, especially when the receivers are located at lower heights (\"Person\" listener). Figure 31: Triple Y-cap. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**‘Car’ (Road Traffic)**} & \multicolumn{2}{c}{**‘Ambulance’ (Ambulance Siren)**} \\  & **Lw (dBA) = 110.2** & **Lw (dBA) = 122.6** \\  & **Lp(dBA) = 99.2** & **Lp(dBA) = 111.6** \\ \cline{2-5}
**Scenario** & **LdBArx** & **LdBArx** & **LdBArx** & **LdBArx** \\  & **‘Person’** & **‘Balcony’** & **‘Person’** & **‘Balcony’** \\  & **Conf 1** & **Conf 2** & **Conf 3** & **Conf 4** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Global SPL (dBA) at the receiver positions for Y-shaped triple caps. The authors intend to focus their future research on reviewing, extending, and validating the present work by considering other diffracting structures (e.g., branched caps) and performing the corresponding measurement analyses. The impact of absorbing materials on the surfaces of the cap elements on noise abatement will also be analyzed. Conceptualization, D.P.-Q. and J.-V.R. ; Methodology, D.P.-Q. ; Software, D.P.-Q. ; Validation, D.P.-Q. ; Formal analysis, D.P.-Q. ; Investigation, D.P.-Q. and J.-V.R. ; Resources, D.P.-Q., J.-V.R., J.-M.M.G.-P., and L.J.-L.; Data curation, D.P.-Q. ; Writing--original draft preparation, D.P.-Q. ; Writing--review and editing, D.P.-Q., J.-V.R., J.-M.M.G.-P., and L.J.-L.; Visualization, D.P.-Q., J.-V.R., J.-M.M.G.-P., and L.J.-L.; Supervision, J.V.R. ; Project administration, J.-M.M.G.-P., and L.J.-L.; Funding acquisition, J.-M.M.G.-[PERSON], and [PERSON]. All authors have read and agreed to the published version of the manuscript.