# Impact and Control of Reflected Noise from an Overpass Bottom
## 3 Results and Discussion
### Reflected Noise Analysis and Improvement Results with Different Overpass Heights
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{**High Section**} & \multicolumn{2}{c}{**Low Section**} \\ \hline  & **Floor** & \(\mathbf{L_{Aeq}}\)**1h dB(A)** & \begin{tabular}{c} **Reflected Noise** \\ **dB(A)** \\ \end{tabular} & \(\mathbf{L_{Aeq}}\)**,1h dB(A)** & \begin{tabular}{c} **Reflected Noise** \\ **dB(A)** \\ \end{tabular} \\ \hline  & 1F & 66.1 & 1.1 & 66.7 & 1.8 \\ Noise level on each & 2F & 68.6 & 0.8 & 69.3 & 1.4 \\ Moise level on each & 3F & 71.7 & 0.5 & 71.7 & 0 \\ floor & 4F & 74.8 & 0 & 75.4 & 0 \\  & 5F & 76.3 & 0 & 69.2 & 0 \\ \hline  & Floor & \begin{tabular}{c} Sound absorption \\ rate 0.36 \\ \end{tabular} & \begin{tabular}{c} Sound absorption \\ rate 0.85 \\ \end{tabular} & \
\begin{tabular}{c} Sound absorption \\ rate 0.36 \\ \end{tabular} &
\begin{tabular}{c} Sound absorption \\ rate 0.85 \\ \end{tabular} \\ \hline Decrease in reflected & 1F & \(-0.7\) & \(-1.8\) & \(-0.8\) & \(-2.1\) \\ noise by sound & 2F & \(-0.5\) & \(-1.3\) & \(-0.6\) & \(-1.6\) \\ absorbing material & 3F & \(-0.3\) & \(-0.8\) & 0 & 0 \\ on the bottom of the & 4F & 0 & 0 & 0 & 0 \\ overpass & 5F & 0 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Simulation results for the overall noise level, reflected noise, and noise reduction results for overpasses of different heights. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sound Receiving Point** & \(\mathbf{L_{Aeq}}\)**,1h Simulation** & \(\mathbf{L_{Aeq}}\)**,1h Measurement** &
\begin{tabular}{c} **Deviation (1)–(2)** \\ **Value (2) dB(A)** \\ \end{tabular} \\ \hline Overpass height (17 m) & 67.6 & 69.1 & \(-1.5\) \\ (sound meter is 1.5 m above the ground) & 70.1 & 70.0 & 0.1 \\ \hline Overpass height (10 m) & 69.2 & 69.9 & \(-0.7\) \\ (sound meter is 4.5 m above the ground) & 72.8 & 71.0 & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation values and measurement values with different overpass heights (overpass is parallel to the road). Reflected Noise Analysis and Improvement Results for Different Road Structures (Overpass Going across the Noise Source Road)  
Table 5 shows simulation and measurement values for different road structures. Table S3 shows Cadna A calibration input parameters for Site C. When subtracting measurement values from simulation values (L\({}_{\text{Aeq}}\),1h), the deviation values at the four measurement points were 0.2-2.6 dB(A), meeting the \(\pm\)3 dB(A) requirement. Thus, the sound field model can be used for simulation assessment at Site C. The noise sources at Site C were mainly northbound and southbound traffic on National Freeway No. 1, with traffic volumes at 4560 vehicles/hour and 4287 vehicles/hour, respectively. The noise level at 30 m from the overpass was lower than that at 15 m from the overpass. The reason is that the 30 m location was farther from where reflected noise was generated and therefore reflected noise was lower. On the other hand, at the same measurement location, both measurement and simulation values (L\({}_{\text{Aeq}}\),1h) were higher when the sound meter was 4.5 m above the ground (roughly sound receiving point on 2F) than when it was 1.5 m above the ground (roughly sound receiving point on 1F). The likely reason is the same as that for Site A. Simulation results show that when the overpass crossed the freeway, the noise source reflected noise was generated only at the intersection of the overpass and freeway, increasing the noise level significantly in this area. That is, when the overpass crossed the freeway, reflected noise was \"localized\" with a small area of influence, compared with the affected areas at sites A and B. Table 6 shows simulated results for the overall noise level, reflected noise, and improvement results for different road structures. Since the first floor of a nearby building was most vulnerable to reflected noise, change in reflected noise at different distances from the overpass was simulated for the first floor. Simulation values show that the reflected noise level was around 0.9-5.4 dB(A).