# Impact and Control of Reflected Noise from an Overpass Bottom
## 4 Conclusions  
This study examined the effects of noise reflected from the overpass bottom under various conditions (different bottom materials, different overpass heights, and different road structures (the overpass is perpendicular or parallel to the source road)) using onsite measurements and Cadna A simulation. The noise reduction effect of sound absorbing materials on the overpass bottom was also evaluated. Conclusions and recommendations are as follows. 1. Reflected noise from the bottom of an elevated road may be as high as 7.8 dB(A) and reflected noise increased the noise level on low floors more than on high floors. 2. Overpass height had an effect on the reflected noise level. As overpass height increased, the level of reflected noise decreased. 3. When the distance between the overpass side and a receiver exceeded 30 m, the level of reflected noise reduced significantly. 4. As the sound absorption rate of the sound absorbing material installed on the overpass bottom increased, the noise reduction effect increased. A sound absorbing material with a sound absorption rate of at least 0.60 is recommended to reduce reflected noise level. \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Distance from Overpass** & **L\({}_{\text{Aeq}}\):1h dB(A)** & **Reflected Noise dB(A)** \\ \hline  & 15 m & 70.2 & 5.4 \\ Noise level at each & 30 m & 69.0 & 2.8 \\ Noise level at each & 45 m & 68.0 & 1.9 \\ distance & 60 m & 67.5 & 1.4 \\  & 75 m & 67.4 & 0.9 \\ \hline  & Distance from overpass & Sound absorption rate 0.36 & Sound absorption rate 0.85 \\ \hline Decrease in reflected & 15 m & \(-\)1.1 & \(-\)3.6 \\ noise by absorbing & 30 m & \(-\)0.7 & \(-\)1.9 \\ materials on the bottom & 45 m & \(-\)0.5 & \(-\)1.3 \\ of the overpass & 60 m & \(-\)0.4 & \(-\)1.0 \\  & 75 m & \(-\)0.2 & \(-\)0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Simulation results for overall noise level, reflected noise, and for different road structures (overpass crossing the road). \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sound Receiving Point** & **L\({}_{\text{Aeq}}\),1h Simulation** & **L\({}_{\text{Aeq}}\),1h Measurement** & **Deviation (1)–(2)** \\  & **Value (1) dB(A)** & **Value (2) dB(A)** & **dB(A)** \\ \hline Distance from overpass side: & & & \\
15 m (sound meter is 1.5 m & 69.9 & 69.7 & 0.2 \\ above the ground) & & & \\ \hline Distance from overpass side: & & & \\
15 m (sound meter is 4.5 m & 72.0 & 71.6 & 0.4 \\ above the ground) & & & \\ \hline Distance from overpass side: & & & \\
30 m (sound meter is 1.5 m & 68.9 & 66.3 & 2.6 \\ above the ground) & & & \\ \hline Distance from overpass side: & & & \\
30 m (sound meter is 4.5 m & 71.4 & 69.8 & 1.6 \\ above the ground) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Simulation values and measurement values with different road structures (overpass crossing the road). By installing sound absorbing materials on the overpass bottom and providing an adequate green space buffer zone (distance between a residential area and an overpass), namely, adopting the approach of \"distance attenuation + control of propagation path\", the impact of reflection noises on nearby residents will be further reduced. 6. At all three investigation sites, reflected noise crossed over acoustic barriers, such that a feasible method to reduce reflected noise is to increase the height of acoustic barriers on both sides of the freeway. The results of this study will be helpful to control existed reflected noise near overpasses and provide useful information to urban design regarding the construction of overpasses.