# Impact and Control of Reflected Noise from an Overpass Bottom
## 3 Results and Discussion
### Reflected Noise Analysis and Improvement Results with Different Overpass Heights  
Table 3 shows simulation values and measurement values for different overpass heights. Table S2 shows Cadna A calibration input parameters for Site B. When subtracting measurement values from simulation values (L\({}_{\text{Aeq},1}\)h), the deviation values at the four measurement points were \(-\)2.7-1.8 dB(A). Since all deviations were less than \(\pm\)3 dB(A), the sound field model can be used for simulation at Site B. At Site B, the noise sources were mainly northbound and southbound traffic on National Freeway No. 1, with traffic volumes of 2208 and 2768 vehicles/hour, respectively. Both simulation and measurement values indicate that the noise level was around 70 dB(A). The noise level at the low overpass location (10 m) was slightly higher than that at the high overpass location (17 m), indicating overpass height had an effect on the overall noise level. We assume sound waves cannot spread quickly due to the confined space, such that the noise level was slightly higher at the low overpass location. Additionally, at the same measurement location, measurement and simulation values (L\({}_{\text{Aeq},1}\)h) 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 main reason for this was the same as that at Site A. Simulation results show that when the overpass was parallel to the freeway, the noise source, the noise level in areas near the overpass was higher via the addition of reflected noise. In other words, when the overpass and highway were parallel, the area influenced by reflected noise tended to be large. The increase in noise level on different floors by reflected noise was 0-1.8 dB(A). Since traffic flow at this site was much lower than that at Site A, the overall noise level and reflected noise level were also much lower. Simulation results demonstrate that reflected noise had a larger effect on low floors. At high and low overpass sections, reflected noise did not affect 4F and 5F, and 3F-5F, respectively (Table 4), mainly because the heights of these floors were close to that of the overpass, such that reflected noise on these floors was insignificant. Additionally, at the low overpass section, the overall noise level on the fifth floor was significantly reduced, mainly because the fifth floor's height was the same as the overpass, and therefore the effects of reflected noises and traffic noise were  
Figure 5: Simulation results for reduction in reflected noise by different materials on the bottom of the overpass (Site A). not noticeable. Although the fifth floor receives traffic noise from the overpass, it has been effectively reduced by acoustic barriers on both sides of the overpass. When sound absorbing material on the overpass bottom had a sound absorption rate of 0.36 and 0.85, the decrease in reflected noise at 1F-5F was 0.3-0.8 and 0.8-2.1 dB(A), respectively. Simulation results show as the sound absorption rate of the sound absorbing material increased, noise reduction improved. The material with a sound absorption rate of 0.85 would be sufficient to eliminate reflected noise. To elucidate the effect of overpass height on reflected noise, reflected noise with different overpass heights at the low overpass section of Site B was simulated. Figure 6 shows simulation results. Reflected noise decreased as overpass height increased. With the first and second floors as examples, when overpass height was increased from the original 9.6 m to 25 m and 31 m, respectively, reflected noise was eliminated. However, increasing overpass height can be costly. Therefore, for economic reasons, we advise installing sound absorbing materials on the overpass bottom.