# Impact and Control of Reflected Noise from an Overpass Bottom
## 3 Results and Discussion  
Reflection Noise Analysis and Improvement Results with Different Overpass Bottom Materials (Steel and RC)  
Table 1 shows simulated values and measurement values with different overpass bottom materials. Table S1 shows Cadna A calibration input parameters at Site A. When subtracting measurement values from simulation values (\(\mathrm{L_{Aeq,1h}}\)), the values at four measurement points differed by 0.5-1.9 dB(A). Since deviations at all four measurement points were \(\pm\)3 dB(A), the sound field model can be used for simulation assessments at Site A. The noise sources at Site A were mainly northbound and southbound traffic flow on National Freeway No. 1, with traffic volumes of 3903 and 4605 vehicles/hour, respectively. The noise level was around 80 dB, which was sufficiently high to interfere with conversation and decrease work efficiency. Thus, the nearby residential area was adversely impacted by the noise. The noise level at the steel overpass section was slightly higher than that at the RC overpass section, suggesting that the effect of overpass bottom materials on the overall noise level was insignificant. Conversely, at the same measurement location, the measurement values and simulation values (\(\mathrm{L_{Aeq,1h}}\)) were higher when the sound meter was 4.5 m above the ground (roughly sound receiving point on the second floor (2F)) than those when it was 1.5 m above the ground (roughly sound receiving point on the first floor (1F)), indicating that the overall noise level tended to typically increased as the number of floors increased due to the different noise sources at different  
floors, including reflected noise, diffracted noise (occurred on top of acoustic barriers) and direct noise. Typically, higher floors received more noise from the noise sources mentioned above. Further, as demonstrated by simulation results, traffic noise (direct noise) from the freeway hit the bottom of the overpass, and reflected noise was generated, evidently increasing the noise level in the residential area along the overpass parallel to the freeway. In other words, when the overpass was parallel to the freeway, reflected noise was largely \"area-based\" with a large impact area. To identify the effect of reflected noise on the overall noise level and assess the success of noise mitigation by sound absorbing material on the bottom of the overpass, the noise level on different floors of nearby buildings was assessed with Cadna A. Table 2 and Figure 5 list simulation results. The increase in the noise level at different floors due to reflected noise was 1.0-7.8 dB(A) (Table 2). Simulation results also show that reflected noise increased the noise level on low floors more than on high floors. [PERSON] et al. [26] showed that reflected noise was the main cause of increased noise around a highway culvert, and can increase the noise level by \(\geq\)5 dB(A). In addition, the WSDOT [34] showed that reflected noise from a double-decker road may be as high as 11 dB(A). [PERSON] et al. [41] assessed the effect of an overpass built across a railway track on the increase in noise level. The results showed that reflected noise from the bottom of the overpass contributed up to 10 dB increase in wayside noise level. Therefore, our findings are in agreement with the results of other studies. Moreover, since the sound absorption rates of steel and RC overpass bottoms were very close (0 for steel and 0.02 for the RC), the levels of noise reflected by these overpass bottoms were similar. Therefore, the effect of these two materials on reflected noise was insignificant. Figure 5 shows simulation results for reflected noise mitigation when the overpass bottom was equipped with different sound absorbing materials. When the sound absorbing material had a sound absorption rate of 0.36, reflected noise on 1F-5F decreased by 0.3-1.5 dB(A). When the sound absorbing material had a sound absorption rate of 0.6 and 0.85, reflected noise on 1F-5F decreased by 0.6-3.0