## 4 Results and Discussion
### Test Results of Fixed-Point Pure Tone in the Tunnel  
Case I studied the sound absorption performance of closed-cell aluminum foam board under fixed pure tone. The method adopted combined the loudspeaker test method with the typical tunnel measurement. Considering the process of field test and the key factors affecting the sound field distribution of the tunnel, the position, distance and frequency were taken as the key points to analyze which may have the effects on sound absorption under the fixed-point pure sound source. (1) Effect of location and distance  
Figure 9a-e not only show the distribution law of the sound field inside the tunnel, but also show the sound absorption effect of closed-cell aluminum foam boards under different sound field positions and distances. On the whole, the sound pressure at the center line of the tunnel is slightly greater than that at the tunnel shoulder. This is because the sound source of the tunnel is arranged on the center line of the tunnel. The linear distance between the measuring point on the center line is larger than the measuring point at the shoulder of the same cross section, so the sound energy has a longer dissipation distance. In addition, it can be found that the farther away from the sound source, the smaller the sound pressure  
Figure 8: Test flow chart of Case II. level measured no matter the measurement is on the centerline of the tunnel or the shoulder of the road. This is because the longer the distance, the more the sound energy is dissipated. However, there are special cases that do not conform to the general rules in the data points measured this time. For example, the equivalent continuous A-weighted sound pressure level measured at the measurement which is on the center line of the tunnel and 4 m away from the sound source at 125 Hz is obviously greater than the distance is 2 m and 6 m; the sound level measured at the shoulder 6 m longitudinal distance from the sound source at 1000 Hz is higher than that measured at the tunnel midline. This is because the pure tone signal released is a sine wave. The positive superposition of the amplitude of the reflected sound wave and the direct sound wave at these measuring points enhances the sound pressure level, while the superposition of the reflected acoustic wave and the direct acoustic wave at other locations is very weak or the superposition is reversed. Figure 9: _Cont._  
From these five pictures, it can be seen that after the aluminum foamed boards were installed on one side, the results measured at each measuring point have a certain decrease compared with the unlaid aluminum foamed boards but the reduction is not large. After the double-sided aluminum foam boards were installed, the measured results at each measuring point were significantly reduced. This is because the sound waves reflected from the side where the aluminum foam boards were not installed, and interfere with the direct sound waves. When the double-sided aluminum foam boards were installed, the reflected sound waves on both sides were affected. Comparing the slopes of the curves of the three test results, it can be seen that the slope of the results measured after the aluminum foam boards were installed on both sides is significantly greater than that of the single-sided installation and non-installation. From this analysis, we can see that the equivalent continuous A-weighted sound pressure level in the tunnel that the aluminum foam boards were installed on both sides drops faster along the longitudinal direction of the tunnel, which means that the installation of aluminum foam boards on both sides has the best noise reduction effect on the sound in the tunnel. Analyzed from the position, the measured results at the center line of the tunnel vary significantly more than the shoulder of the tunnel.