## 4 Results and Discussion
### Test Results of Fixed-Point Pure Tone in the Tunnel
From the results measured at the road shoulder, the closed-cell aluminum foam also has a certain sound absorption effect on the noise at the road shoulder, but the sound absorption capacity is limited, so the line graph shows the characteristics of small slope and flatness. However, the noise reduction is more obvious at the center line of the tunnel. The possible reason is that the interference between the reflected sound wave and the reflected sound wave at the shoulder of the road closer to the side wall is more obvious, and the interference between the reflected sound wave and the direct sound wave is also more obvious, but at the center line of the tunnel, the interference effect between the reflected sound wave and the reflected sound wave is weakened. From the distance point of view, the farther away from the sound source, the reflected sound wave can go through more reflections, and there is enough distance for the sound energy to be dissipated, whether it is in the center line of the tunnel or at the shoulder of the road. Figure 9: Influence of location and distance. In general, the noise reduction effect at the center line of the tunnel is better than that at the tunnel shoulder in the noise reduction of the tunnel, and the longer the distance, the more obvious the noise reduction effect. The noise reduction effect of the double-sided aluminum foam board is better than that of the single-sided aluminum foam board and the unmounted aluminum foam board. The single-sided aluminum foam board has a small noise reduction effect. Effect of frequency  
Figure 10a-f reveal the sound absorption performance of closed-cell aluminum foam boards for pure sounds at different frequencies. In general, the equivalent continuous A-weighted sound pressure level measured in the tunnel presents a M-shaped distribution, that is, the equivalent continuous A-weighted sound pressure level is higher at the frequency of 250 Hz and 1000 Hz, and the results measured at the frequency of 125 Hz and 2000 Hz are very low. Figure 10: _Cont._  
As for the insertion loss, the closed-cell aluminum foam board has the best sound absorption effect for pure sound with a frequency of 250-1000 Hz. According to the relationship between the speed of sound, frequency and wavelength, for pure sound with a frequency lower than 250 Hz, since the speed of sound is equal to the product of wavelength and frequency, the sound wave has a longer wavelength at low frequencies. However, the average pore diameter of closed-cell aluminum foam is only 5 mm, so the scattering effect of aluminum foam surface on low frequency acoustic wave is very weak. In addition, there are cavities behind the aluminum foam boards that were drilled, so the entire installation structure of the aluminum foam boards can be regarded as a structure in which multiple Helmholtz resonators are connected in parallel. According to the principle of resonance sound absorption, the natural frequency of the aluminum foam board is just close to the testing frequency, so when sound waves of the same frequency are transmitted to the aluminum foam boards, it can achieve the effect of sound absorption and noise reduction by resonate. From the discount slope in Figure 10, it can be seen that the sound absorption performance of the closed-cell aluminum foam for pure tones with frequencies between 250-1000 Hz is relatively similar, while the sound absorption effect of closed-cell aluminum foam board increases rapidly from 125 Hz to 250 Hz. Besides, from 1000 Hz to 2000 Hz, the sound absorption effect of closed-cell aluminum foam board decreased obviously. By comparing the figures of left column and right column, it can be seen that the influence of frequency on sound absorption effect of closed-cell aluminum foam board is more obvious at the center line of tunnel. At the shoulder, the influence of frequency on sound absorption effect is relatively small.