## 3 Field Test
### Case II  
Case II mainly studies the sound absorption performance of closed-cell aluminum foam under moving sound sources, that is, the research on the insertion loss of closed-cell aluminum foam boards in tunnels. By comparing the maximum sound pressure level of the vehicle from entering the tunnel to leaving the tunnel before and after the installation of the aluminum foam board, then the effect of the aluminum foam board in the noise reduction application of the tunnel was obtained. The noise generated by the vehicle was affected by the type of vehicle, use time, brand and speed. In the field measurement of highway tunnels, it is impossible to adopt a unified standard for past vehicles. Therefore, the vehicle is divided into large, middle and small categories according to the size of the vehicle, limit the speed of the vehicle on the lane, collect large amounts of data and other methods to reduce the impact on the test results caused by the above reasons.  
The test of Case II is similar to that of Case I. Following the installation progress of the aluminum foamed board, the test was also carried out in three stages before installation, one-sided installation is completed, and two-sided installation is completed, as shown in Figure 8. Before the test, the two lanes were blocked at 500 m from the entrance and only allowed to pass through the only lane, which can achieve the effect of limiting the speed of the vehicle and avoid some accidental noise interference caused by the lane change when the car is driving. During the test, turn on the continuous sampling function in the sound level meter when the vehicle enters the tunnel. The sampling interval of the sound level meter is 1S. When the vehicle exits the tunnel, stop sampling and read the maximum instantaneous A-weighted sound pressure level during the sampling period. For each vehicle type, at least 100 data samples were taken in each test, and the 100 data samples were divided into 10 groups evenly, and the average value of each group was calculated as the representative value of this group. Finally, the representative values were analyzed to explore the noise reduction law of closed-cell aluminum boards in tunnels.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Condition.**} & \multirow{2}{*}{
\begin{tabular}{c} **Frequency/Position/** \\ **Distance** \\ \end{tabular} } & \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Frequency/Position/**} & \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Frequency/Position/**} \\  & & & & **Distance** & & **Distance** \\ \hline
1 & 125 Hz/midline/2 m & 11 & 250 Hz/shoulder/4 m & 21 & 1000 Hz/midline/6 m \\
2 & 125 Hz/midline/4 m & 12 & 250 Hz/shoulder/6 m & 22 & 1000 Hz/shoulder/2 m \\
3 & 125 Hz/midline/6 m & 13 & 500 Hz/midline/2 m & 23 & 1000 Hz/shoulder/4 m \\
4 & 125 Hz/shoulder/2 m & 14 & 500 Hz/midline/4 m & 24 & 1000 Hz/shoulder/6 m \\
5 & 125 Hz/shoulder/4 m & 15 & 500 Hz/midline/6 m & 25 & 2000 Hz/midline/2 m \\
6 & 125 Hz/shoulder/6 m & 16 & 500 Hz/shoulder/2 m & 26 & 2000 Hz/midline/4 m \\
7 & 250 Hz/midline/2 m & 17 & 500 Hz/shoulder/4 m & 27 & 2000 Hz/midline/6 m \\
8 & 250 Hz/midline/4 m & 18 & 500 Hz/shoulder/6 m & 28 & 2000 Hz/shoulder/2 m \\
9 & 250 Hz/midline/6 m & 19 & 1000 Hz/midline/2 m & 29 & 2000 Hz/shoulder/4 m \\
10 & 250 Hz/shoulder/2 m & 20 & 1000 Hz/midline/4 m & 30 & 2000 Hz/shoulder/6 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test condition of Case I.