## 4 Identification of the Causes of Abnormal Noise  
To identify the causes of abnormal noise due to rail corrugation and analyze the noise transmission path, noise measurements were taken not only inside the vehicle, but also on the wheels during operation. Figure 6 shows the positions of the vibration and noise sensors, while Figure 7 shows the noise sensors installed on the left and right wheels of the first axle in the direction of running. Additionally, to analyze the noise transmission path, vibrations were measured at \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Item** & **Case 1** & **Case 2** & **Case 3** & **Case 4** \\ \hline Mean & 80.6 & 79.9 & 79.6 & 81.8 \\ \hline Maximum & 89.2 & 85.9 & 86.0 & 89.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of interior noise measurements according to vehicle maintenance status (dBA). Figure 5: Interior noise levels according to vehicle maintenance status. Figure 9: \(Cont\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Item**} & \multicolumn{1}{c}{**Measurement Location**} & \multicolumn{1}{c}{**Measurement Direction**} & \multicolumn{1}{c}{**Sensor Specification**} \\ \hline \multirow{4}{*}{Accelerometer} & Wheel & Longitudinal front axle right side journal box & Longitudinal & Kistler triaxial & accelerometer, \(\pm\)50 g \\  & Bogie & Longitudinal front bogie right side frame top & Lateral Vertical & Kistler triaxial & accelerometer, \(\pm\)10 g \\  & Car body & Longitudinal front bogie center body bottom & Longitudinal & Kistler triaxial & accelerometer, \(\pm\)2 g \\ \hline \multirow{4}{*}{Microphone} & Right wheel & Longitudinal front axle right side journal box & - & PCB 1/2 prepolarized \\  & Left wheel & Longitudinal front axle left side journal box & - & free-field condenser \\  & Car body & Center of the car body, height 1.6 m & - & microphone, 50 mV/Pa, \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Accelerometer and microphone specifications. To analyze the relationship between vibration and noise in the rail corrugation section, the correlation between the vibration acceleration of various vehicle components and interior noise was examined at the maximum noise point (located at 425-525 m). The results of the correlation analysis between the RMS of axle box, bogie, and car body vibrations, measured every 10 m, and interior noise are shown in Table 6 and Figure 11. Vibrations with relatively high correlations included the vertical vibration of the axle box and the longitudinal, lateral, and vertical vibrations of the car body. Among these, the vertical vibration of the car body showed the highest correlation with interior noise, at 0.95. These results indicate that interior noise is strongly influenced by car body vibrations. Figure 10: Acceleration and noise data analyzed at 10 m intervals. Figure 9: Acceleration and noise data measured in the rail corrugation section (). For a more detailed analysis, the frequency characteristics of the noise and vibrations were analyzed based on the presence of corrugation while the vehicle was operating at 80 km/h. Figure 12 shows the frequency analysis results for interior noise. Compared to sections without corrugation, the frequency characteristics in the corrugated section increased significantly in the 920 Hz band, and its multiples, 1840 Hz, 2760 Hz, and 3680 Hz, also increased. Figure 13 shows the frequency analysis results of the vertical vibrations of the axle box and car body, which exhibited a high correlation with the interior noise. Similar to the interior noise frequency analysis, it was confirmed that the vertical vibrations of the axle box and car body increased significantly in the 920 Hz band when passing through the rail corrugation section. This aligns with the previous correlation analysis results, confirming that the vertical vibrations of the axle box and car body strongly correlate with interior noise.