## 5 Analysis of Interior Noise after Rail Corrugation Treatment
### Analysis of Interior Noise after Track Stiffness Adjustment and Rail Grinding  
The D-E section with serious interior noise was selected as the regulation section. After measurement, the lateral stiffness of the section was 9.8-11.3 kN/mm, less than the stiffness of other lines with no rail corrugation. Its capacity to maintain the gauge was poor; therefore, the lateral vibration of the rail could not be restrained effectively. It is evident from the literature [39; 40] that the low lateral stiffness of the track is the direct cause of abnormal rail corrugation in the curve section. Therefore, the following measures were adopted to reconstruct the from K38 + 424 to K38 + 692 section with serious corrugation.  
In the first treatment, the fasteners were replaced. The DTVI2 fasteners were replaced with Spring clip-II fasteners, which could enhance the gauge keeping capacity. After the treatment, the vertical static stiffness of the track had little change, while the lateral static stiffness improved significantly, and increased to 13.1-14.2 kN/mm. The second treatment was to grind the rail after the first treatment.  
The line condition before and after treatments is shown in Figure 16.  
Figure 16: Metro line condition before and after the treatments. (**a**) Before the treatments, (**b**) After the treatments.  
Figure 15: One-third octave diagram of rail irregularity in test section.  
One month after each treatment, the noise inside the train was tested again to study the change in the A-weighted sound pressure level. The results from the analysis of two noise test results following the treatment are shown in Figure 17. It is shown in Figure 17a that the time-variation trend of interior noise is essentially the same before and after the treatments, but the value of interior noise is evidently reduced. Before the treatments, the maximum interior noise of the test section reaches 104.4 dB(A), and after two treatments, the maximum value of the noise in the vehicle is 96.5 dB(A), which is 7.9 dB(A) lower than that before the treatments.  
As shown in Figure 17b, after replacing the fasteners, the wheel-rail relationship improves. The interior noise is 95.6 dB(A) before the treatments, while it is 87.9 dB(A) after replacing the fasteners. The interior noise is reduced by 7.7 dB(A), and the interior noise at all frequencies is reduced, with a decrease of 5-10 dB(A). The interior noise is reduced to 84.2 dB(A) after rail grinding and is reduced largely in the 400-1600 Hz frequency range, which is the main frequency of interior noise.  
Since only the most serious rail corrugation segment of the D-E section is selected for transformation, and the transformation section is short, the interior noise of the whole section remains large. Due to the hysteresis of the interior noise, the noise cannot be quickly attenuated. Therefore, the interior noise in the test section still exceeds the limit.  
Even so, after the treatments of rail corrugation, the interior noise of the test section is reduced from 95.6 dB(A) to 84.2 dB(A), and the reduction range is up to 11.4 dB(A). The effect of the treatments is clear.