## 4 Results and Discussion
### Acoustic Characteristics of Different Configurations
#### 4.2.2 Spectral Characteristics of SPL  
This section studies the effect of different diversion strategies on the far-field radiated noise of pantographs. The standard observing points should be 25 m from the central line of rail track and 3.5 m above the surface of rail track. In view of the train roof model which is used in this study, the far-field acoustic measurement points are arranged at a height of 1.5 m from the ground and 25 m from the center line of rail track. As depicted in Figure 11, 20 monitoring points are equally spaced along the longitudinal direction of the train (x direction), each observer point is 5 m apart, and the origin is at the bottom center of train. Therefore, the range of monitoring points is [\(-\)35, 60] m. The pantograph is located near the 6 th monitoring point along the x direction. Figure 12 demonstrates the comparison curves of the SPL at monitoring points under different pantograph diversion strategies when the train model and pantograph are set as the sound sources. As shown in Figure 12, along the direction of train running, the aerodynamic noise SPL tends to increase first and then decrease, and the maximum SPL appears near the  
Figure 10: Pressure fluctuation of the computational models. (**a**) Case 1; (**b**) case 2; (**c**) case 3.  
pantograph area. Compared with the SPL when pantograph is installed on the train roof alone, it is clear that the SPL is reduced by about 2 dBA when the pantograph is installed on the sinking platform, indicating a better aeroacoustic performance. In addition, the maximum SPL of cases 1 and 2 occurs at the 6 th monitoring point. When the pantograph is installed with the fairing, the location changes, which is the 4 th monitoring point. Meanwhile, the SPL at the monitoring point before the 6 th monitoring point increases, after which the SPL at almost all monitoring points decreases along the train running direction, with an increasingly lower decrease in degree. This can be explained by the fact that the fairing itself brings great noise, and the airflow is disturbed by the leading edge of the fairing first, resulting in some noise reduction at the front end of the fairing cavity. After a period of time, the turbulent flow continues to impact the rear part of the pantograph fairing area, together with the vortex shedding of the pantograph rod, which weakens the noise reduction effect of the fairing. Importantly, compared with case 1, the fairing mainly decreases the noise near the pantograph, and the largest SPL reduction is up to 3 dBA at the 8 th monitoring point. Figure 13 illustrates the 1/3 octave band frequency curves at the maximum SPL monitoring points of the abovementioned three cases. As can be seen in Figure 13, the distribution of aerodynamic noise energy in all the cases is concentrated in 630-2500 Hz, and the SPL gradually decays toward the high frequency and low frequency regions. Furthermore, the noise reduction effect of the sinking platform in the frequency band of 200-1000 Hz is not clear, while the magnitude above 1250 Hz is almost less than case 1 by 2 dBA, which is consistent with the behavior in Figure 12. With regard to case 3, the presence of the fairing greatly increases the noise in the frequency band below 500 Hz, and decreases the noise in the frequency band above 1000 Hz. Consequently, a large SPL maximum is obtained for case 3 in Figure 12.  
To further investigate the causes of the difference in SPL energy between the three cases, the spectral characteristics of three critical components of the pantograph, namely, the panhead, the lower arm rod, and the base frame, are analyzed in this section. Figure 14 shows the noise spectrum curves at the maximum SPL monitoring point of each case when the panhead, the lower arm rod, and the base frame are considered as noise sources, respectively. Figure 14a illustrates that when the pantograph is placed on the train roof alone, the main-order and the second-order frequency of 1767  
Figure 11: Locations of the standard sound monitoring points along the x-direction.