## 4 Results and Discussion
### Aerodynamic Noise Characteristics
#### 4.2.2 Aerodynamic Noise Radiation and Attenuation Characteristics  
To study aerodynamic noise radiation and attenuation characteristics induced by the up- and down-pantograph with the different base configurations, the contours of spatial distribution in near-field and far-field noise at three planes in the y-direction i.e. y = 0 (near-field), 10 and 20 m (in far-field) are compared, as depicted in Figure 16. In this analysis, the acoustic finite element method is directly used to deal with the whole field noise, including near-field y = 0 plane noise, without considering the effect of quadrupole noise and corresponding convection effect. The main reason is that Mach number of the incoming flow is only 0.278 in this calculation. As it is known that the ratio of quadrupole noise to dipole noise is proportional to the square of Mach number [32], the influence of the quadrupole noise at this speed is relatively small, and it is reasonable to ignore it in the present study. As the pantograph is the noise source, the sound pressure decreases with the y distance. The contours also indicate that the regions with higher sound levels for both frequencies are in the vicinity of the pantograph. These distributions are more uniform at the high frequency. It is shown that, as frequency increases, the sound pressure amplitude decreases. Figure 15: Normalized aerodynamic pressure fluctuation amplitude at 200 and 1000 Hz on: (**a**) down-pantograph with flush base; (**b**) up-pantograph with flush base; (**c**) down-pantograph with sunken base; and (**d**) up-pantograph with sunken base. To obtain the noise characteristics in the far field, two positions in the y-direction, i.e. at y = 5 m and y = 25 m from the center line of the train, as shown previously in Figure 4, are evaluated, as plotted in Figure 17. Some phenomena are observed. Firstly, the noise amplitude at y = 25 m is lower than that at y = 5 m at all frequencies. Next, it is shown that the noise caused by the down-pantograph is higher than that by the up-pantograph. Finally, the noise from the pantograph with the flush base configuration is of higher amplitude than that with the sunken base configuration irrespective of the pump. Figure 16: Spatial distribution of near-field (Y = 0 m) and far-field (Y = 10 m, Y = 20 m) noise at 200 Hz and 1000 Hz on: (**a**) down-pantograph with flush base; (**b**) up-pantograph with flush base; (**c**) down-pantograph with sunken base; and (**d**) up-pantograph with sunken base. whether the pantograph is under the up or down condition. These phenomena could be explained by the fact that, for the sunken configuration, the vortices generated in the rear of the insulator and the base move inside the cavity and the movement to the surroundings is blocked by the cavity surface. This may explain the predicted weakening of the far-field noise. Additionally, one can see that, for frequencies below 200 Hz, the noise caused by the sunken type configuration is almost the same as that from the flush. However, at higher frequencies, the noise caused by the sunken configuration of the pantograph is lower than that by the flush configuration in general. This is because the large vortices produced by the sunken insulator are blocked by the cavity, therefore their streamwise growth and spread are constrained. As a result, large eddies are not easily broken into small ones. To further study the trend of the overall sound pressure levels (OASPL) at different distances along x (parallel to the train) and y (normal to the train) directions, Figure 18a,b shows the OASPL in the far field along the x and y directions, respectively. Figure 18a indicates that the OASPL is inversely proportional to the distance of sound source for all cases.