# A Review of Recent Research into the Causes and Control of Noise during High-Speed Train Movement
## 2 High-Speed Train Noise Composition
### High-Speed Train Aerodynamic Noise  
The airborne noise generated by high-speed trains is divided into external airborne noise and internal airborne noise, of which the external airborne noise of high-speed trains is the main source of noise. When the driving speed of high-speed trains exceeds 300 km/h, the external airborne noise accounts for more than 50% of the total noise of the train [9], and as the speed of the train increases, the external aerodynamic noise of the train will increase in the ratio of six to eight times the speed of the train [10]. The aerodynamic noise sources are monopoles, dipoles, and quadrupoles. The aerodynamic noise generated by high-speed trains during the driving process is mainly caused by the dipole and quadrupole sound sources generated by the surface pressure fluctuations around high-speed trains. When running, the aerodynamic noise will change due to the speed and the surrounding environment [9]. The train's aerodynamic noise mainly comes from the pantograph area, bogie area, connection area, and the concave structural areas of the train's body surface. Smoothing the design of the train body [11], flow-field control, laying sound-absorbing material [12], and other measures are the main ways to reduce noise generation on the train body. Noise reduction in the pantograph region is currently achieved by optimizing the pantograph shape. Pantograph shape optimization can control the scale of eddy current shedding and thus reduce noise [13]. The addition of shields on both sides of the pantograph effectively diverts the airflow and prevents noise diffusion. The contribution of radiated noise from pantographs accounts for more than 10% of the total noise generated. Researchers mainly study the sound generation mechanism of pantographs and optimize them through experiments and numerical simulations. The bogie fairing can reduce the development of turbulence outside the fairing, thus reducing the noise from the bogie. Meanwhile, the use of a reasonable skirt design also reduces the influence of the train bottom spill on the car body and decreases noise propagation therefrom [14]. The inter-coach windshield region is considered one of the main sources of aerodynamic noise. At present, a fully enclosed outer windscreen is used to ensure that the external airflow and the inner windscreen do not come into contact, thus eliminating the vortex in the inner windscreen part, which can control the aerodynamic noise generated on the windscreen of the train [15]. The pantograph is the main noise source of aerodynamic noise in high-speed trains. Compared with the cavity and bogie, the pantograph is located on the top of the train, and the aerodynamic noise generated by it is difficult to be isolated by sound barriers. This paper counts some representative pantograph noise reduction measures in recent years, as shown in Table 1, which are found through comparison and analysis. For the noise reduction measures in the pantograph area, the current research can be summarized into two levels: one is to reduce the number of rods of the pantograph for the pantograph itself; second, passive noise reduction is applied to the shunt area. In Table 1, we analyze the noise reduction measurements, research methods, frequency variation, and sound pressure level variation of each research. Through comparison, we find that researchers generally study the pantograph noise reduction problem by wind tunnel tests and CFD analysis, among which CFD analysis is the most widely used research method, which indicates that CFD technology has strong application value in the field of high-speed trains at present. In some of the studies, the results of wind tunnel tests also fully prove the reliability of CFD analysis. In the future, the flow simulation methods and computer performance algorithms will be the focus area of the researcher. At present, optimizing the rod structure of the pantograph is a feasible measure to reduce the pantograph noise. By analyzing the noise frequencies of each study, we can find