# 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
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Methods to Reduce Noise** & **Research Methods** & \begin{tabular}{c} **Frequency** \\ **Characteristics** \\ \end{tabular} & \
\begin{tabular}{c} **Sound Pressure Level** \\ \end{tabular} & **Reference** \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Different strip spacing of \\ the pantograph \\ \end{tabular} } & \multirow{4}{*}{CFD analysis} & \multirow{4}{*}{No effect} & The sound pressure level of the \\  & & & standard noise measuring \\  & & & point is reduced by 2.8 dB with \\  & & & a spacing of 540 mm \\  & & Around 1000 Hz \\  & & (original model) & The total noise decreased \\  & & Around 100 Hz & by 10 dB \\  & & (optimized model) \\  & & & Total peaks are & The peak sound pressure level \\  & & & gradually reduced from \\  & & & 2 kHz \\  & & & The peak sound pressure level \\  & & CFD analysis, Wind \\  & porous sound absorption \\  & material & & No effect \\  & Covering the fairings with \\  & a porous material & CFD analysis, Wind \\  & & & No effect \\  & & tunnel test \\  & & & \\  & Covering the circular \\  & cylinder with metal foam \\  & & Wind tunnel test \\  & & & lower frequencies \\  & & & The difference in OASPL \\  & & & between the pantographs with \\  & without the cavity \\  & & & and without the cavity was \\  & & & approximately 4 dB at the side \\  & & & A pair of baffles with half of \\  & & & the height of the pantograph \\  & & & on both sides can lessen noise \\  & & & by about 3 dB \\  & & & The flat roof with side \\  & & & insulation plates has the \\  & & & lowest overall noise levels. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of noise reduction measures for pantographs. that optimizing the cylindrical rod and push rod of the pantograph will change the frequency of pantograph noise so that the peak frequency shifts to the low-frequency direction. Compared with high-frequency noise, low-frequency noise is more difficult to be absorbed by porous materials (polyurethane fiber, glass fiber wool, etc.). Using metamaterials to absorb low-frequency noise may be a feasible method. In Section 4, we further introduce the metamaterial for sound absorption. Bionic pigeon feathers shaped pantograph rods and covering the fairings with the porous material bring the most significant noise reduction effect, but the shape of the pigeon feathers will significantly increase the processing cost, and the porous material will change its pore structure under high wind conditions. Compared with other noise reduction measures, changing the strip spacing of the pantograph is the easiest to implement, but this has only been analyzed by CFD analysis, and the specific effect has yet to be experimented with. An overall analysis of the studies in Table 1 shows that all pantograph noise reduction measures can improve the aerodynamic noise of high-speed trains to some extent, but the improvement is limited and difficult to be applied in actual high-speed train manufacturing. Perhaps there will be better measurements to improve the pantograph noise in the future, but at present, it is difficult to optimize the pantograph to reduce the noise. Therefore, in future research, improving the train body panels and sound barriers to stop noise propagation may be the best way to reduce aerodynamic noise.