# A Review of Recent Research into the Causes and Control of Noise during High-Speed Train Movement
## 4 High-Speed Train Noise Control Methods
### Acoustic Metamaterials Applied to High-Speed Train Noise Control
#### 4.2.2 Metamaterials Applied to Railroad Sound Barriers  
At present, railroad sound barriers are mainly upright insert-type sound barriers. The wind pressure fluctuations, when passing through such sound barriers, can lead to the loosening and breaking of bolts and the destruction of sound barrier panels [103]. When designing high-speed railroad sound barriers, not only should the sound insulation and sound absorption characteristics of the barrier be considered, but also the dynamic response of the sound barrier structure. Conventional sound barriers produce sound reflections that lead to increased noise levels within high-speed trains. For future high-speed railroad sound barriers, they inevitably need to meet codified sound insulation criteria and, at the same time, have both ventilation and noise absorption capabilities.  
There is always a balance between the thickness of a sound insulation device and its ventilation capacity. [PERSON] et al. [104] designed a binary structure consisting of a coiled unit and a hollow tube with a thickness of less than one-fifth of the wavelength, which can block low-frequency sound from different directions while allowing 63% of the airflow to pass through. [PERSON] et al. [105] designed a vented metamaterial absorber operating at low frequencies (<500 Hz) with only two absorption units, achieving high-efficiency absorptionunder vented conditions by using weak coupling of two identical split-tube resonators. [PERSON] et al. (2016) combined spiral channels and embedded apertures; this metamaterial structure can absorb low-frequency noise while maintaining the requisite thickness. [PERSON] et al. (2017) proposed a deep sub-acoustic wavelength metamaterial cell that includes nearly 60% of the open area allowing passage of air and also serves as a high-performance selective sound muffler. [PERSON] et al. (2018) proposed an acoustic metamaterial composed of many cells, and this open metamaterial contains a large hole in each cell to ensure airflow. [PERSON] et al. (2019) integrated eight labyrinth cells of different configurations to form an acoustic metamaterial and introduced a herringbone channel to achieve ventilation. [PERSON] et al. (2019) proposed a metamaterial with a conchoidal cavity structure and embedded this metamaterial into a conventional concrete or metal sound barrier, which can significantly improve the sound absorption capacity of the sound barrier and prevent sound pollution of the surrounding high-rise buildings by high-speed railroads.  
As shown in Figure 7, acoustic metamaterials can ensure better sound insulation and absorption effects under ventilation, but they have not been widely used in sound barriers due to their complicated production process and high cost of production. To increase the potential for wider engineering applications of acoustic metamaterials, researchers should further simplify their structure while considering the production cost in the design process and, furthermore, fit the specific use scenario for optimization.