## 4 Comparative Analysis of Acoustic Vibration Characteristics of BGBs for Three Working Conditions  
The curves of the vibration acceleration spectrum taken at the center of the span of the top and bottom plates of the BGB are used to compare and analyze the vibrational characteristics of the two top plate-thickening design solutions. After this, the linear sound pressure levels at different points of the transverse sound field in the span of the BGB are selected, as shown in Figure 5, to analyze the noise reduction effects of the top plate-thickening design solutions, and the field points are selected as shown in Figure 5. The relationship between the two is used to infer the relationship between vibration reduction and noise reduction. As can be seen in Figure 6, under the same excitation, the vibration spectra of the three BGBs are similar. The severe vibration band is concentrated at 80-100 Hz, and the vibration at 0-80 Hz is the small-amplitude vibration. The difference is that the top plate-thickening design not only weakens the vibration amplitude of the BGB in this band, but  
Figure 4: In situ measurement of the noise radiation of high-speed railway BGB: (**a**) photograph of measurement site; (**b**) calculated and measured sound pressure levels. the small-amplitude vibration band is also significantly shifted. The vibrational shift in the center of the bottom plate in the BGB with a thickened top plate is more obvious and is in the very low-frequency band of the initial BGB vibration. In the two BGB thickening designs, the vibrational amplitudes of different plates in the BGB are significantly reduced, and the reduction in the vibrational amplitude of the top plate is significantly greater than that seen in the rest of the plate, thus indicating that both designs will be very effective in vibration damping, with significant vibration reduction. Compared with the design with a partially thickened top plate under the rail, the vibration reduction effect of the whole top plate-thickening design is better, especially in the main vibrational frequency band. However, attention needs to be paid to the significant increase in vibration in the bottom plate at 20-40 Hz when the whole top plate of the BGB is thickened, so the effects of this phenomenon on acoustic radiation characteristics and noise reduction are studied in detail in the following. Figure 5: Schematic diagram of sound field points to be used in the BGB sound radiation study. Figure 6: The vibration acceleration comparison of the top plat-thickening design schemes: (**a**) top plate center point; (**b**) bottom plate center point. Figure 7 shows a comparison of the linear sound pressure levels in three types of BGBs. From Figure 7, the acoustic radiation characteristics of BGBs can be summarized as follows:  
* The stronger frequency band of RGB acoustic radiation can, in general, be divided into two bands, namely, the 0-40 Hz lower-frequency band and the 80-100 Hz severe vibration band. In these two bands, the RGB shows strong acoustic radiation characteristics. The phenomenon of concern is that the acoustic radiation of BGBs in the lower frequency band of different sound fields is very strong--even stronger than that seen in BGBs under the effects of severe external excitation. This phenomenon is contradictory to the general law of strong acoustic radiation caused by severe vibration. Therefore, the reasons for this phenomenon and the key issues to be addressed when designing a thickened top plate to reduce noise are analyzed in detail in the following, providing a basis for the further optimization of the subsequent design;
* The two noise reduction designs with top plate thickening have obvious noise reduction effects, the most significant being in the lower frequency bands, and there are also substantial reductions in sound pressure at certain frequencies.