## 5 Key Issues of BGB Top Plate Thickening Design in Acoustic Vibration Control
### Exploration of the Key Influence of Vibration Distribution of BGB on Its Sound Radiation
The vibration pattern and external excitation vibration patterns take the form of first-order overall vertical bending, forming overall resonance in the BGBs. This vibration pattern does not correlate with strong acoustic radiation efficiency, and the BGBs under the three conditions show a high acoustic radiation capacity when subjected to very small vibration instead. This shows that, at this time, in the BGB subjected to resonance, a small vibration can give rise to a strong acoustic radiation capacity. Although the vibration of the BGB at 6.8 Hz is small, the thickened top plate also attenuates the vibration amplitude of the BGB, which can serve to reduce the acoustic radiation capacity of the BGB to a certain extent; (2) The 10 th order vibration patterns of the BGBs under the three operating conditions change significantly, with the BGB with a thickened top plate showing bottom plate local self-oscillation characteristics. The acoustic radiation efficiency of the BGB with a thickened top plate is substantially reduced, which in turn reduces the acoustic radiation capacity of the BGB--for example, the maximum amplitudes of the initial BGB and that with a thickened top plate are 0.60 m/s\({}^{2}\) and 0.54 m/s\({}^{2}\), respectively, which is a smaller difference, but with a 9.6 dB sound power attenuation. Further, the sound radiation efficiency and the maximum vibration amplitude of the locally thickened BGB under the top rail plate have been reduced to a greater extent compared with the initial BGB, so the noise reduction effect is better. The comparison of the vibroacoustic characteristics between the two types of top plate-thickened BGBs and the initial BGB shows that the sound radiation efficiency of the attenuated BGBs in the low frequency band is better in terms of noise reduction as opposed to vibration reduction; (3) The vibroacoustic characteristics of the BGB in the 12 th order mode under three operating conditions further affirm the results of the second analysis, and the significant reduction in sound radiation efficiency directly reduces the sound radiation capacity of the BGB. The web of locally thickened BGBs under the top rail plate also shows a certain resonance under these conditions, so the noise reduction effect of the design with the thickening of the whole top plate is better; (4) The natural vibration characteristics of the BGBs in the 18 th order mode under the three operating conditions are quite different. As discussed above, the small-amplitude vibration of BGBs in this frequency band under external excitation will be shifted, and the maximum vibration amplitude of the two BGBs with thickened top plates will be increased substantially. Still, the problem is that the acoustic radiation capacity of the BGB with a thickened top plate under the rail will be slightly enhanced. In contrast, the sound radiation capacity of the BGB with a whole thickened top plate will be weakened. At this time, the natural vibration characteristics of the two top plate-thickened BGBs can be shown as the local natural vibration characteristics of the bottom plate and the top wing plate, respectively, and these affect the sound radiation efficiency of the BGB. This shows that the top plate of the BGB plays a particularly important role in its sound radiation and is a key area to control when aspiring to noise reduction. It also shows that, in the low-frequency band, the vibration of the BGB is not the most critical factor contributing to sound radiation; (5) The vibration patterns of the BGBs subjected to severe external excitation are similar under each of the three working conditions, and their acoustic radiation capacities decrease as the vibration is reduced. Therefore, the acoustic radiation of BGBs in the severe vibration band can be controlled mainly by reducing vibration.