## 5 Key Issues of BGB Top Plate Thickening Design in Acoustic Vibration Control  
### Analysis of Self-Vibration Characteristics of BGB under Three Working Conditions  
Firstly, the self-vibration characteristics of the BGB before and after the design changes have been implemented are analyzed. Table 2 and Figure 10 give the partial self-vibrational characteristics and vibration patterns of the three types of BGBs, respectively, and analyze the effects of the two design options on the BGB's self-vibration characteristics. From Table 2 and Figure 10, it can be seen that there is a certain increase in the natural frequency of the BGB with a thickened top plate and in that of the BGB with a locally thickened top plate, at the same order, between the initial and the natural frequency of the BGB with its whole top plate thickened. The modalities of the first two orders of the BGB under the three conditions give the overall self-oscillation characteristics; the initial BGB shows more local self-oscillation characteristics in the top plate, including the (\(m\), \(n\)) order vibration patterns, which have a greater impact on the low-frequency acoustic radiation capability of the BGB. The whole and local self-oscillation characteristics in the higher-order modes occur simultaneously in most cases, forming the overall local self-oscillation characteristics of the BGB, and the local vibration characteristics of the top plate are more prominent in comparison. In regard to the design of BGB top plate thickening, the biggest change in the low-order vibration pattern is that the local self-oscillation characteristics of the top plate are weakened. In contrast, the bottom plate will show local self-oscillation characteristics, which will occur in both thickening designs. The local self-excitation of the wing plate will be enhanced in the process of vibrational energy transfer. At the same time, thickening the whole plate has less of an effect on the vibration of the wing plate, and a more significant effect on the vibration of the bottom plate, because the top plate under the rail that is being thickened is located above the BGB cavity and the wing plate, respectively. In summary, the self-vibration characteristics of BGBs under all three operating conditions comprise the overall self-vibration characteristics, the local top plate self-vibration characteristics, and the overall local self-vibration characteristics of all plate members of the BGB. The top plate local vibration characteristics have the greatest influence on the acoustic radiation of the BGB. The top plate thickening design can reduce the probability of top plate local self-oscillation. This phenomenon plays a very important role in reducing the acoustic radiation of the BGB. Figure 9: The distribution of sound field patterns of BGBSs under the same reference values for three case conditions (Unit: dB). \begin{table}
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Vibration Acceleration Distribution Characteristics of BGB Subjected to External Excitation under Three Working Conditions  
In order to reflect the overall vibration characteristics of the BGB subjected to external excitation at different frequencies under the three working conditions, the vibration acceleration distribution clouds of a BGB under external excitation are given in Figure 11.  
From the comparison of the vibration acceleration clouds of the BGB shown in Figure 11, it can be seen that when thickening the top plate of the BGB, although the structure will still be dominated by the local vibration of the top plate subjected to external excitation, the bending vibration of the top plate will change significantly, i.e., the bending vibration characteristics of the BGB will be changed. At the same time, the vibration of the rest of the plate and of the wing plate of the BGB will be strengthened to a certain extent, and the vibrational amplitude will be significantly weakened.