## 2 Experimental Analysis
By comparing the natural frequency variations between Cases 1 and 2, where the guide bumpers were removed, a significant decrease in the first to third natural frequencies was observed. This result indicated a substantial impact of the guide bumpers on the dynamic characteristics, confirming their dominant influence. Conversely, in the experimental results of Cases 3-5, the changes were negligible compared to the baseline Case 1, suggesting their limited influence. In conclusion, the experimental analysis demonstrated that, among the various connecting structures between the tailgate and the vehicle, guide bumpers had the most significant impact on the dynamic characteristics of the tailgate. Figure 1:3 Impact and measurement points for the experimental modal tests. Figure 1:4 Mode shapes of the tailgate at the natural frequencies: (**a**) 23.7 Hz; (**b**) 28.6 Hz; and (**c**) 37.6 Hz. By analyzing the results from the noise, vibration, and tailgate modal tests conducted to date, the origins of the booming noise within the vehicle while driving can be explained as follows. Through driving tests, significant noise frequencies originating from the rear-seat area were identified as being 25, 28, and 37 Hz, with 28 Hz being the dominant component. Furthermore, when examining the vibration signals of the tailgate measured simultaneously with the noise signals during the driving tests, clear vibration components at 25 and 28 Hz were evident in the central part, while a component at 37 Hz appeared on the left and right sides. Comparing these observed tailgate behavioral characteristics obtained through the ODS analysis with the results of the modal tests yielded meaningful conclusions. Upon examining the primary frequencies identified in the ODSs of the tailgate to analyze the sources of the booming noise, several observations can be deduced. We found that booming noise frequencies correspond to the natural frequencies of the tailgate. When examining the ODSs, the behavior of the tailgate similar to the natural modes was observed at 24.0 and 28.4 Hz, where the tailgate exhibited significant back-and-forth movements. Additionally, at 38.6 Hz, the entire edge of the tailgate twisted, indicating that the twisting mode of the tailgate was expressed as the actual movement through it. When comparing the noise and vibration frequencies, similarities exist at 25, 28, and 37 Hz. Therefore, it can be assumed that tailgate behavior influences booming noise generation dominantly in both frequencies. However, to clearly elucidate the mechanism of booming noise generation, not only the similarity in natural frequencies but also the tailgate behavior must be considered. The behavior of the tailgate, characterized by the complex deformation of the panel in both the front and rear directions at 23.7 and 28.4 Hz, is closely related to the expansion and contraction of the vehicle's interior air space, thus having a significant connection with booming noise. However, at 38.6 Hz, the tailgate's behavior involves overall twisting, which has minimal potential to expand or contract the vehicle's interior, making it less likely to contribute significantly to noise generation. Therefore, it is reasonable to conclude that the occurrence of similarities in the frequencies and booming noise is independent in this frequency.