## 2 Experimental Analysis
At 28.4 Hz, a rigid body movement of the tailgate in the front-to-back direction was observed, along with significant deformation and movement at the central part of the tailgate when compared with other points. These movements can induce a booming noise through the expansion and contraction of air inside the vehicle compartment. These experimental results confirm that the noise occurring at approximately 25 and 28 Hz is manifested by the behavior of the tailgate. Conversely, significant acceleration elements were detected at the edges of the tailgate, specifically at 38.6 Hz. ODS analysis of the tailgate in this frequency revealed that movements in both longitudinal directions were seen in certain areas of the tailgate. However, the tailgate exhibited an overall twisting motion, with phases alternating laterally and vertically. Although local expansions and contractions were observed in some regions of the tailgate, their contribution to the generation of booming noise is expected to be negligible. Additionally, modal tests were conducted on a tailgate in its actual installed state to understand the relationship between the behavior of the tailgate during driving tests and its modes. Mode shapes represent the unique configurations of a system in the absence of external forces. Modal experiments were carried out within the 100 Hz range, where tailgate behavior was observed, as indicated by well-known modal analysis procedures. Accelerometers were attached to the glass of the tailgate, and at 50 specific points marked by red dots (Figure 8), impact hammers were applied sequentially to obtain the frequency response functions (FRFs) from the accelerometer output signals in response to the hammer input signals. This process allows the extraction of natural frequencies and mode shapes. Figure 1:1 Operational deflection shapes of the tailgate at different noise frequencies: (**a**) 24.0 Hz; (**b**) 28.4 Hz; and (**c**) 38.6 Hz. These experiments utilized PCB's impact hammer (Model 086D20). The test results obtained for the tailgate through these modal experiments are illustrated in Figure 1:2  
In summarizing the results of the tailgate modal tests, the following observations were made: The tailgate exhibits its primary mode at 23.7 Hz, characterized by a predominant back-and-forth translational motion of the upper part of the tailgate, with some bending deformations evident in the panels. In contrast, at the second mode, 28.6 Hz, the tailgate predominantly exhibited bending deformations, particularly at its central lower part, accompanied by some translational motion of the entire tailgate in the same direction. Conversely, the third mode at 37.6 Hz indicates a global twisting deformation of the tailgate in the fore-aft direction. Combining the findings from the modal experiments, it can be concluded that the predominant translational motion of the entire tailgate and the bending deformations of the tailgate panels at 23.7 and 28.6 Hz could significantly impact the interior cabin noise. However, the twisting mode observed at 37.6 Hz is expected to have a comparatively minor effect on the interior noise of the vehicle. The influence of the separable boundary conditions between the tailgate and the vehicle was also experimentally analyzed. As shown in Table 1, this experimental setup was configured based on the presence or absence of removable boundary components, including guide bumpers, gas lifts, spindles, over-slam bumpers, and weather strips. Case 1 represented the baseline condition, with all connecting structures intact. Cases 2-5 involved the measurement of the natural frequencies of the tailgate after removing one component each from the initial setup, namely, the guide bumpers, gas lifts, spindles, over-slam bumpers, or weather strips. A comparison of the changes in dynamic characteristics when each boundary condition was removed from the initial state provided us with significant results.