## 2 Experimental Analysis
The main noise components were identified at 28 and 37 Hz. Comparing the SPL intensities, it was evident that the noise at 28 Hz dominated over 37 Hz. In addition, at approximately 25 Hz, the SPL was relatively low; however, the noise persisted consistently over specific periods of time. Summarizing these results, the frequencies where noise occurred are 25, 28, and 37 Hz.Below, the behaviors of the tailgate related to these frequencies are analyzed. Figure 4: Measurement points for vibration signals. Figure 3: Microphone positions (red dots) for measuring booming noise. The main components of the tailgate's vibration signal were identified using a method similar to that used to analyze the noise signal. When representing the frequency components of acceleration over time from the vibration signals measured on the left, center, and right panels beneath the tailgate glass simultaneously with the noise signals, the results appears as shown in Figure 6. Similar to the noise analysis results, it was observed that the vibration signal, which was influenced by the impact of the irregular road surface, varied in intensity over time. However, the identification of specific vibration components over time under constant-speed conditions allowed us to understand the major behavior of the tailgate. When examining the characteristics of the vibration signals based on measurement locations, it is evident that the 28 Hz component is prominent in the central part of the tailgate, and at approximately 25 Hz, the magnitude of the vibration signal is weaker than the 28 Hz component but persists consistently over a certain range. However, on the left and right sides, the 25 and 28 Hz components are almost absent, whereas the 37 and 43 Hz components are dominant. The ODS analysis method introduced next is utilized for a more systematic analysis. Figure 5: C-weighted sound pressure level at the rear passenger position. Figure 6: Time spectrogram graphs of the vibration signals: (**a**) left; (**b**) middle; and (**c**) right. The ODS analysis was employed to conduct a more in-depth analysis of the tailgate behavior in the main frequencies. This refers to the deformed shape during operation and serves as an analytical technique for visualizing the movements occurring in real systems. In this study, this method was utilized to visualize the tailgate behavior at specific frequencies during driving. It is commonly used to represent the real movements of a target either in the domain of time or at specific frequencies. Our analysis focused on studying the movements at particular frequencies using frequency spectra. To achieve this, auto-spectra and cross-spectra were acquired from the signals of both the reference accelerometer and the simultaneous measurement of the roving accelerometer. The movement of the roving location in relation to the reference position was visualized by integrating the magnitude information from the auto-spectra and the phase information from the cross-spectra. At each point where the motion data were desired, roving accelerometers were attached to obtain the signals. This study discerned how the actual system behaves by assigning the measured acceleration signals to the corresponding positions on the shape. Through this analytical method, the behavior of the tailgate in response to road-induced forces during driving, which lead to noise generation, was examined at specific frequencies. The behavior of the tailgate in the frequencies in which the booming noise occurred was analyzed using the ODS technique. The ODS results for the primary frequencies used to analyze the relationship between vibration and noise, based on the comparison of vibration signals depending on the noise signal components and the position of the tailgate, are shown in Figure 1:0 At 24.0 Hz, the tailgate exhibited an overall front-to-back movement, with the upper part dominating compared to the lower part.