## 2 Experimental Analysis
This experiment involved measuring the noise and vibrations while driving on a standard rough road at a constant speed. Measurements of booming noise inside the vehicle cabin during these tests can be potentially influenced by road disturbances or wind. To mitigate these effects, our noise measurements were carried out while driving the vehicle on a standardized rough road surface with all windows closed. More information on the impact of road quality and wind factors on vehicle noise and vibration can be found in Refs. [29; 30; 31; 32]. Microphones were used to measure the noise inside the vehicle. Accelerometers were attached to the exterior of the tailgate to capture vibration signals. By comparing these signals, the correlation between the interior vehicle noise during road driving and the tailgate vibration signals was determined. The test conditions for analyzing booming noise and tailgate behavior based on the measured noise and vibration signals were as follows. The experimental vehicle was driven at a constant speed of 40 km/h for approximately 22 s on a test track designed to sustain continuous impact forces within the broadband range, following the standardized test methods of the automotive manufacturer. To capture the interior noise, microphones (378B02) from PCB Inc (Depew, NY, USA). were installed at the height of the headrest in the middle of the front and rear seats inside the vehicle, as shown in Figure 3. To understand the behavior of the tailgate, accelerometers were attached to 12 points marked with red dots on the tailgate, as shown in Figure 4, to acquire vibration signals. The accelerometers used in the experiment were three-axis accelerometers (356A15) from PCB Piezotronics, Inc., capable of measuring vibrations from 1.4 to 6.5 kHz. Noise and vibration signals were collected using a spectrum analyzer (LMS SCADAS Mobile SCM05) from SIEMENS Inc. (Munich, Germany), and the gathered data were analyzed using data analysis software (Simcenter Testlab 2019.1) to assess the results using diverse approaches for noise, vibration, and durability testing. Figure 2: Tailgate boundary conditions. The method used to identify the main components of the noise signal acquired during driving is as follows. The time response of the sound pressure level from the microphone was transformed into the frequency domain using the fast Fourier transform method. Subsequently, the noise spectrum within the 60 Hz range was obtained to confirm the major components. To analyze the changes in the frequency components over time, spectra were continuously acquired throughout the duration of the drive and analyzed for variations. Using this method, the noise signal measured at the central area of the rear seat over time represents the frequency components of the noise, as shown in Figure 5. The color map illustrates how the frequency components of noise (\(x\)-axis) change over the driving time (\(y\)-axis), indicating the intensity of the sound pressure (Pa) through colors. The higher the sound pressure, the redder the color, whereas lower sound pressure values are represented using shades of blue. The C-weighted sound pressure level (SPL) was adopted in this study. This weighting was applied because A-weighting, which is commonly used in noise measurements, tends to underestimate low-frequency sound pressure based on the human auditory response curve. In cases such as this one, where low-frequency noise is a crucial measurement target, using the C-weighted SPL, which emphasizes the low-frequency range, for analysis is more reasonable. By analyzing the characteristics of the noise signal following the method described earlier, it was observed that noise occurred in the frequency range of 20 to 50 Hz (Figure 5). Owing to the irregular impact forces acting on the rough road track, the noise level varied over time. However, because the measurements were performed while driving under constant-speed conditions, significant noise components appeared within specific frequency ranges and varied in their levels.