Depending on the contact load, the amount of noise transmitted from the glass and the lower area of the lip can be determined. To estimate the minimum contact load (CLDmin), a few CAE methods have been studied and proposed (Saf et al., 2020). However, since these CAE methods produce inconsistent results, it is more reasonable to measure the proximity sound directly using a pin microphone according to the varied contact load with the aging condition of the inner belt weatherstrip through the wind tunnel. The test conditions for measuring noise were a wind speed of 110 kph and 11.3° yaw angle of the vehicle; the pin microphone was installed at a distance of 100 mm, and the parts that are not necessary to be measured were taped down to minimize the influx of noise. The measured noise was subjected to the fast Fourier transform (FFT) and converted to 1/3 octave band from 10 Hz to 10 KHz.  
The noise measurement results for the lip load are presented in Figure 3. At a lip load higher than 0.96 N/100 mm, there was no change in noise, whereas for values less than 0.96 N/100 mm, the performance dropped sharply in the high frequency area. The contact load decreases with time according to the stress relaxation characteristic represented by the Maxwell model of viscoelastic materials. Therefore, the initial contact load (CLDi) should be designed according to the minimum contact load maintained even after permanent deformation, as shown in Equation (6) and (7), where CLDa is the contact load after permanent deformation.