## 4 Experiment Results and Discussion
### Performance and Feasibility of the ESG in Suppressing Road Noise  
After determining the optimized number and locations of the reference sensors, the real-time ARNC experiment was carried out with the test car on a paved road at a constant speed of 60 kph. Among the six cases of the real-time ARNC experiments to demonstrate the feasibility of the ESG-cowl pair as a secondary actuator, the results of two different experiments, when the numbers of reference signals were 4 and 12, were compared with each other and analyzed, in which 4 reference signals indicate the proper least number obtained from all 6 real-time experiment cases. However, 12 reference signals specify the number of all reference signals, that is, the best control performance to suppress road noise can be acquired with this number. Thus, as plotted in Figure 9, the comparison of the two results shows the differences between \"the best (thick dotted-dashed lines)\" and \"a properly optimized (thick solid lines)\" performances in terms of the measured SPLs before and after controls at the driver's ear in the cabin of the test car. In each ARNC experiment, the convergence coefficients \(\alpha_{\ell}\) in Equation (7) were adjusted according to the amplitude of the corresponding reference signal for maintaining the stability of the control algorithm and achieving more road noise reductions. The weighting factor \(\beta_{\ell}\) was set the same value. Figure 9a shows the comparison of the measured results in terms of the A-weighted SPLs from \"before control\" (a thin dashed line), \"after control with the 4 reference signals\" (a thick solid line), and \"after control with the 12 reference signals\" (a thick dotted-dashed line) in the frequency domain. Figure 9b displays the comparison of their reduction levels after control compared to the level before control. As can be seen from Figure 9a, three dominant booming noises before ARNC were mainly measured in the frequency range of 100-250 Hz. Two of them were road booming noises at about 130 Hz and 175 Hz, and the remaining one at about 230 Hz was a tire cavity booming. As shown in Figure 9a,b, after ARNC using all 12 reference signals, the A-weighted SPLs of these three booming noises dropped by about 6.2, 9.0, and 9.8 dB, and the averaged reduction level of the three booming noises was about \(-8.3\) dB (refer to Figure 8). On  
Figure 9: Comparisons of the measured A-weighted SPLs before control (thin dashed lines), after controls with the 4 reference signals (thick solid lines) or the 12 reference signals (thick dotted-dashed lines), and their reductions with respect to the frequency. (**a**) A-weighted SPLs. (**b**) A-weighted SPL reductions. the other hand, when the four optimal reference signals were applied, reduction levels of about 5.8, 6.3, and 6.0 dB were achieved at the three booming noises, respectively, and their averaged reduction level was about 6.0 dB (refer to Figure 8). This comparison of the two experimental results explains that the more reference signals, the further the averaged reduction levels, and this is coincident with the graph by computer simulation in Figure 8. However, it is also notable in Figure 9b that the two reduction levels at the first booming (130 Hz) were not clearly distinguishable, and this does not support more reference signals always guaranteeing better performances in suppressing the road noise. Despite the fact that the performance with 4 reference signals was inferior to that with 12 reference signals, the achieved reduction levels at the three booming noises were almost 6 dB each in the real-time control experiment. Therefore, the ESG as a secondary actuator used in this study was able to excite the cowl panel to produce anti-sound to suppress the road noise in the cabin.