## 4 Experiment Results and Discussion
### Number of Reference Signals  
In the consideration of the purpose of using the ESG as the secondary actuator in this approach, computation cost is an important factor for implementation of the ARNC system. Hence, an intensive investigation based on computer simulation to optimize the proper least number of reference signals at the chosen locations was carried out. For the optimization, the control performances obtained from computer simulation using the proposed ARNC algorithm against all possible acceleration directions (\(x\), \(y\), and \(z\) axes) and number variations of the reference signals from 1 to 12 were compared and assessed. The actual causality condition and the actual electro-acoustic features including the property of the ESG-cowl pair measured from the primary and secondary paths in the test car while driving were considered in the simulations. The control performance was estimated by averaging the A-weighed SPL reductions of the three dominant resonances in the frequency range between 100 and 250 Hz. When the number was greater than 3, a condition that accounts for at least one reference signal from each wheel in the combination of the reference signals was applied in the computer simulation. Figure 7: Multiple coherence using the 12 measured reference acceleration signals and the acoustic signal measured at the driver’s ear. After an intensive computer simulation, the averaged reduction levels of road noise at the driver's ear position were estimated. Then, each best control performance in terms of the A-weighed SPL reduction in dB at each number (1-12) of the reference signals was acquired, and this is plotted (dashed line with triangles) in Figure 8.  
The plot for the computer simulation shows that the reduction levels increased gradually with more reference signals, as 3 dB with 1 reference signal and 12 dB with 12 reference signals. It is noted that there was a large increase of about 4 dB in reduction level when the reference signal number increased from 2 to 3. Then, only a 2 dB increase from 8 dB to 10 dB was achieved even though the number of reference signals was augmented to 10 from 3. Based on the computer simulation results, the relevant combinations of the reference signals for the actual real-time ARNC experiment were decided. Then, the actual real-time ARNC experiment was carried out with the test car at a speed of 60 kph. The experimental results using the suggested combinations of even numbers of reference signals are illustrated (solid line with circles) in Figure 8. Thus, there were six actual real-time ARNC experimental cases with different numbers of reference signals. The comparison of the two lines (solid line for experiment and dashed line for simulation) in Figure 8 shows that they had similar trends that the reduction levels increased as more reference signals were used. Similar to the simulation, the slope of the reduction levels against the number of reference signals by the experimental results became less steep after the number of the signals reached a certain number, in this case, six. Especially, it was remarkable that the averaged A-weighted SPL reduction of 6 dB was achieved with only four reference signals in the ARNC experiment. This reduction level with 4 signals was just 2.3 dB less than that with all 12 signals in the experiment. As a consequence, these four reference signals were considered the best cost-effective case for the proposed MR-SISO ARNC system. It is noted in this case that the four chosen reference signals were from each wheel and their acceleration directions were all lateral (\(y\)-axis). However, the reduction levels in the actual control experiment were worse by about 1-4 dB than those in the computer simulation, as displayed in Figure 8. This is because there are some real-world limitations that were not considered in the computer simulation such as variations and uncertainties in the primary and secondary paths against driving conditions on roads in the actual ARNC experiment using the ESG-cowl pair.