## 4 Experimental Results
### Experiment 1: Noise Reduction Experiment to Evaluate the Multichannel Feedforward ANC System  
We first conducted a noise reduction experiment under the conditions shown in Table 3 to evaluate the multichannel feedforward ANC system with multiple reference microphones. The step size parameter is set to 0.01 to converge the noise control filter because the background noise exists in the real environment. In this experiment, we used one to four reference microphones and the ANC system was started after 5 s. The average reduction (22) per second was used as the evaluation score. The experimental results are shown in Figures 10-12. Figure 10 shows the noise reduction performance of the single-channel ANC system. From Figure 10, the ANC system with a single reference microphone can achieve noise reduction when the primary path is longer than the reference path. Moreover, the noise reduction ability is greatest when the reference microphone used is the one closest to the noise. In this arrangement, reference microphone 1 is the closest to noise sources 1 and 2, reference microphone 2 is the closest to noise source 3, and reference microphone 3 is the closest to noise source 4. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Noise 1** & **Noise 2** & **Noise 3** & **Noise 4** \\ \hline Reference mic. 1 & 70.7 & 61.8 & 29.3 & 0.0 \\ Reference mic. 2 & 29.6 & 41.1 & 50.0 & 41.1 \\ Reference mic. 3 & \(-16.7\) & 0.0 & 29.3 & 61.8 \\ Reference mic. 4 & \(-64.7\) & \(-46.3\) & \(-11.8\) & 41.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Differences in distance between primary path and reference path [cm]. \begin{table}
\begin{tabular}{l l} \hline \hline Input signal & White noise \\ Sampling frequency & 8000 Hz \\ Cutoff frequency of analogue low-pass filter (LPF) & 2000 Hz \\ Tap length of secondary path model \(L\) & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Identification conditions of the secondary path. Figure 9: Impulse response and frequency response of the secondary path. Figure 11 shows the noise reduction performance of the case (2, 1, 1) multichannel ANC system with two reference microphones: the most effective reference microphone in Figure 10 and another one. From Figure 11, the multichannel ANC system can reduce the unwanted noise more effectively the single-channel ANC system. Moreover, the multichannel ANC system with the nearest and next-nearest reference microphones shows the best noise reduction performance and highest convergence speed. Figure 12 shows the noise reduction performance of the case (3, 1, 1) and (4, 1, 1) multichannel ANC systems with three or four reference microphones. The case (3, 1, 1) system uses the two most effective reference microphones in Figure 11 and one other reference microphone. From Figure 12, the noise reduction ability of the case (3, 1, 1) multichannel ANC system is almost the same as that of the case (2, 1, 1) system. This is because some reference microphones do not satisfy the causality constraint, which affects the noise reduction performance and convergence speed. Therefore, the reference microphones that satisfy the causality constraint should be selected to obtain better noise reduction performance of the multichannel ANC system. Figure 11: Noise reduction performance of a two-channel feedforward ANC system in the experiment. Figure 12: Noise reduction performance of three- or four-channel feedforward ANC system in experiment. Experiment 2: Detection of Change in Noise Environment and Selection of Reference Microphones in the Proposed ANC System  
Next, we conducted a numerical experiment to evaluate the noise reduction ability and the selection of the optimal reference microphones of the proposed ANC system in the case that the noise environment changes.