## 4 Experimental Results  
We conducted an experiment in a soundproof room (width: 3.1 m, depth: 2.9 m, height: 2.1 m) to evaluate the effectiveness of the proposed method.  
In this experiment, we assumed a similar noise environment to that inside a factory or an incubator in a hospital, in which the noise sources have various locations and the unwanted noises are generated with different timings. In a factory, there are various industrial machines around the workers. We focus on a situation in which the workers sit in front of a production line and the ANC system reduces the unwanted noise around the workers' ears. On the other hand, a baby inside an incubator does not move significantly or quickly. There are many medical devices around the incubator that generate  
Figure 5: Flow chart of the reference microphone selector.  
unwanted noise with different timings. In this scenario, the user's ears as the noise control point do not move significantly or quickly. Hence, the movement of the head does not affect the TDOA in this experiment. Moreover, in these environments, the direction of the noise does not frequently change. Therefore, we focused on only gradual changes in the noise environment.  
The average sound pressure level of the unwanted noise was about 80 dB at the error microphone. We implemented the ANC system as shown in Figure 6. In this experiment, we used a head-mounted ANC system [18] that measured the primary and secondary paths for the multichannel ANC arrangement shown in Figure 7. The pictures of the measurement arrangements are shown in Figure 8. The secondary loudspeaker used for the head-mounted ANC system is the headphone (PFR-V1, Sony, Tokyo, Japan). Unwanted noises are emitted from the loudspeakers (Eclipse TD-510, Fujitsu-ten, Hyogo, Japan). The reference microphones and error microphone are the electret condenser microphones (AT9904, Audio-Technica, Machida, Tokyo, Japan). Analogue low pass filters are implemented on MSPAMP800 (Hiratsuka Enginieering, Kanagawa, Japan) shown in Figure 6. In Figure 7, the error microphone on the head-mounted ANC does not move significantly or quickly as a result of the head movement. In other words, the head movement does not affect the TDOA in this arrangement. In this arrangement, the differences in the distance between the primary path and the reference path are shown in Table 1. In Table 1, a positive value indicates that the primary path is longer than the reference path (the reference microphone is closer to the noise source than the error microphone) and the causality constraint is satisfied. On the other hand, a negative value indicates the opposite case (the error microphone is nearer the noise source than the reference microphone) and the causality constraint is not satisfied. The secondary path model was identified by NLMS under the conditions shown in Table 2. Here, Figure 9a shows the identified impulse response of the secondary loudspeaker, Figure 9b,\(c\) represent the amplitude-frequency response and phase-frequency response of the secondary loudspeaker. Figure 9d shows the group delay which represents the time delay at each frequency. From Figure 9d, the response delay \(D_{\text{S}}\) is confirmed to know that the causality constraint is satisfied or not in the experimental arrangement. In the following experiments, the step size parameter and regularization parameter were determined via preliminary experiments to obtain the highest noise reduction performance under each condition.  
Figure 6: Implementation of the system.  
Figure 8: Pictures of the experimental arrangement.  
Figure 7: Arrangement of equipment in the experiment.