# Adjustable Structure for Feedback Active Headest System Using the Virtual Microphone Method
## 5 Real-Time Experimental Results and Discussion
### Experiment 1: Varying the Distances between the Secondary Loudspeakers and the Ears  
To justify the necessity of the proposed adjustable structure, a reasonable number of comparison experiments were performed using three VS methods by moving the support frame on each side of the headrest away from the ears horizontally to the left or right. All plant responses including secondary paths were remodeled after each movement. In this experiment, the fixed structure, as described in Section 4, was considered identical to the proposed adjustable structure because in these two structures, the secondary loudspeakers and error microphone were fixed together and moved with the secondary loudspeakers. For the movable microphone structure, as illustrated in Section 4, the only difference with the fixed structure was that the physical monitoring microphone was always placed close to the ear. The first machine noise was employed as the disturbance signal to drive the No.1 primary loudspeaker.  
Figure 6 shows the noise reduction at the left ear when the secondary loudspeakers were moved to various distances with different VS methods and headrest structures. The X-axis represents the distance that the left-hand side secondary loudspeakers were moved to the left, the Y-axis represents the distance that the right-hand side secondary loudspeakers were moved to the right, and the Z-axis indicates the attenuation of the total sound pressure level (SPL) at the left ear. As the noise reduction effectiveness was almost identical in both ears, only the attenuation results of the left ear are illustrated. It was clear that, as the secondary loudspeakers moved away from the ears, there was only a slight degradation in the performance for the AFM and RMM, regardless of the headrest structures. With the fixed structure/proposed adjustable structure, the performance of the VMM was inferior to that of the RMM by 5 dB attenuation when both sides of the secondary loudspeakers and the associated physical monitoring microphone were moved by no more than 2 cm, whereas a substantial performance degradation was observed when moved farther away. This was because of the enhanced difference in the primary disturbance at the physical microphone and the target microphone as the distance increased. Thus, the assumption \(d_{m}(n)=d_{v}(n)\) became increasingly incompatible. In contrast, the performance of the VMM in the movable microphone structure was slightly better than that of the fixed structure/proposed adjustable structure and was affected less by the secondary loudspeaker movement. This was due to the assumption \(d_{m}(n)=d_{v}(n)\) being essentially satisfied and almost unaffected in the movable microphone structure. However, in some cases, placing microphones near the ears was impractical and uncomfortable [6]. Hence, the user had to adjust the position of the support frame as a trade-off between the movable area and the attenuation performance when applying the proposed adjustable  
headrest structure. For the same distance between the secondary loudspeaker and the ear, the performance of the AFM and RMM was similar and superior to that of the VMM.