# Adjustable Structure for Feedback Active Headest System Using the Virtual Microphone Method
## 6 Conclusions  
In this paper, a multichannel feedback active headrest system combined with the VMM and a manually adjustable headrest structure for users was presented. The VMM transfers the noise reduction target from the physical monitoring microphone to the ear and avoids placing the microphone at the target during the control stage. The proposed headrest structure was developed to allow the secondary loudspeakers and the corresponding error microphone to be flexibly moved as a single unit by the users. This arrangement ensured that the relative position between each unit and the ear was constant, and thus maintained the stability of the secondary paths and other related plant responses. A series of experiments were conducted to validate the noise attenuation performance of the proposed system. The results confirm the necessity and superiority of the proposed headrest structure compared to the conventional fixed structure and the movable microphone structure. With the proposed adjustable structure utilized, the AFM and RMM achieved the same noise reduction performance level, and they outperformed the VMM when the primary source was stable. When encountering the varying primary source (including the characteristics, position, and quantity) and the practical issue where it was not feasible to re-measure any transfer function, the effectiveness of the AFM and RMM decreased while the performance of the VMM remained stable as a trade-off. This was because, when applying the AFM and RMM, the additional filters and observation filters that need to be trained in advance were dependent on the primary source. Hence, it is necessary to select an appropriate VS method according to the details of the application. Moreover, the comparative results of the trials involving 10 volunteers of different statures validated that the proposed active headrest system is robust and applicable to users of varying statures. This research will contribute to the design of active headrest for people working around engines, where primary sources are dominantly multi-sinusoidal and continuously changing.  
In this study, all disturbance signals were considered for narrowband noise only. In the future, the attenuation performance of the proposed system against broadband interference will be explored.  
Conceptualization, Z.Z. and M.W.; methodology, Z.Z.; software, Z.Z.; validation, M.W., C.G., L.Y. and J.Y.; formal analysis, Z.Z.; investigation, M.W.; resources, J.Y.; data curation, Z.Z., C.G. and L.Y.; writing--original draft preparation, Z.Z. and M.W.; writing--review and editing, Z.Z., M.W., C.G., L.Y. and J.Y.; visualization, Z.Z.; supervision, J.Y.; project administration, M.W. and J.Y.; funding acquisition, M.W. and J.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (grant No. 11804365).  
The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Review Board of the Key Laboratory of Noise and Vibration Research, Institute of Acoustics, Chinese Academy of Sciences (NV2021-0502).  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.