## 6 Conclusions  
A study on an ANC headrest system mock-up was conducted to qualify the effectiveness of signal processing algorithms and the related implementation techniques for successful application onboard regional aircraft cabin seats. Although physical implementation constraints and economic considerations were not addressed in the present study, the proposed concept showed efficiency in attenuating low-frequency noise in a controlled environment that replicated the turboprop harmonics of a regional aircraft cabin. The ANC scheme uses two secondary actuators (e.g., loudspeakers) to generate a secondary sound field and two error sensors (e.g., microphones) to pick up residual noise, which serves as the error signal for updating the adaptive filters. A head mannequin equipped with two microphones was used to measure the noise reduction. The results showed that the ANC headrest system achieved sound level reduction of up to 19.92 dB and 17.36 dB at the mannequin's left and right ears, respectively, while the error microphones were placed at the headrest. These moderate but realistic achievements were obtained under free-field conditions (semi-anechoic conditions) by using a representative active seat arrangement. Sound intensity measurements showed an artificial comfort zone of about 0.35 \(\times\) 0.40 \(\times\) 0.5 m around the passenger's head. This volume could be considered large enough to include both the lateral and vertical head movements of a passenger, who would therefore be able to benefit from the active noise cancelling system without the need to wear an active headset. Our experimental activities fully demonstrated the technical feasibility of the proposed concept, opening the way for possible upscaling to TRL5 in a real cabin demonstrator. However, implementation considerations such as system complexity, physical constraints, and cost reduction may affect the system's performance in practical applications. Furthermore, ANC systems on adjacent seats may induce instabilities. Although the ANC system provides promising benefits in terms of overall noise reduction, these benefits cannot be realized without causing a significant fuselage weight increase due to the massive equipment, actuators, and sensors integrated into the cabin seats. A comparison with other methods used in experimental and some rare commercial systems will be made in future studies. Virtual sensing methods will also be investigated to reposition the point of cancellation and thus increase the upper frequency of control by using head tracking to monitor the passenger's head movement. Numerical optimizations are also envisaged to predict active noise control capabilities at the cabin level and compare them with the experimental data obtained within the project for passive noise control solutions. **Author Contributions:** Conceptualization, I.D. ; methodology, I.D. ; software, I.D. ; validation, I.D. and P.V. ; formal analysis, I.D. and C.C. ; investigation, I.D., C.C. and J.C.; resources, C.C. and J.C.; data curation, M.B. ; writing--original draft preparation, I.D. ; writing--review and editing, I.D. and J.C.; visualization, I.D. ; supervision, I.D. ; project administration, M.B. ; funding acquisition, M.B. All authors have read and agreed to the published version of the manuscript. **Funding:** The research leading to these results was developed within the framework of CASTLE (cabin systems design toward passenger wellbeing) Core Partnership of the Clean Sky 2 Program. CASTLE received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under the grant agreement no. CS2-AIR-GAM-2014-2015-01. The contents of this paper reflect only the authors' views, and the Clean Sky 2 Joint Undertaking and the European Commission are not liable for any use that may be made of the information contained therein. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable.