## 5 Conclusions  
The paper presented a chair equipped with active noise control. Noise-canceling chairs can be the competitors of the well-established active headphones, as their long-time wear is uncomfortable. Several attempts to build such a chair have been reported, often referred to as an active headrest system. Our literature survey has shown that the main development has been focused on the acquisition of the error signals, as the error microphones built into the headrest cannot result in sufficient suppression at the ears. Virtual and remote microphone technology are preferred in active headtests, while the controller is usually unable to cancel broadband noise.  
Our intention was to also suppress broadband noise; therefore, feedforward control was chosen. Preliminary experiments have shown that the chair should be surrounded by multiple reference microphones. Such an arrangement is prepared for the cancellation of the primary noise generated at different positions and results in greater suppression, compared to the single reference case. Simulations have proven that physical microphones at a comfortable distance from the ears can provide fairly high suppression.  
Then, the construction details of the noise-canceling chair were described. We have used two flexible gooseneck microphones fastened to the headrest. The microphones can be set by the user to the appropriate position. The two loudspeakers are controlled by the signals of the two error microphones and four reference signals. The controller is based on the normalized filtered error least mean squared algorithm, implemented on an Analog Devices ADSP-21262 signal processor-based hardware. The MIMO controller can manage four reference signals with an adaptive filter length of 2000 each, at a sampling frequency of 8 kHz. The application of the ELMS algorithm is justified by its faster convergence, less computational demand and memory usage, compared to the XLMS or MLMS algorithms. In the last section, experimental results were reported that showed the efficient suppression of tonal, as well as broadband disturbances in a moderately damped room.  
The development of the noise-canceling chair is not complete, but the results are promising. Application of the remote microphone technique besides the proposed multiple reference control and the utilization of wireless reference microphones are subjects of further research.  
Figure 23: Power spectral density of broadband noise at the measurement point. Blue line: control off. Red line: control on.  
**Author Contributions:** Conceptualization, L.S.; Methodology, L.S.; Software, A.S.; Validation, L.S., and A.S.; Investigation, L.S.; Supervision, L.S.; Writing-original draft, L.S., and A.S.  
**Funding:** The research was co-sponsored by the EU and Hungary under the project GOP-1.3.1-11/C-2012-0054 and carried out by Euronet Inc.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
AD Analog to digital ANC Active noise control DA Digital to analog ELMS Filtered error least mean squared FFT Fast Fourier transform JTAG Joint Test Action Group LMS Least mean squared MIMO Multiple input-multiple output MLMS Multiple error least mean squared PSD Power spectral density RMT Remote microphone technique XLMS Filtered reference least mean squared VMT Virtual microphone technique