## 4 Experimental Results  
The chair has been tested in various environments, and the measurement method was similar to that described in Section 2. This section presents the results of additional experiments carried out in a room.  
The test environment was a 5 by 4.3 by 2.8 m room furnished like an office, exhibiting significant reverberation. This room was also located in a multi-story building and had a concrete floor and ceiling and brick wall. Along the longer side, it had a wall-to-wall glass window, on the opposite side a door and a built-in wardrobe made of solid wood. The floor plan with the measurement arrangement can be seen in Figure 17. The area surrounding the chair is depicted in Figure 18. The figure shows the exact positions of the reference and the error microphones. The suppression of the sound had been measured by a dummy head fastened to the headrest, which is also drafted in the figure by a rectangle. The symbols are the same as listed in Figure 2.  
The measurement equipment can be seen in Figure 19. The main components were already described in the previous section. Three reference microphones have been placed on tables, and the overhead one has been placed on a stand. In addition to the chair and the controller, there is a dummy head and a sound level meter. Note that the probe is detachable and has been placed inside the dummy head. A Cakewalk UA-101 USB audio interface has been used to generate the primary noise and simultaneously record the residual noise with a sampling frequency of 44.1 kHz. The suppression has been measured by the sound level meter and by evaluating the records in MATLAB.  
Figure 16: Identification of the secondary path for the ELMS algorithm. Illustration for the two by two case.  
Figure 19: Control equipment for the test experiments.  
Figure 17: Measurement arrangement in the office.  
Figure 18: Detailed map of the neighborhood of the chair.  
The spectra were also calculated, which enabled the evaluation of the control in the frequency domain. The power spectral density (PSD) has been calculated by the fast Fourier transform (FFT) using the Welch method [17]. We used 2048 wide Hamming windows in order to suppress the phenomenon of picket fence and leakage. Five second-long periods have been selected from the records to calculate the spectra for the 'ANC off' and the 'ANC on' state, respectively.  
The measurement results are summarized in Table 3. Although only one setting for the location of the primary source and the reference microphones is introduced here, these parameters are also included in the table. Thus, the test results can be easily compared to those of the preliminary experiments presented in Table 2.  
The suppression of the tonal components of 200, 500 and 800 Hz was excellent, as expected. The appropriate spectra can be seen in Figures 20-22, respectively.  
The suppression of the broadband random noise with a bandwidth of \(B=2000\) Hz was also successful. The spectra can be seen in Figure 23. The controller was able to cancel the noise even above 1 kHz, which is a promising result. Unfortunately, at low frequencies, below about 300 Hz, there was no suppression. This is because of the non-linear amplifiers that deteriorated the correlation between the error signal and the actual sound pressure at the microphones. This phenomenon is in accordance with the moderate suppression of a tonal sound of 200 Hz introduced above.  
Figure 21: Power spectral density of a tonal sound of 500 Hz at the measurement point. Blue line: control off. Red line: control on.  
Figure 22: Power spectral density of a tonal sound of 800 Hz at the measurement point. Blue line: control off. Red line: control on.