# Combination of Robust Algorithm and Head-Tracking for a Feedforward Active Headrest
## 4 Experiments and Noise Control Performance  
Experiments were conducted in a semi-anechoic room. As shown in Figure 3, a dummy head with torso was placed on the model of a sleeper on a high-speed railway. The sampling frequency of the noise control system was 16 kHz. The primary sound source was placed approximately 2 m from the ears of the dummy head. Measured noise on the high-speed railway denoted that the main frequency component of the broadband noise in the cabin was below 700 Hz. Therefore band-limited noise (0-700 Hz) was used as the primary noise. Microphones at the eardrum of the dummy head were used as error microphones to record the primary noise and the residual noise. To demonstrate the capacity of the noise reduction in the active headrest with the head-tracking system whenever the head moved and rotated, we assumed that an ideal reference signal could be obtained. The reference signal could be obtained directly from the signal to the primary loudspeaker. In practice, there is no such reference signal. The selection of the reference signal will be discussed in detail in future work. The head-tracking algorithm was conducted on a single chip microcomputer, and the noise control algorithm was conducted on a digital signal processor (DSP) development board. The region number of the location of the head center was sent to the DSP development board through the USB serial port from the head-tracking system, and, thus, the noise control system retrieved the precalculated filter coefficient at the current region. After the filter coefficients at 25 regions were calculated, the proposed active headrest could work. Error microphones were no longer needed. Figure 3: Photograph of experimental setup. The noise control performance of the active headrest system with the robust algorithm was compared to the noise control performance with the traditional system. The dummy head was placed at the center of region No. 13 with no rotation. Figure 4 shows the noise control performance of with and without robust algorithm. Table 1 shows the noise reduction with traditional and robust algorithm when the head was at the center of No. 13. The sound pressure at the left and right ears level was reduced by 16.7 dB and 17.0 dB without algorithm, respectively. Reduction of 11.8 dB and 13.4 dB was achieved with robust algorithm at two ears. Because more error positions are considered in the robust algorithm, the noise reduction with robust algorithm when the head had no rotation was lower than the the noise reduction with the traditional algorithm. However, the noise reduction remained 10-15 dB when the head was rotated \(30^{\circ}\), \(45^{\circ}\), and even \(90^{\circ}\) leftwards or rightwards. Furthermore, experiments were conducted to show the noise reduction performance when the head moved to different positions with the robust algorithm. When the head-tracking system was not switched on, the filter coefficient precalculated at region No. 13 was retrieved. Figure 5 shows the noise control performance of the active headrest at the left ear with the robust algorithm but without the headtracking system. The red dotted lines indicate the boundaries of each region, which are consistent with the red lines in Figure 2. The numbers in Figure 5 are the region numbers. The head was placed \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Noise Reduction (dB)**} & \multicolumn{2}{c}{**With Traditional Algorithm**} & \multicolumn{2}{c}{**With Robust Algorithm**} \\ \cline{2-5}  & **Left Ear** & **Right Ear** & **Left Ear** & **Right Ear** \\ \hline no rotation & 16.4 & 17.0 & 11.8 & 13.4 \\ \hline leftward & 30\({}^{\circ}\) & 15.7 & 11.9 & 10.9 & 12.3 \\  & 60\({}^{\circ}\) & 14.4 & 8.1 & 11.6 & 11.4 \\  & 90\({}^{\circ}\) & 12.9 & 6.3 & 12.7 & 11.0 \\ \hline rightward & 30\({}^{\circ}\) & 9.6 & 14.8 & 11.3 & 13.5 \\  & 60\({}^{\circ}\) & 7.2 & 14.0 & 10.8 & 13.7 \\  & 90\({}^{\circ}\) & 6.0 & 13.4 & 10.6 & 14.1 \\ \hline \hline \end{tabular}
\end{table}