# Adjustable Structure for Feedback Active Headest System Using the Virtual Microphone Method
## 4 Experimental Setup  
Figure 5 shows the active headrest system and the overall schematic setup in a room. The reverberation time of this room is \(T_{60}=0.42\) s. Two microphones at the ears of a dummy head (G.R.A.S., KEMAR Head & Torso) were employed as target error microphones. The left-side and right-side support frames in the proposed headrest structure were initially positioned 3 cm away from the left and right ears of the dummy head. The primary disturbance signals originate from machine noise at different speeds. At the first speed, there were three dominant narrowband noise components at 168 Hz, 210 Hz and 252 Hz, and at the second speed, the noise components were at 171 Hz, 228 Hz and 285 Hz. There were two primary loudspeakers placed in front and to the left of the dummy head at a distance of 2.5 m.  
The controller was implemented on a digital signal processor (TI, TMS320C6678) platform with a sampling frequency of 8 kHz, the analog to digital converter was MAXIM MAX11049 with 16-bit resolution, and the digital to analog converter was TI DAC7644 with 16-bit resolution. Their conversion times were short enough to meet the low-latency requirements of the ANC.  
All algorithms in this study require modeling the secondary paths in advance. Using bandlimited white noise \(\leq\) 2 kHz, the secondary paths were modeled as FIR filters by an adaptive system identification method based on the NLMS algorithm [24]. Table 1 lists the common measurement conditions.  
To verify the necessity and superiority of the proposed headrest structure, two more forms of the headrest structure were compared. The first comparative structure typically incorporates secondary loudspeakers and error microphone fixed together at a certain distance from the left and right sides of the head. In the second type of comparative structure, the user can adapt the positions of the monitoring error microphones to be  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Tap length of the estimated secondary paths & 512 \\ Tap length of control filters \(L_{w}\) & 512 \\ Tap length of additional filters \(L_{\text{AF}}\) & 512 \\ Tap length of observation filters \(L_{\text{O}}\) & 512 \\ Step size \(\mu\), \(\mu_{\text{AE,T}}\), \(\mu_{\text{AF,C}}\) & 0.001, 0.01, 0.001 \\ Regularization factor \(\beta\), \(\beta_{\text{AE,T}}\), \(\beta_{\text{A,E,C}}\), \(\beta_{\text{O}}\) & \(10^{-6},10^{-6},10^{-6},10^{-8}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Common measurement conditions in all experiments.  
Figure 5: Overall setup of the proposed active headrest system with two primary loudspeakers. (**a**) Active headrest system with two secondary loudspeakers and a physical microphone on each side. (**b**) Schematic of experimental measurement configuration with two primary loudspeakers at \(0^{\circ}\) and \(90^{\circ}\) relative to the dummy head.  
close to the ears while maintaining the positions of the secondary sources fixed. The two comparative structures mentioned above are denoted as a fixed structure [4] and a movable microphone structure [22], respectively.