# Some Practical Acoustic Design and Typical Control Strategies for Multichannel Active Noise Control
## 2 Practical Acoustic Design
### ANC in Open Areas around Head  
In a lot of applications, the multichannel ANC system either has to generate a quiet zone surrounding a human's head [14,15] or prevent the noise radiation from a compact noise source [16]. A well-designed 16-channel cylindrical ANC system has been proposed to generate a \"quiet zone\" within an area as large as a human head [17,18]. The diffraction of the head and locations of the error microphones and loudspeakers affect the performance of such systems significantly. In [17], the effect of a diffracting sphere on this system was studied. As shown in Figure 3(a) (cited from [PERSON]'s 2008 paper [17]), experiments were carried out in an irregular room of about 80 m\({}^{3}\). The primary noise sources were located in three different directions and generated by the same amplified pure-tone signal. Sixteen speakers and sixteen microphones, distributed on two layers, were used as control sources and error sensors, respectively. The filtered-x algorithm was used to generate the anti-sound of the 16-channel ANC system. To study the effect of the human head on the control performance,  
Figure 1: Experimental setup in [PERSON]’s 2017 paper [13]: (a) a 3-channel ANC system setup with a wooden plate as the reflecting surface; (b) a 3-channel ANC system setup in free field; (c) a configuration of a 2-channel ANC system; (d) a configuration of a 3-channel ANC system.  
Figure 2: Optimal noise reduction with and without the wooden plate (a) measured results (b) simulation results. Figures are cited from [PERSON]’s 2017 paper [13].  
a hollow iron sphere of a radius about 0.09 m was used to approximate the human head. The experimental results show that the NR can be improved by the diffracting sphere, especially in higher frequency as shown in Figure 3b.  
In a similar experiment reported in [18] where the diffracting sphere is not considered, it was shown that a larger radius of error sensors (a\({}_{\text{e}}\)) may cause noise reduction to decrease, as shown in Figure 4a (cited from [PERSON]'s 2007 paper [18]). It also reported that the control performance improves rapidly with an increase in the radius of controllers (a\({}_{\text{c}}\)) if a\({}_{\text{c}}\) is close to a\({}_{\text{e}}\), indicating that secondary sources should not be too close to error sensors as shown in Figure 4b from [18]. As explained in [18], the main reason is that when the loudspeaker is close to one of the error sensors, the contribution of the anti-sound is dominated by the closest loudspeaker only. In this way, a global optimization is hardly achieved. As the distance increases to a certain value, the control sources affect the entire cylindrical surface and the noise control is more efficient.  
Figure 4: Control performance (**a**) with respect to a\({}_{\text{e}}\), \(f\) = 250 Hz, a\({}_{\text{c}}\) = 1.22 m; (**b**) with respect to a\({}_{\text{c}}\)–a\({}_{\text{e}}\), \(f\) = 250 Hz, a\({}_{\text{e}}\) = 0.38 m. Figures are cited from [PERSON]’s 2007 paper [18].  
Figure 3: (**a**) Experimental setup; (**b**) experimental result of the control performance with or without a sphere. Figures are cited from [PERSON]’s 2008 paper [17].