## 6 Active Control System Performance
The major conclusion is that when the active casing is distant from the corner, the Setup 1 with error microphones surrounding the casing performs better than the Setup 2, where error microphones are moved away from the corner. On the other hand, when the active casing is placed close to the walls in the corner (\(d_{1}\) = 0.1 m), it is the other way around--Setup 2 provides substantially better results. This leads to a conclusion that when the casing approaches the corner, there is a threshold distance when it is beneficial to move the error microphones out from the gap between the casing and the corner, and place them on the other side of the casing, complementing the error microphones there. For the studied setup the threshold distance seems to be \(d_{4}\) = 0.4 m, although, the general conclusion is to expect such threshold value. Moreover, it is worth mentioning that when the active casing is placed close to the walls in the corner (\(d_{1}\) = 0.1 m), the error microphones placed in the gap at locations R and B in Setup 1 actually impede the correct adaptation of the control system. Even unplugging them, leaving only three error microphones operating, improves the control performance, as shown in Figure 16 (the specific configuration with disabled R and B error microphones is marked with green color). It is important to keep this conclusion in mind, when an obstacle with reflecting surface is in the proximity of an active casing, as such location between the casing and the obstacle may be disadvantageous for error microphones. Another conclusion follows from a comparison of control performance for the casing placed at the distance \(d_{1}\) = 0.1 m in Setup 2, and placed at the distance \(d_{8}\) = 0.8 m in Setup 1. Both of these two opposite scenarios provide very satisfying performance, reaching very high mean reduction levels. However, the reduction levels observed for the casing placed in the corner are higher and are over a wider frequency band. Although the difference is not high, it is clearly visible and leads to a conclusion that placing the active casing in a corner can be beneficial compared to being placed at locations distant from enclosure walls. At least, such configuration, if technically feasible, should provide results of the same quality. Moreover, it follows from the research that such configuration, which for many  
Figure 16: (**a**) The average variance of the room microphone signals for each of the primary noise tones with and without active control and (**b**) the averaged noise reduction for the noise sensed by the room microphones for each of the primary noise tones. For Setup 1, Setup 2 and Setup 1 with the error microphones B and R disabled, \(d_{1}\) = 0.1 m.  
applications is more realistic, is not less efficient. These conclusions are some of the major contributions of this paper, as such configuration of the active casing has not yet been evaluated. In addition, one more test has been done to check how much the actuators mounted to the casing panels facing the corner contribute to the overall noise reduction. Hence, in addition to secondary paths investigation, a control experiment has been performed with specific actuators disabled. Firstly, the actuators mounted to the left and front panels were disabled. Then, the actuators mounted to the right and back panels were disabled. Obtained results compared with results for all actuators operating are shown in Figure 17. What is interesting is that no significant difference can be noticed between disabling front and left panels, or back and right panels (the same number of actuators were operating). This proves that the actuators actually excite the whole casing with such efficiency that even placing them at panels facing the corner is equally beneficial as placing them at panels facing the enclosure interior. This can be explained by very strong coupling between casing panels, which was studied in detail in [12].