## 7 Conclusions  
The active casing approach has been proven in previous research of the authors to provide significant global noise reduction. However, the active casing was placed at a distance from the enclosure walls. In this paper, the active casing was located in a corner and such placement is intentionally used to facilitate the active control system operation. On the other hand, such arrangement is common for many applications. Extensive active control experiments were carried out and a thorough study on control system performance was presented, which is the main contribution of this paper. In the opinion of the authors, the presented research confirms the hypothesis that placing the active casing in a corner and appropriately rearranging the microphones can lead to both enhanced noise reduction levels and a wider frequency range of global noise reduction. The analysis of both primary and secondary paths was also given as an additional insight into the investigated plant. Figure 17: **(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. Observe, only active control with Setup 2, Setup 2 with the actuators on the left and the front panel disabled and Setup 2 with the actuators on the right and the back panel disable, \(d_{1}=0.1\) m.  
The following conclusions can be drawn from the presented results. When the active 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, supporting the error microphones there (change from the Setup 1 to Setup 2). Then, the distance between the error microphones becomes smaller and the frequency range, in which acoustic emission is properly observable, becomes greater [25]. This results in higher mean reduction levels and in a wider frequency band, where the global noise reduction is achieved. For better representation of this behavior, let us assume that the noise source is surrounded by a sphere, at which a limited number of error microphones are placed. The effectiveness of the global noise reduction is limited in the frequency range by the distance between the error microphones--it should be smaller than the wavelength of noise to avoid spatial aliasing and to provide a proper observability of the emitted noise. In the previous research, when the active casing was distant from the enclosure walls, the introduced sphere was actually limited to a hemisphere, as the casing was placed on the floor. However, when the active casing is placed in a corner, the hemisphere is further reduced to approximately one-eighth of the sphere. Therefore, the remaining spherical surface which has to be covered with error microphones is smaller, hence the same number of microphones can be more densely distributed. This leads to smaller distances between error microphones, facilitating the operation of the active control system. Besides this major conclusion, two more observations are worth emphasizing. Firstly, an error microphone placed in a narrow gap between the casing and a sound-reflecting obstacle may not only be useless, but it may even impede the adaptation of the control system. Secondly, if the cross couplings between casing panels are strong, placing the actuators at panels facing the corner walls is as beneficial as placing them at panels facing the interior of the enclosure. This may be important when actuators placement is decided, and the surface available for actuators can be limited due to the device occupying most of the space inside the casing. This research also shows that placing the casing at the corner does not limit the performance in terms of globally reducing noise generated by the enclosed source.