## 6 Active Control System Performance  
In this section, experimental evaluation of the investigated active control system is presented. The loudspeaker placed inside the active casing generates a tonal primary noise of frequency incremented by 5 Hz in the range from 1 Hz to 350 Hz. The considered frequency range includes the low frequencies where the speaker starts to transmit sound. To achieve the goal of global noise reduction, instantaneous square values of error signals are minimized by feedforward adaptive control system (introduced in the previous Section), controlling together 21 inertial actuators. The error signals are obtained by the error microphones (their arrangement is discussed in Section 3.3). The control performance is evaluated as noise reduction levels observed by the room microphones. Performance of the control system is tested in multiple configurations, including a range of distances from the corner \(d_{i}\) and different error microphones' arrangement (Setup 1 or Setup 2). For each frequency of the primary disturbance, a 60 s experiment was performed. In its initial 5 s, the active control was off, and variance of signals acquired by different sensors was estimated as the reference point. Then, the active control was turned on. When the adaptive control algorithm converged, the final 5 s of the experiment were used to estimate the variance of signals acquired by corresponding sensors. In Figures 14 and 15, frequency characteristics for the active control experiments are presented. Time plots of signals obtained by individual sensors are not shown in this paper, as such insight into the control system behavior has been previously presented, e.g., in [12]. In the previous experiments, the active casing was placed distant from the enclosure walls and several control approaches were examined and described, e.g., in [12; 14]. In the previous research, the maximum global noise reduction evaluated by mean noise reduction measured at all room microphones exceeded 15 dB. In this research, even higher performance has been achieved and the maximum global noise reduction level often exceeded even 25 dB, although these results rely on the specific circumstances of the experimental settings, which evolved over time. Hence, such direct comparison of noise reduction levels obtained for different settings is unjustified. However, that is not the aim of this research. The purpose of this paper is to evaluate the overall control performance of the active casing placed in a corner. Hence, the main point of investigation is the mean global noise reduction affected solely by the distance \(d_{i}\) and the error microphone arrangement (Setup 1 or Setup 2). The other experimental settings remained unchanged. Figure 14: The average variance of the room microphone signals for each of the primary noise tones in the range 1 Hz to 350 Hz with a step of 5 Hz, with and without active control, for both Setup 1 and Setup 2. In Figure 14, the mean variance estimated based on the room microphones measurements is given for different distances \(d_{i}\), ranging from 0.1 m to 0.8 m. For each distance \(d_{i}\) a plot is given, where black color represents primary noise without active control; red and blue colors represent residual noise. Figure 15: The averaged noise reduction for the noise sensed by the room microphones for each of the primary noise tones in the range 1 Hz to 350 Hz with a step of 5 Hz, for both Setup 1 and Setup 2.  
noise when active reduction is turned on with error microphones arranged in Setup 1 and Setup 2, respectively. To facilitate the comparison of control performance obtained for different configurations, in Figure 15 the noise reduction characteristics are also given, calculated as a difference between noise level without and with control (reduction for Setup 1 and Setup 2 is marked with red or blue color, respectively). It follows from the analysis of Figures 14 and 15 that the error microphone arrangement affects strongly the overall performance of the control system.