## 3 Results and Discussion
The first row (a) of Figure 6 shows that the fundamental tone with the blade passing frequency 119.4 Hz is strongly suppressed in all three configurations. In configuration 2, the highest SPL reduction is achieved around the two virtual error microphones. As before, the mean SPL reduction is maximised for the maximum number of virtual error sensors. A similar behavior is observed for the second (149.2 HZ) and the third (268.6 Hz) frequency in rows two (b) and three (c). However, the mean SPL reductions are approximately 10 dB less than for the fundamental tone. A mean SPL reduction at the fourth (388 Hz) and fifth (417.9 Hz) frequency in rows four (d) and five (e) is only observed in configuration 5. All configurations achieve local SPL reductions with SPL increases elsewhere. A local error sensing scheme like in configuration 2 leads to strong tonal SPL increases up to 40 dB locally and 22 dB globally as can be seen in subfigure (2e). It is noted for configuration 2 that the strong local SPL reduction around the two virtual error microphones (quiet zones), which are located in plane \(\alpha\), is less pronounced because the SPL reduction is averaged over both planes. From the results shown in Figures 5 and 6, it is concluded that a local error sensing scheme is generally undesirable (apart from special localized application scenarios) because it leads to strong spatial variations of the SPL. Furthermore, as can be seen in Figure 7, the vibration levels of the lining structure increase for a local error sensing scheme. Figure 5: SPL distribution in front of the lining evaluated on two planes \(\alpha\) and \(\beta\). Four different virtual microphone configurations (2-5) are compared to the uncontrolled case (1). The maximum, minimum and mean SPL are provided for each plot. The locations of the virtual error microphones are indicated by a green dot. Figure 6: Tonal SPL reduction in front of the lining averaged over two planes. The row designation (**a**) to (**e**) refers to the five CROR frequencies. Three different virtual microphone configurations (2, 3 and 5) are evaluated relative to the uncontrolled case (1). The maximum, minimum and mean SPL reductions are provided for each plot. The locations of the virtual error microphones are indicated by a green dot. Table 1 lists the mean and the maximum reductions of the SPL for the investigated configurations. The assignment of the microphone numbers from column two of Table 1 to the microphone positions is possible with Figure 3. The two actuators indicated in Figure 2 are used in all configurations. The SPL values are calculated from the root mean square (rms) values of the virtual microphone signals. The mean/maximum SPL reduction equals the mean/maximum value of the difference between the SPL values of the uncontrolled and the controlled case. Figure 7 shows the vibration level distribution on the lining surface calculated from the signals of the sixteen accelerometers shown in Figure 2. It is noted that the vibration level of the window panes is not visualized correctly because the signals of the two corresponding sensors are not evaluated. However, it can be assumed that the vibration levels of the window panes are hardly changed due to the weak feedback of the actuators to the panes. A significant increase of the vibration levels is observed for both active control system configurations. The local error sensing scheme of configuration 2 leads to an increase in the mean vibration level by 26 dB and the global error sensing scheme of configuration 5 increases the mean vibration level by 11 dB. It is concluded that both configurations act as so-called active noise vibration control (ANVC) systems. In an ANVC system, the structure acts as a loudspeaker to produce anti-sound at the error sensor locations. The radiated sound power is not necessarily reduced in such a system which could be seen as a drawback.