## 2 Methods  
### Experimental Setup  
In Figure 1, a scheme of the laboratory setup in the sound transmission loss facility of the German Aerospace Center (DLR) is shown. The loudspeaker array (LSA) which is used for the disturbance excitation is positioned in front of the fuselage structure having an offset of approximately 0.15 m. Of the 112 loudspeakers of the LSA, 72 are used. The remaining loudspeakers are not used because they are beyond the fuselage panel surface. The 72 loudspeakers are arranged in an array of nine rows and eight columns. Two different acoustic load cases are considered. One acoustic load case is typical for a counter rotating open rotor (CROR) engine. It is characterized by multiple tones with frequencies 119.4 Hz, 149.2 HZ, 268.6 Hz, 388 Hz and 417.9 Hz [12]. The bandwidth is limited to the first five harmonics because these tones are most critical for the interior noise. The second acoustic load case is realized by driving the 72 loudspeakers with uncorrelated bandlimited white noise signals. The resulting broadband sound field is intended to roughly approximate the characteristics of a turbulent boundary layer. Both load cases are relevant for propeller-driven aircraft, but the tonal parts dominate. This can be attributed to the higher sound pressures and the higher spatial correlation of the CRO load case. It is assumed that the propeller induced pressure components are up to 30 dB higher than the TBL induced pressure components. This is especially true in or near the rotor plane, the part of the fuselage closest to the engines. Absolute measured auto-spectra of a TBL excitation can be taken from [PERSON] et al. ([PERSON] et al. (2013), Figure 11) and numerically determined absolute pressure values of a CROR engine are provided in [PERSON] and Akkermans (2014). The test specimen consists of a carbon fiber reinforced plastics (CFRP) fuselage structure with a coupled lining. Figure 2 shows the experimental setup seen from the semi-anechoic room. The fuselage structure (not visible) is supported by shock mounts placed in the corners of the outer frames (yellow boxes in Figure 2b). From a set of 19 evenly distributed accelerometers (indicated in blue), 16 are selected as potential remote sensors. The three discarded sensors are marked with an X. The reason for the exclusion of these three sensors is the low coherence due to the unfavourable position on the window pane or due to a sensor defect. The 20 microphones (indicated in green) are arranged in two vertical lines as shown in Figure 2b. It is assumed that the vertical locations of the passengers ears will usually be between these two lines. The 20 microphones are positioned at three distances \(z=-0.5\) m, \(z=-1\) m and \(z=-1.5\) m from the lining to capture SPL variations from the window to the aisle. The 60 locations of the virtual microphones are defined in Figure 3. All sensor signals plus an analog reference signal from the LSA and plus the two actuator control voltages are captured simultaneously with a sampling rate of 5 kHz and a time duration of 100 s. The reference signal is sampled from an analog output signal of the LSA containing the five CROR harmonics. Time data are sampled for both load cases. The data of the CROR load case are used for the identification of the observer filter and for the simulation of the active control system. Time data of the broadband load case are used for the selection of the remote sensors. The time data which are needed for the calculation of the frequency response functions (FRF) from the actuators to the remote sensors and to the microphones are obtained from a multi-reference test. For this purpose, the loudspeaker field is switched off and all actuators are  
Figure 1: Top view schematic of the experimental setup in the transmission loss facility. The loudspeaker array is located in the reverberation room and the fuselage with coupled lining is mounted in the test opening of the facility. The used loudspeakers are highlighted in yellow.