## 1 Introduction  
The problem of active noise control in propeller-driven aircraft has been thoroughly investigated in research during the last few decades. Several systems have been successfully implemented for example in the aircraft Saab 2000 or Bombardier DHC-8 Q400. One approach applies active noise control (ANC) using anti-sound generated by loudspeakers to reduce the sound pressure in the cabin. Early results of ANC in aircraft are published by [PERSON] et al. [1]. In this work, the interior sound pressure level (SPL) associated with the blade passing frequency (88 Hz) is reduced up to 13 dB. A different method to reduce interior SPL is the active structural acoustic control (ASAC). There, shakers or piezoelectric patch actuators are used to control the sound radiating structural vibration. [PERSON] and [PERSON] [2] published early results on this topic. To implement an ASAC system in aircraft, the fuselage or the interior parts are augmented with structural actuators and sensors. One approach uses active sidewall panels (linings) to realize active noise control in aircraft. In [PERSON] and [PERSON] [3], experiments with active linings are documented. The active linings are mounted to a stiffened fuselage barrel. A loudspeaker is used for the acoustic excitation of the barrel. The SPL reduction achieved by the active linings is up to 5 dB. Ground tests in a McDonnell Douglas DC-9 aircraft are documented in [PERSON] and Mathur [4]. The linings in the aft section are augmented with 16 piezoelectric patch actuators. The 32 error microphones are positioned at the headtests and in the aisle. The results of this approach are considered unsatisfactory by the authors because systems with loudspeakers and systems with actuators mounted on the fuselage implemented on the same aircraft showed a higher performance (see [4], Figure 4). Active linings using electrodynamic exciters as actuators are successfully realized by [PERSON] et al. [5] and by [PERSON] [6]. In [5], tests of an active lining in a sound transmission loss facility and, in [6], full-scale tests of two active lining modules mounted in the cabin of a Dornier Do728 aircraft, are reported. A loudspeaker array with 112 sources is used to mimic the acoustic excitation of the engine. A maximum SPL reduction of 11.3 dB and a mean SPL reduction of 6.8 dB are achieved in front of the active linings inside the Do728 cabin. The active noise control systems mentioned so far have in common that they use microphones for error sensing. However, the requirement of having distributed and closely adjacent microphones in the whole cabin is undesirable because it requires additional wiring and prevents flexible cabin layouts. The so-called smart lining concept pursued by [PERSON] et al. tries to overcome these drawbacks by modular active linings with structurally integrated actuators, sensors and control. This concept requires a substitution of the physical error microphones by so-called virtual error microphones. Different methods are applicable to set up a noise reduction system with virtual microphones. In the work of [PERSON] and [PERSON] [7], a virtual microphone control for an active noise-cancelling casing is realized. The active vibration control system is tuned in a two-stage approach to reduce the SPL at specific locations in the acoustic field solely based on vibration sensor data. A related approach is pursued in [PERSON] and [PERSON] [8], where the locations of vibration sensors of an active vibration control system are optimized regarding the reduction of the radiated sound power of a fuselage structure. Another method is the remote microphone technique for active control proposed by [PERSON] and [PERSON] [9]. In this technique, the error microphones are substituted by remote microphones and an observer filter. In a similar approach, [PERSON] and [PERSON] [10] replace the remote microphones by accelerometers mounted on the radiating structure.