Robust tunable acoustic impedance control on electroacoustic resonators for aircraft noise reduction

Air traffic noise emissions are responsible for a significant part of the overall environmental noise, especially in the vicinity of airports. Exposure to environmental noise is known to negatively impact health and to be associated with cardiovascular diseases, sleep disturbance, cognitive impairment of children, and annoyance. To improve the energetical efficiency of turbofan engines, UHBR engines are being developed, whose tonal noise emissions are predominant and with lower frequency content, and in which the inlet diameter is increased, and the nacelle shortened. With the planned evolution of turbofan engine designs, the passive sound-absorbing liners conventionally embedded in their nacelle are no longer satisfactory and new designs for acoustic absorbers must be researched.This thesis aims at the design and optimization of active electroacoustic absorbers specifically intended for being embedded in these future aircraft engines. These absorbers are electrodynamic loudspeakers whose resonant behavior of their membrane is actively modified by a digital controller to optimally absorb the noise emission of the engine. The design and the experimental assessment of an active liner based on the state-of-the-art hybrid sensor-/shunt-based control method in a flow duct, and in a one-to-four scale UHBR engines test facility are first presented in this work. The results obtained with this active liner show that the technology is promising but requires more robust controllers to improve its stability and its reliability. Several ways of improving the reliability of an electroacoustic absorber are then presented. The first proposed improvement consists of a better method for the estimation of the required parameters of the transducer. Two novel control architectures of the electroacoustic absorber are then presented, which both rely on the use of an additional microphone placed in the enclosure behind the loudspeaker. The first design is purely based on feedback control and no longer depends on the analytical model of the transducer. It is however limited in the range of achievable target behaviors of the membrane. The second design is a combination of feedforward and feedback control, which enables a wide range of tunability while reducing the sensitivity to uncertainties in the analytical model of the transducer. The stability limits of the improved electroacoustic absorber in its acoustic environment are then analyzed and exploited to select an optimal configuration of the controller. The overall contribution of this thesis opens the way for more robust and reliable active electroacoustic absorbers and liners. The thesis concludes by proposing some future perspectives, regarding, among others, the optimal target behavior of the liner, real-time estimation of the parameters, and taking advantage of the lattice configuration of the unit cell absorber in a liner design.