EXPERIMENTS ON NOISE REDUCTION IN AIRCRAFT WITH ACTIVE SIDEWALL PANELS

Abstract

The present work focuses on the active reduction of aircraft interior noise by means of smart lining (sidewall panel) modules. In general, the interior noise floor of jet-driven aircraft is on an acceptable level. This, however, might change, if energy-efficient engines with open rotors are used in combination with stiff lightweight fuselage structures. In such a case, unacceptably high noise levels might arise especially at low frequencies (< 500 Hz). In this frequency range, passive sound insulation methods are usually incompatible with the mass and volume restrictions of aircraft. The linings are coupled to the fuselage and radiate sound into the cabin. They have large sound-emitting surfaces and many passengers are sitting directly in front of them. Therefore, it is reasonable to reduce the low-frequency sound emission of linings with active control. Furthermore, smart linings could be used for secondary tasks like passenger announcements or noise masking. Multiple functionalities can be realized in parallel. This contribution describes recent experimental research work on smart linings conducted in the DLR test aircraft Dornier Do728. The Do728 provides a fully equipped cabin with a realistic acoustic environment. The external acoustic excitation is realized with a loudspeaker array placed directly in front of the fuselage. A synthesized counter-rotating open rotor (CROR) noise containing the first five harmonics (< 500 Hz) is used to emulate the acoustic fuselage excitation of a real CROR engine. Two smart lining modules are driven in parallel, each with two inertial actuators and an independent adaptive feedforward control unit. Microphones are used as error sensors and for capturing the sound Emission and noise reduction capability of the smart linings. Mean sound pressure level reductions up to 10 dB are achieved in the monitored area of the cabin.