Active Structural Acoustic Control of Rotor Noise Transmission

Abstract

The fuel efficiency of engines is of particular importance to the development of modern aircraft. To meet this future requirement contra rotating open rotor (CROR) propulsion systems are currently discussed. Their fuel efficiency offers respectable benefits compared to common jet engines. But up to now their high noise emission in certain frequency bands impede dissemination on the market. Today, modern methods like Active Structural Acoustic Control (ASAC) allow the research and development of noise reduction strategies in the presence of complex acoustic excitations. In this paper an ASAC system for the reduction of noise transmission through a section of a curved 1700 x 1300mm^2 carbon fiber reinforced plastics (CFRP) fuselage is presented. The fuselage is mounted in the Acoustic Transmission Loss Test Facility of the DLR which consists of a reverberation and an anechoic room connected by a test window. The external excitation of the fuselage is simulated by a 112-channel loudspeaker array which is placed in front of the fuselage. Driving each loudspeaker by a particular signal, the complex sound pressure field of a CROR engine is synthesized on the fuselage. Sensors on the fuselage measure the normal acceleration in specified points of the structure. A robust controller calculates the desired actuator signals to realize the reduction of sound transmission in the bandwidth from 1 to 500 Hz. Beside a detailed description of the realization and the algorithms, the results of the experiments and the effectiveness of the control concept are shown.