Investigation of engine tones in flight

Acoustic fly-over measurements with Boeing 747-400 aircraft have been performed in order to extend an existing noise data base for Airbus A319/20 and MD-11 aircraft with another widely used wide-body aircraft. The data were recorded with a large multi-arm spiral microphone array with 238 microphones which extended over an area of 42 by 35 m. The flight trajectories were determined using a combination of onboard flight recorder and GPS data and an array of laser distance meters on the ground. A linearised trajectory of the aircraft was determined for every fly-over in order to calculate frequency spectra for different emission angles at emission time by compensating the Doppler effect. Sound source maps were calculated using the standard beamforming algorithm and a deconvo- lution method for moving sources. Fly-over measurements were performed with different configurations in order to analyse airframe and engine noise sources separately. In this paper, the main focus is on the analysis of fly-overs with different engine speeds, but also, some results for landing gear noise are presented.

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