The Generation and Suppression of Aircraft Noise

Flyover noise levels of turbojet-powered airplanes are dominated by the low-frequency roar of the jet-exhaust noise at both takeoff and approach power settings. Jet-noise suppressors developed for these turbojet engines did reduce some-what the noise levels under the takeoff flight path but did not reduce the levels under the approach flight path.

Low-bypass-ratio turbofan engines produced less jet-exhaust noise than the turbojet engines, but the discrete-frequency components of the turbomachinery noise were more prominent. A research program, conducted for NASA, showed that the installation of acoustically absorptive duct linings could yield significant reduction in the level of the turbomachinery noise, on the JT3D-powered transports at approach power settings. Expectations relating to installations of similar material on JT8D-powered transports are described.

The new high-bypass-ratio turbofan engines produce substantially less jet-exhaust noise than the predecessor low-bypass-ratio turbofans. Turbomachinery noise on these new engines was reduced by careful selection of various engine parameters and by installation of acoustic treatment. The use of jet transports powered by these new engines should result in a significant reduction in noise levels for virtually all airport neighbors.