Propulsion system noise: Turbomachinery

Turbomachinery sources are important contributors to whole aircraft noise both at takeoff and approach. The turbomachinery contribution contains both tone and broadband components and is generated by a variety of mechanisms that occur in the fan, turbine, compressor, and combustor. These different sources are reviewed along with technologies that have been developed to mitigate their effect, either by reducing noise at source or by applying sound-absorbing treatments within the engine. Discussion is limited here to the exterior impact of turbofan noise on communities near airports. The effect on passengers and crew within the aircraft is dealt with separately elsewhere. Attempts to mitigate the environmental impact of aircraft noise are dominated by the metrics that are used to certify and regulate aircraft noise. These are introduced and their implications are discussed.