What are the main noise sources in a turbofan engine and how are they reduced?

Main noise sources: Fan noise (rotor wakes interacting with stators, buzz-saw at supersonic tips), Jet noise (turbulent mixing of exhaust with ambient air), Combustor noise (direct and indirect, through entropy waves), and Turbine noise (less significant due to lower velocities). Reduction methods: Increased BPR (lower jet velocity), Chevron nozzles (enhanced mixing, jet noise reduction), Acoustic liners in nacelle (fan/turbine noise attenuation), Reduced fan tip speed (eliminates buzz-saw), Optimized blade counts (cut-off design), and Swept/leaned blades. Modern engines are 25dB quieter than 1960s designs. ICAO Chapter 14 sets current limits; future eNoise standards will require further reduction.