Aircraft Noise Reduction Technology

Engine Noise Sources

Modern turbofan engines generate noise from several distinct mechanisms:

• Fan noise: The dominant source on high-bypass ratio turbofans; generated by fan blade interaction with inlet and outlet guide vanes. Higher bypass ratio (BPR) engines — like the GE9X with BPR of 10:1 — move more air at lower velocity, significantly reducing jet noise while increasing fan noise relative to earlier engines
• Jet noise: Caused by turbulent mixing of hot exhaust with ambient air; proportional to jet velocity to the eighth power (Lighthill's law). Very high BPR engines dramatically reduce jet velocity and this source
• Turbine noise: Tonal noise from blade passing frequencies; typically masked by fan and jet noise on modern aircraft
• Combustor noise: Broadband noise from the combustion process; becomes more significant as fan and jet noise decrease

Overall, modern high-bypass engines are 75% quieter than the first-generation jets of the 1960s, measured in perceived noise decibels (PNdB). The 787 with GEnx engines is approximately 60% quieter than the 747-200 it replaced on many routes.

Chevron Nozzles

Chevron nozzles are serrated, sawtooth-shaped edges on the trailing edge of the engine's fan duct and/or core nozzle. They work by encouraging rapid mixing of hot exhaust with cooler ambient air at the nozzle exit, breaking up large turbulent eddies into smaller ones that dissipate at higher frequencies — which are less efficiently radiated as sound.

The 787's GEnx engine uses chevrons on both core and fan nozzles, contributing approximately 3–4 EPNdB reduction on takeoff and 1–2 EPNdB on approach. The trade-off is a small thrust penalty of approximately 0.3%, translating to a modest fuel cost. Some operators select engines without chevrons to avoid this penalty on high-frequency routes.

Acoustic Liners

Acoustic liners are honeycomb-structured panels installed in the engine intake duct, bypass duct, and exhaust duct. They work via Helmholtz resonator absorption — the cellular structure traps sound waves and converts their energy to heat through viscous losses. Different cell sizes target different frequency ranges. The CFM56 (737 NG) has approximately 4 m² of acoustic liner; the LEAP-1B (737 MAX) uses advanced composite liner structures with 10–15% greater absorption area in the same space.

Airframe Noise

At approach speeds (typically 150–160 kt), airframe noise — generated by landing gear, flaps, slats, and fuselage — becomes comparable to engine noise. Landing gear turbulent flow generates broadband noise; landing gear fairings have been tested on the 777X. Slat cove noise arises from the gap between slat and wing; Airbus's morphing slat concept eliminates this gap using flexible composite structures. NASA's ACTE programme demonstrated that flexible, gap-free flaps reduce airframe noise by up to 5 dB with no aerodynamic penalty.

Community Impact

Noise certification is performed under ICAO Annex 16, Chapter 14, which sets limits at three reference points: takeoff (flyover), sideline, and approach. The Boeing 787-9 is approximately 40 EPNdB below Chapter 14 limits. The area exposed to Lden ≥ 55 dB around Heathrow has shrunk by approximately 90% since 1975 despite a doubling of aircraft movements — almost entirely due to quieter engines and procedures.

Regulations

ICAO CAEP is developing Chapter 15 noise standards, targeting another 7 EPNdB reduction beyond Chapter 14 for new designs. Research programmes including Clean Sky 2 (EU) and NASA's AATF are pursuing open rotor engines (counter-rotating propfans) that could offer 25–30% fuel savings but present significant noise challenges. The trajectory is consistent: each new aircraft generation achieves meaningful noise reductions, with the A220 and 737 MAX representing the quietest narrowbodies yet in service.