landing led to fewer people experiencing aircraft noise and that noise relief could be shared by using more precise flight paths, allowing control of the noise footprint of departing aircraft. Noise relief could be enhanced by switching flight paths, for example by using one flight path in the morning and another in the afternoon. Technological advances Engine design Modern High bypass turbofans are not only more fuel efficient, but also much quieter than older turbojet and low-bypass turbofan engines. On newer engines noise-reducing chevrons further reduce the engine's noise, while on older engines hush kits are used to help mitigate their excessive noise. Engine location The ability to reduce noise may be limited if engines remain below aircraft's wings. NASA expects a cumulative 20–30 dB below Stage 4 limits by 2026–2031, but keeping aircraft noise within airport boundaries requires at least a 40–50 dB reduction. Landing gear, wing slats and wing flaps also produce noise and may have to be shielded from the ground with new configurations. NASA found over-wing and mid-fuselage nacelles could reduce noise by 30–40 dB, even 40–50 dB for hybrid wing body which may be essential for open rotors. By 2020, helicopter technologies now in development, plus new procedures could reduce noise levels by 10 dB and noise footprints by 50%, but more advances are needed to preserve or expand heliports. Package delivery UAS will need to characterize their noise, establish limits and reduce their impact. See also Aviation taxation and subsidies Electric airplane Farley v Skinner Hush kit Helicopter noise reduction Jet noise Noise barrier Rotor-stator interaction Silent Aircraft Initiative Train noise XF-84H Thunderscreech, the loudest aircraft ever built. General: Health effects from noise Noise pollution Noise regulation Aviation and the environment ReferencesU.S. Noise Control Act of 1972 United States Code Citation: 42 U.S.C. 4901 to 4918 S. Rosen and P. Olin, Hearing loss and coronary heart disease, Archives of Otolaryngology'', 82:236 (1965) External links Aircraft noise Noise Sound Environment and health