Noise Reduction

The "roar" of a jetliner is becoming a thing of the past. Boeing has been integrating noise-reducing innovations into aircraft designs for five decades, making each new airplane quieter than its predecessor.

Noise is a form of pollution. The noise generated by air traffic may not have any long-term effects on the environment, but as air travel becomes a part of our lives in nearly every part of the world, reducing it certainly improves the quality of life for all of us. For communities and citizens near airports, noise pollution negatively affects property values and personal health. Inside the aircraft, noise impacts the comfort of passengers as well as the work environment of cabin and flight crews. Noise is unwanted sound. The unwanted sound of a neighbor’s stereo or a construction site’s heavy machinery creates pressure fluctuations in the air detectable by the human ear. The intensity of these sounds is measured in decibels. The decibel (dB) uses a logarithmic scale to include a wide spectrum of perceivable sound. For the human ear, near silence is zero dB, a whisper is 15 dB, and a rock concert measures in at 120 dB. A 10 dB increase is perceived by most people to be a doubling in loudness, meaning a 120 dB jet engine is perceived to be twice as loud as a 110 dB car horn. In response, civil aviation authorities around the world continue to update regulations on aircraft noise, including noise-related restrictions and economic incentives for airlines and other aircraft operators. For all these reasons, the investments necessary to continually advance new technologies are environmentally and economically rewarding. Quieter airplanes are better for our communities, improve the flying experience for passengers and crews, and help airlines meet important noise restrictions. So what makes a jet loud? Airplane noise is comprised of both engine noise and airframe noise. Engine noise, which is typically most pronounced during takeoff, has several contributors. The turbulent mixing of flow streams—when the hot core efflux (high-velocity exhaust) and the fan exhaust flow meet free-streaming air—produces noise from the engine’s jets. The fan within the engine makes noise that escapes from both the front and back of the engine. The fan is also responsible for the “buzz saw” noise during takeoff, created when the tips of the fan blades are traveling close to the speed of sound. Airframe noise, unlike engine noise, is most noticeable during landing. The lowering of landing gear creates turbulence that produces noise. Similarly, the interaction of flaps and slats on the wing during descent influences airstream flow and thus creates noise. Boeing is continually engineering nozzles and airframes to lower overall noise and let communities live better with air traffic.