Anyone who has traveled extensively by plane or lived near an airport may wonder why airplanes make so much noise. Interestingly, using the same amount of engine power, airplanes are considered to be 100 times quieter today than they were 40 years ago.
Sanjiva Lele, AA, professor of aeronautics and astronautics and of mechanical engineering at Stanford University, addressed this topic in a paper he published in June 2008 titled "Do airplanes have to be so loud?"
To summarize, airplane noise is derived from both the plane's engine and its "body".
During takeoff, the noise we hear comes primarily from the engine. As the plane lands, the noise is attributed to the plane's landing gear and airframe components. Airframe components include mechanisms on the aircraft's wings that flutter in and out to ensure a smooth landing (if you've sat in a seat near the wing of a plane, you may have noticed this flapping motion as the plane descended).
Why are airplanes so much quieter today than they were 40 years ago? By changing the airflow through the engines through the use of a turbo-fan engine, engineers have been able to increase the engine's efficiency and lower the noise output. According to Mr. Lele, increasing the size of the fan permits greater capacity for air and bypasses the hot and high-pressure airflow through the core engine. Unfortunately, due to ease of inspection and maintenance, there are not many solutions to reducing the noise of landing gear. It's designed to be safe; thus, it's designed to be noisy.
Increased research and innovation is expected to reduce airplane noise in the future. Using computer models that measure airflow and other noise-reduction physics, engineers have learned to modify the shape of jet exhaust nozzles by incorporating "chevrons", which are wavy cutouts that alter the flow of air and diminish the sound of airflow in certain sustained frequencies.
Looking to nature, another innovation is to model aircraft design after that of birds. By designing slats on aircraft wings to be more "brush-like", simulating feathers, noise is reduced by minimizing sharp changes in airflow.
Until the invention of the noiseless aircraft, we can expect that airplanes will generate a fair amount of noise. However, experts like Mr. Lele concur that through the use of technology such as high-fidelity computer simulation tools, we are that much closer to reducing airplane noise by altering airplane design, particularly via airflow dynamics and modifications to the body of the airplane.