As you might expect, most aviation noise is caused by the engines. As well as moving parts making sounds, hot air is pushed out of the engine at a high speed, causing friction as it moves through the stiller, colder air around it.
Some modern bypass engines (engines with a large turbofan) are quieter than older aircraft engines because they interrupt the friction between the hot and cold air with a layer of cold air that moves at a medium speed.
The aircraft body (or airframe)
Even an aircraft without an engine, such as a glider, produces some noise. As an aircraft flies, its airframe causes friction and turbulence with the air. This produces sound waves. The faster an aircraft flies, the more friction it produces and more noise. When an aircraft uses its flaps (on the wings) and landing gear (wheels), there is increased surface area to create more friction which in turn generates more noise.
Temperature, humidity, rain, cloud and wind speed all affect the sound produced by an aircraft in flight and how we perceive the noise on the ground. Different types of weather can cause sound waves to echo, making aircraft seem louder. This echoing of sound can sometimes mean that aircraft can be heard from an altitude when they might normally be too high to hear.
Aircraft generally take-off into the wind, and land into the wind too, meaning that wind direction can change an aircraft’s flight path. A light breeze allows them to take off and land in the same direction, as it reduces the aircraft’s weight. Sometimes a stronger wind can cause an aircraft’s engines to work harder as it is necessary for it to push against the wind. For safety reasons heavy aircraft must depart and land in the same direction.
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