In the early days of aviation, the effects of noise were primarily considered in terms of their influence on an aircraft's passengers and crew. However, airline travel has since become much more common and widely available, resulting in an increased amount of air traffic and thus an increased number of people who are affected by aircraft noise, both as passengers and as members of communities located near airports.
Aircraft noise derives primarily from airplanes' propulsion systems and from aerodynamic noise. First, there are two major propulsion systems to consider, the turbojet and the turbofan. In the turbojet engine, air is first compressed, then heated in a combustion chamber, and finally accelerated by exansion through a jet nozzle. This results in the production of three types of noise: noise radiated from the air intake, noise radiated from vibrations of the engine shell, and exhaust noise. The turbofan engine differs primarily from the turbojet engine in that it utilizes a thrust-producing fan and that it produces a lower exhaust velocity, resulting in quieter operation for a given total thrust. (Raney & Cawthorn, 1979)
The second type of noise that is dominant in aircraft noise is aerodynamic noise, which includes the noise that is produced by the passing of air around the aircraft. Under normal cruising conditions, this type of noise predominates, especially among higher frequencies. However, propulsion noise tends to predominate at the lower takeoff and landing speeds. (Raney & Cawthorn, 1979)
In subsonic flight, which is typical for the vast majority of passenger flights, aircraft noise peaks when the aircraft is approximately overhead and then gradually diminishes to the ambient noise level. Additionally, noise that is heard as the aircraft approaches tends to be dominated by higher frequency sound, whereas noise that is heard after the aircraft has passed tends to include more lower-frequency sound (Raney & Cawthorn, 1979). However, this also depends on whether the aircraft is powered by a turbojet engine or a turbofan engine. In a turbojet engine, lower-frequency exhaust noise predominates over the rearward-propagating, higher-frequency noise produced by the air compressor. In the turbofan engine, on the other hand, higher-frequency sound radiating from the internal fans predominates over exhaust noise and propagates both forward and rearward from the aircraft (Bugliarello et al., 1976). This may be a moot point, however, since the vast majority of commercial aircraft now use turbofan engines. Thus, contemporary aircraft noise pollution generally includes more higher-frequency noise than aircraft noise pollution of yesteryear.
In addition to subsonic airplanes, other types of aircraft also contribute to noise pollution, such as helicopters and supersonic airplanes. In helicopter noise, there is both a periodic noise produced by the "blade slaps" and a nonperiodic broadband noise that results from air interacting aerodynamically with the rotor. Helicopter noise depends primarily on the speed of the blade tips, but also to a lesser extent on the number of blades, with both factors exhibiting positive correlations with noise production. (Raney & Cawthorn, 1979)
Supersonic airplanes produce a unique noise phenomenon: the sonic boom (view image). Sonic booms occur as a result of an aircraft flying faster than the speed at which sound waves travel. In most cases, a sonic boom is experienced as a shock wave sweeping across the ground below the aircraft. However, sonic booms can also occur during acceleration periods such as diving, and in such cases are more transient. The intensity of a sonic boom is primarily influenced by distance from the aircraft (i.e. altitude, as far as the observer on the ground is concerned). To a lesser extent, sonic booms are also influenced by how fast an aircraft is traveling beyond the speed of sound. Though sonic booms are obviously much more damaging than noise produced during subsonic flight, they are generally restricted to military aircraft and are banned above most urban areas, and thus contribute less to the noise pollution experienced by most people. (Raney & Cawthorn, 1979)
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