Upon takeoff, aircraft can make a number of noises that can disturb unknowing residents in the area around an airport. Engine noise can be sourced from the fan, compressor, or the air discharge from the core of the engine; however, the noise generated by the engine exhaust is the loudest. While not all the noise produced by aircraft can be mitigated, aerospace engineers have crafted a solution: noise suppressors. Noise suppressors are commonly an integral, airborne part of the aircraft engine installation or engine exhaust nozzle.
The noise produced by the engine exhaust is the result of a high degree of turbulence caused by a high-velocity jet stream traveling through a relatively quiet atmosphere. Just behind the engine, the high velocity of the jet stream produces a high frequency noise. The resulting noise is caused by the turbulent mixing of the exhaust gases with the atmosphere.
Over time and distance, the velocity of the jet stream slows down, it combines with the atmosphere, and violent turbulence begins. The noise generated as the exhaust gases dissipate is at a much lower frequency. The lower the frequency of the noise, the greater distance it is able to travel, making it so that individuals on the ground can hear the noise in greater volume. In contrast, high frequency noise is weakened more rapidly due to the interference of buildings, terrain, and other various atmospheric disturbances.
An engine with low airflow but high thrust due to high exhaust gas temperature, pressure, and/or afterburning produces a high velocity gas stream resulting in high noise levels. In the case of a larger engine, such powerplant types can handle more air and are quieter at the same thrust. The noise can be reduced by operating the engine at a lower power setting. Additionally, large engines that operate at partial thrust are less noisy than smaller engines operating at full thrust. In the case of a turbojet, the turbofan engine is much quieter during takeoff. Fan-type engines are able to produce noises at a lower volume because the exhaust gases ejected at the engine tailpipe have a slower velocity as compared to those of a turbojet.
Fan engines need a larger turbine to provide power to drive the fan. A larger turbine is able to reduce the velocity of the gas as well as reduce the noise generated because the exhaust gas noise and velocity are proportional. The exhaust from the fan is expelled at a low velocity so it does not create a noise problem.
Now that we have covered the way noise is produced in different engine types, it is important to get familiar with noise suppressors. The most common types are either a corrugated-perimeter type or multi-tube type. Both of these noise suppressors are able to break up the main jet exhaust stream into smaller jet streams. In doing so, the perimeter of the jet nozzle area is increased and the size of the air stream eddies are reduced as the gases are released into the atmosphere. Though the total noise-energy stays the same, the frequency is raised and the size of the air stream eddies and exhaust stream are scaled down. This is able to successfully bring the noise to the level beyond the audible range of the human ear, and these audible frequencies can be absorbed by the atmosphere more successfully than lower frequencies.
Additionally, the engine nacelle has acoustic linings surrounding the engine. This noise-absorbing lining is able to convert the acoustic energy into heat. The linings consist of a permeable skin supported by a honeycomb backing which provides a little room between the fact sheet and the engine duct. For optimal noise suppression, the acoustic properties of the skin and the liner can be matched.
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