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Sparked by this question:

Take any non-propeller powered aircraft, when idle, the sound produced by the engines are fairly low pitched, and as the engine spins up, increasing in pitch before becoming a full on roar.

I figured that the major sources of engine noise could be

  • The air rushing out of the exhaust,
  • The noise of combustion in the engine,
  • The mechanical noise of engine parts

My question is what causes the majority of the engine noise at idle vs full power.

To stop this question being too broad, lets discount noise coming from any source that's not the engine's of the aircraft in question. I just want to know about engine noise.

I had a look at this question, but that doesn't have any information about the source of engine noise, just that the turbine itself will also produce noise which is something I didn't consider.

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  • $\begingroup$ This is just a guess, but its possible that a good portion of the noise is the air being sucked into the nacelle, then on the other side when the compressed air expands through the exhaust could create more noise as well. I'm sure there are noise studies of jet engines somewhere, but I can't find them... $\endgroup$
    – Ron Beyer
    Feb 15 '16 at 13:39
  • $\begingroup$ The exhaust makes most of the noise. Engines with higher by-pass ratios generally makes less noise. The shape of the exhaust nozzle also has a great impact. That's why most hush-kits look a bit funny. $\endgroup$
    – Sami
    Feb 15 '16 at 14:30
  • $\begingroup$ Related: Sources of Aviation Noise --- Why are aircraft noisy and what can be done about it? --- Technology for a quieter America with this visual comparison between 60s and 90s. $\endgroup$
    – mins
    May 31 '16 at 8:47
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At full power it is the mixing process of the fast exhaust flow with the surrounding air. Note that noise increases with the sixth to eighth power of jet velocity, so the loudest engines are those with a low bypass ratio and reheat on.

Plot of exhaust noise level over jet velocity Diagram from a Stanford web page on noise. The jet velocity is in ft/s.

At idle the biggest noise source of the whole aircraft is the flow around the lowered landing gear, gear cavities and the slotted flaps that you asked us to disregard. The engine noise is a combination of intake noise, bearing noise and nozzle noise, and which of those contributes most to the engine noise depends on the individual engine and its installation into the aircraft.

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    $\begingroup$ I am so sorry I never marked this as correct. Have +15 $\endgroup$
    – JamesENL
    Apr 7 '17 at 1:40
  • $\begingroup$ I'm trying to understand your answer, but it's not clear to me. Most of the noise comes from the "mixing process of the fast exhaust flow with the surrounding air". And for a turbofan the diagram shows a 20-35 dB "delta" (the link to the document is dead). I guess 20 dB is not the sound pressure of a turbofan since a hair dryer is already 90 dBA, so what is it and how much loud is this turbofan mixing process? $\endgroup$
    – mins
    Sep 29 at 12:30
  • $\begingroup$ @mins Bummer that the page is offline. I read it such that the exhaust flow of a turbofan adds another 20 - 36 dB to all other noise sources (gears, intake etc.). What is missing is the distance at which that has been measured! Without the distance the comparison to a hair dryer becomes moot. I intended the graph only as an illustration for the influence the exhaust speed has on noise, not as a reference on absolute noise levels. $\endgroup$ Sep 29 at 16:00
  • $\begingroup$ dB are logs: 30 dB + to 30dB is not 60 dB, but only 33 dB, and this is still a whisper. If you add 20 dB and obtain the total noise pressure of, say 115 dB for a turbofan, then you need to add this to 115 dB already (115+20=115). So where the 115 did come from in the first place.... dB(A) are usually measured at 3 m and integrated. $\endgroup$
    – mins
    Sep 29 at 17:35
  • $\begingroup$ @mins You are so right. The person who drew the chart probably added logs like integers. I cannot find the source of the diagram (even with TinEye), so I cannot provide any more background. $\endgroup$ Oct 2 at 10:09

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