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We're all familiar of the sound planes flying quite high overhead and you hear a faint drone, even when planes are lower to the ground you hear them scream at a slightly higher pitch, but what exactly are you hearing?

I would guess it's either the sound of the fan blades spinning extremely fast or the noise of the air (thrust) being forced out the rear of the engine?

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It depends on the type of operation:

Approach (close to airport):

  • Much aerodynamic broadband noise from the air-frame
  • Modern turbofan engines are less noisy than the air frame during approach. But engines emit most of the tonal components in the sound.
  • Depending on the type of aircraft and angle of descent the pilot might need to use air-brakes to reduce speed. That will cause turbulence and noise.
  • On some Airbuses you hear a typical high pitch annoying sound that is caused by air flowing across cavities under the wing (Fuel Over Pressure Protector to be exact, not from the engine).

Departure (close to airport):

  • Pretty much all the noise you hear comes from the engines, such as the buzzsaw noise generated when the tips of the fan blades reach supersonic speeds. Hot high speed gases causes turbulence and that creates low frequency noise, especially behind the aircraft.

Overflight at 35.000-40.000 ft:

  • What you hear is low-frequency broad band noise that originates from turbulence behind the engines. All the high and medium frequencies are gone because the air between aircraft and ground absorbs much of the sound. The sound will also fluctuate due to differences in wind speed, humidity and temperature, along with the flight path and along the sound propagation path.

Further reading / sources

1. Conference lecture by Prof. Dr. Ing. J. Delfs, Head of Technical Acoustics, German Aerospace Center

2. ECAC doc 29 (Technical documentation)

3. An Overview of Aircraft Noise Reduction Technologies

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  • $\begingroup$ On the approach, might a lot of the noise comes from the flaps (part of the airframe bit you mentioned, but if you're going to call out the air brakes specifically, then flaps might be fair game, too). At least when I'm on the plane, there is a significant increase in volume once the flaps are extended, even to the first position. $\endgroup$
    – yshavit
    Nov 30, 2017 at 17:54
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    $\begingroup$ Wow that's amazing, while it definitely was a thought, I really didn't expect that air flowing around the aircraft or its brakes could generate such loud noises. $\endgroup$
    – Kxy
    Nov 30, 2017 at 18:13
  • $\begingroup$ @Ksery: You would understand the airframe noise if you've flown light planes, or even (maybe especially) sailplanes. $\endgroup$
    – jamesqf
    Nov 30, 2017 at 18:41
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    $\begingroup$ The main improvement that could be done to this answer is to provide sources so that this is no longer your personal opinion. $\endgroup$
    – CGCampbell
    Nov 30, 2017 at 19:19
  • $\begingroup$ @CGCampbell - That could be done! What is the appropriate way? Edit the answer or provide them in the comment section? $\endgroup$
    – Meatball
    Nov 30, 2017 at 20:15
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Here's NASA Glenn's breakdown of the noise emitted by a typical commercial airliner with 1992-level technology.

enter image description here

Image source

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    $\begingroup$ This could be improved by explaining the different items, and also citing the image source. $\endgroup$
    – fooot
    Dec 1, 2017 at 17:12
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    $\begingroup$ Your graph indicates that approaches makes more noise then take offs. That is not true and I think we all agree on that. Could you have missunderstood the graph? I would guess that what is shown here is sound pressure level at a location closer to the approach path then the take off path? $\endgroup$
    – Meatball
    Dec 1, 2017 at 20:29
  • $\begingroup$ Also, how old is the chart and what was the "typical commercial airliner" that it is based on? $\endgroup$
    – Sports Racer
    Dec 3, 2017 at 16:43
  • $\begingroup$ @Sports Racer : Clearly the chart is wrong. There are no engines that emits less noise during take off. $\endgroup$
    – Meatball
    Dec 4, 2017 at 9:12
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There are many great YouTube videos on aircraft noise reduction such as this NASA noise reduction program

It won't surprise anyone, the majority of noise during takeoff is the engines.

What is interesting is the engines are now quite enough that during landing, most of the noise comes from the wheels and flaps whistling and rumbling!

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