# (Why) does the sound of propeller planes change with increasing distance?

I recently heard a propeller plane, and then very shortly after a jet like noise. After I did some probability calculations, I conjectured that probably these two events are timely correlated.

I also have a hypothesis on why: When the propeller plane is near, the "damping factor" due to the medium (air) not being perfectly "frictionless" is approximately 1, so the experienced loudness of the different frequencies ~ log(amplitude) is approximately described by some formula like the free space path loss formula, so especially the dependance upon the distance is the same for all frequencies, all loose log(d²) DB compared to if you were standing in 1m proximity to the aircraft. Therefore you hear the classical propeller noise.

When the plane distances itself enough, and flies low enough, i suppose the following three effects may change the noise into a white noise jet like noise:

(1) the frequency dependant damping now plays a major role, the air damps the different frequencies differently, damping the classical "propeller frequencies" more than some ohter frequencies that then make up that white noise.

(2) the higher frequencies of the noise are heavily affected by obstacles (the plane flies low and is far away now, so there are obstacles in the way) while the lower frequencies "bend around them" (due to diffraction).

(3) it may also be that with increasing distance and therefore increasing travel time in a real medium like air, sharp frequency distributions naturally get smoothed out over time.

Can/Does the noise of propeller planes change like this? If yes, why does that happen?

• It's probably just the doppler effect combined with the fact high frequencies dissipate more quickly. Jul 28, 2022 at 19:26
• Sounds like a reasonable explanation on on why this happens. (if it does) What im still unshure of is if that happens at all, or if I just heard a propeller plane and then a jet instead of a propeller planes noise changing.
– KGM
Jul 28, 2022 at 19:34
• Did it sound like this? youtube.com/watch?v=C6DWBkF7NUI Jul 28, 2022 at 19:39
• Yep, just the propeller noise was unnoticable in the end... It sounded like a jet... But the same pattern, at first the propeller frequency dominates the sound, then there is this white noise like sound afterwards...
– KGM
Jul 28, 2022 at 19:49
• Can't say for sure but I would think that's just the sound of the moving air, separate from the beating of the propeller (or the whirring of a turbine). Jul 28, 2022 at 19:54

As noted in the comments, what you hear is a combination of engine noise as well as the sound of air over the aircraft and propeller. This will slightly complicate the circumstances, but only slightly. The movement of the aircraft results in a doppler effect.

Sound is compression and rarefaction of air and travels in waves. There is a YouTube video explaining that the waves will be subjectively compressed when the source is traveling towards you and will be expanded/rarefied when traveling away from you. This change in frequency means a change in pitch.

The complication arises from the multiple sources of the sound, unlike the automobile horn sound in the video.

One typically experiences a more "pure" example when at a railway crossing and the train is activating the horn during approach and departure. The observer hears a higher pitch at first, an "accurate" pitch abeam of the observer and a lower pitch as the train engine (and horn) departs the crossing.

As a related anecdote, I attended a sailplane competition many stars ago. The racing portion resulted in the sailplanes returning at very high speed and low altitude, as they were carrying water ballast to improve penetration (see sailplane wing loading) and would burn off the speed as well as dump the ballast.

The sound of the sailplane was impressively intense and created the same doppler effect as a jet aircraft at high speed, but without engine noise. Although sailplanes are aerodynamically efficient and aerodynamically "clean," there's still plenty of noise/sound generated at high speed.

• So the sailplanes made the whoosh but not the scream or roar? Jul 28, 2022 at 22:23
• it's a seriously loud whoosh, but no roar or scream as there is no engine (in most). There is also the doppler effect of the whoosh, which is pretty darn cool. Jul 28, 2022 at 23:39