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I had this discussion with my dad about a question he had asked. Basically he said:

If a plane flies faster than the speed of sound, will he be able to overtake his own sound and hear itself?

(let's leave behind the fact that he can run out of fuel or that the sound finishes before he hears it)

I disagreed because it seemed illogical to me, but we never actually found hard evidence to determine if the statement is true or false.

Can a plane actually do this?

Jamiec
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BRHSM
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    If the plane is already flying faster than sound, then the sound is never "ahead" of the plane to be overtaken - it is behind you as soon as it is generated. You would still hear sound as transmitted through the airframe though. If you start below the speed of sound and accelerate through it, you do indeed "catch up" with the sound you've already generated, resulting in a sonic boom. – IanF1 Apr 16 '15 at 07:00
  • As you've probably gathered from the accepted answer, your dad has it backwards. When not moving the plane always hears itself ("hear" means the same as you can hear yourself talking). When it's faster than the speed of sound it can no longer hear itself because it left the sound behind it (minus whatever sound transmitted via the airframe). – slebetman Apr 17 '15 at 17:09
  • @slebetman thanks for clearing that up but what do you mean with the ariframe? – BRHSM Apr 18 '15 at 09:10
  • Airframe = body, wing, tail etc. Basically the physical structure of the airplane. Of course the airplane is more than just the airframe. An airplane = airframe + control surfaces + electronics + engine etc. – slebetman Apr 18 '15 at 09:12
  • @slebetman ahh i understand, it's the frame on which the aircraft's housing is build and the part that keeps the plane together – BRHSM Apr 18 '15 at 09:17
  • Your dad does have a fairly correct intuition about hearing sound twice though. But generally you can't hear it in the airplane itself. Or you can but by then it would be moments before you die and the aircraft disintegrating. Hearing sound produced by the airplane multiple times simultaneously is actually what the sonic boom is. A sonic boom is merely the normal sounds the airplane makes (engine rumble, the swish of flowing air etc.) arriving all at once (see picture from Fedrico's answer). – slebetman Apr 18 '15 at 09:18
  • Sonic boom at source is very destructive. It causes control surface failure, engines to fail, violent vibrations etc. It's why fast jets have long noses and swept wings. To delay the pressure wave from touching the aircraft body (or at least important parts of the aircraft) as late as possible. That's also what practically limits airplane speeds. The faster you want to go (and survive) the longer nose and more wing sweep you need. – slebetman Apr 18 '15 at 09:22
  • Here's a link to a handy demo video my friend made: https://www.youtube.com/watch?v=TklvFQsBxRM ;) – Michael Hall Jan 28 '19 at 23:22
  • @MichaelHall wow that's actually really cool! – BRHSM Jan 29 '19 at 20:25
  • @BRHSM, FYI that was actually a joke video he made, it doesn't actually work like that. The air package inside the cockpit is moving along at the same speed as the occupant so the sound waves would propagate uniformly from the source. It may be useful to think of it in terms of ripples on a body of water. Drop a rock and ripples are circular. A slow moving boat will push some waves ahead of it. But a speed boat will get there before the ripples (wake) it generates. The sonic boom is just the sound equivalent to a wake hitting the shore after a boat passes. – Michael Hall Jan 29 '19 at 20:46

3 Answers3

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I think you are missing some physics knowledge, so let's start from here:

If a plane flies faster than the speed of sound

Sound is a compression wave that travels through air. Given a certain air temperature, the sound will travel at different speeds through the air.

If a plane flies faster than the speed of sound means that the compression waves it is generating will remain behind the aircraft:

enter image description here

image from here

will he be able to overtake his own sound and hear itself?

As you can see on the right picture, if the aircraft is travelling faster that the speed of sound, the compression waves it generated in the past will remain behind. If it would stop/slow down, the waves would then catch up with the plane and a person on board would be theoretically capable of hearing them.

Federico
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  • does this also means that the dictance between plane and his sound will become greater over time? – BRHSM Apr 16 '15 at 07:05
  • @CoderGuy depends on the velocity, if it is not Mach 1, yes. – Federico Apr 16 '15 at 07:07
  • @Frederico mach 1 is the speed of sound right? – BRHSM Apr 16 '15 at 07:20
  • @CoderGuy yes, but labeled so that you do not have to know the temperature. (and it is FEderico, no extra r) – Federico Apr 16 '15 at 07:22
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    I don't know much about planes, but I know the "stopped" phase is really bad for planes. :o – corsiKa Apr 16 '15 at 15:07
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    @corsiKa I find it difficult to board a non-stopped plane. – Federico Apr 16 '15 at 15:08
  • Would I be correct in thinking that the speed of sound increases with pressure, and that in front of the plane there would be a region where the pressure was sufficient to elevate the speed of sound to that of the aircraft? If a supersonic aircraft were to fly into an obstacle (e.g. a hailstone) I would expect that the hailstone would be hit by a shockwave slightly (somewhere between a microsecond and a millisecond) in advance of being hit by the aircraft itself. Would that intuition be correct? – supercat Jun 17 '15 at 15:59
  • While it is a different question, I think this gets more interesting when you think of a jet flying in a racetrack pattern at a speed greater than Mach 1. I doubt the airframe can handle turning fast enough, but if it could, I expect it would sound the same as a fly-by. – Steven C. Howell Aug 16 '22 at 19:18
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Your father asked:

If a plane flies faster than the speed of sound, will he be able to overtake his own sound and hear itself?

Of course he can!

He can catch up because he is faster. But that does not fully answer this brilliant question. We want to know what would practically happen:

But what does it sound like?

We want to catch up with our own sound, so it needs to get away from us first. That means we fly slower than sound in the beginning. For example, your father shouts

Can you hear me?

in the direction of travel.

Now, we accelerate our aircraft to be faster than sound. With that, we create a shockwave in front of the plane, which can be heard as the sonic boom. Practically, the whole situation is really loud

We hear no difference.

It does get louder, but not noticeable.
But in theory, we can think about what we would hear when it would be quiet, and we could use the air molecules in front of our shockwave as a microphone.

But if we could hear it anyway?

Then, we would actually hear "Can you hear me?", but in very deep voice, so deep that it is to deep to be heard by a human. The words were spoken in some seconds, but we reach the sound waves only slowly over some minutes.

We would hear a very deep voice...

And as we creep up to our sentence from behind, we catch the last syllable first, because that is what we said at the latest time, so it had less time to go away. The direction of the time in the sound is flipped.

Speaking the words backwards!



It's a very deep

¿em raeh uoy naC

Volker Siegel
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-4

Sound does not travel in a straight line. It radiates evenly in all directions. Therefore, even if your moving super fast, and I mean fast, you would not be able to catch it. I know you're trying to think if you made a really quick lap around the planet could you catch it, you would simply blow through the place it was made. But there are echoes that you could hit if you moved fast enough but even then it would not be the sound itself but a vibration left on the surrounding matter.

David Richerby
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Yance
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    Welcome to aviation.SE! Is "a lap around the plant" meant to be "around the plane"? And what does a "vibration left on the surrounding matter" mean? – Pondlife Jan 22 '17 at 19:56
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    @Pondlife Most likely, a lap around the planet. – David Richerby Jan 22 '17 at 23:36
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    @DavidRicherby You're right, that makes a lot more sense! – Pondlife Jan 22 '17 at 23:46
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    Isn't all sound "vibration of the surrounding matter?" Isn't that basically the definition of sound? It's merely a mechanical vibration, no? – reirab Jan 23 '17 at 02:49
  • Shame the downvoter piranhas got to this answer, it is actually correct. You could catch your own sound by lapping the planet, but it is easier to slow down and let the sound catch you. This phenomena is more easily seen on power boats, where the separated high speed wake will slurp back to the transom when the boat slows. – Robert DiGiovanni Feb 04 '20 at 04:44
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    downvotes are because of "even if your moving super fast, and I mean fast, you would not be able to catch it" which is very incorrect. – Hobbes Feb 04 '20 at 15:34