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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?

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    $\begingroup$ 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. $\endgroup$ – IanF1 Apr 16 '15 at 7:00
  • $\begingroup$ 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). $\endgroup$ – slebetman Apr 17 '15 at 17:09
  • $\begingroup$ @slebetman thanks for clearing that up but what do you mean with the ariframe? $\endgroup$ – BRHSM Apr 18 '15 at 9:10
  • $\begingroup$ 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. $\endgroup$ – slebetman Apr 18 '15 at 9:12
  • $\begingroup$ @slebetman ahh i understand, it's the frame on which the aircraft's housing is build and the part that keeps the plane together $\endgroup$ – BRHSM Apr 18 '15 at 9:17
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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.

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  • $\begingroup$ does this also means that the dictance between plane and his sound will become greater over time? $\endgroup$ – BRHSM Apr 16 '15 at 7:05
  • $\begingroup$ @CoderGuy depends on the velocity, if it is not Mach 1, yes. $\endgroup$ – Federico Apr 16 '15 at 7:07
  • $\begingroup$ @Frederico mach 1 is the speed of sound right? $\endgroup$ – BRHSM Apr 16 '15 at 7:20
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    $\begingroup$ I don't know much about planes, but I know the "stopped" phase is really bad for planes. :o $\endgroup$ – corsiKa Apr 16 '15 at 15:07
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    $\begingroup$ @corsiKa I find it difficult to board a non-stopped plane. $\endgroup$ – Federico Apr 16 '15 at 15:08
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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

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  • $\begingroup$ @VolkerSiegel actually what you would hear is ¿em raeh uoy naC $\endgroup$ – Jpe61 Feb 3 at 16:49
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    $\begingroup$ @Jpe61 Oh yes, absolutely! I had thought about the direction for a second or so, and dismissed it. Too early, obviously. Thanks! $\endgroup$ – Volker Siegel Feb 3 at 17:07
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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.

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    $\begingroup$ 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? $\endgroup$ – Pondlife Jan 22 '17 at 19:56
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    $\begingroup$ @Pondlife Most likely, a lap around the planet. $\endgroup$ – David Richerby Jan 22 '17 at 23:36
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    $\begingroup$ @DavidRicherby You're right, that makes a lot more sense! $\endgroup$ – Pondlife Jan 22 '17 at 23:46
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    $\begingroup$ Isn't all sound "vibration of the surrounding matter?" Isn't that basically the definition of sound? It's merely a mechanical vibration, no? $\endgroup$ – reirab Jan 23 '17 at 2:49
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    $\begingroup$ 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. $\endgroup$ – Hobbes Feb 4 at 15:34

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