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totally hypothetical here:

lets say a man is playing a song on a guitar and I begin travelling quickly away from the guitar, if I were to reach the speed of sound, what will I hear? (my assumption is that I will hear a single note humming in a constant state...like pressing a key on a synth).

assume im not in a vehicle and the sound of air wizzing past me isn't involved...not a practical situation, just hypothetical.

total noob here, my apologies.

and to take it a step further...if i can speed up or slow down (move forward or backward) ever so slightly from the current note "im in", then back to the speed of sound at another note, would this be possible?...to move from one note of the song to another?

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  • $\begingroup$ Sorta the opposite position of this question (where you're stationary & sound is coming from a supersonic object). $\endgroup$ – Kyle Kanos Oct 28 '15 at 21:51
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my assumption is that I will hear a single note humming in a constant state.

A sound wave is not a thing that you can hear. Assume for a moment that you are just standing in the coffee shop, enjoying the music. What you are hearing is not the waves. What you are hearing is the guitar.

The waves carry acoustic energy from the guitar to your ear. The guitar causes fluctuations in the pressure of the air that immediately surrounds it. "Wave" is our word for how those fluctuations propagate through the air. Your eardrum experiences the same fluctuations as the wave passes by, and you hear the sound.

If you could somehow magically keep pace with the waves and not feel the supersonic blast of wind in your face then you would hear nothing because the wave is not passing you by. You would experience only the steady-state pressure of one peak of the wave or one trough. As far as your ears are concerned, a steady-state pressure equals silence.

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I'm pretty sure you will hear nothing from the guitar, because you will be traveling at the same speed as the sound so the sound wave will not be able to make vibrations on your eardrum thus you hear nothing.

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  • $\begingroup$ That's wrong. The speed of sound is relative to the medium, not the source. $\endgroup$ – Physicist137 Oct 28 '15 at 21:31
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    $\begingroup$ @Physicist137, Exactly! The sound wave is propagating through the medium at the speed of sound, because that's what sound waves do, and the OP is strapped to a rocket or whatever that is propelling him through the same medium at the same speed. No sound waves from the guitar pass by his ear, so there's nothing to hear (at least, not from the guitar.) $\endgroup$ – Solomon Slow Oct 28 '15 at 21:39
  • $\begingroup$ @jameslarge Apparently I misread the question. Its the receiver which is moving. You are right. Sorry. And out of consequence, this answer is right too =). $\endgroup$ – Physicist137 Oct 29 '15 at 1:06
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You would hear Whoosh... and Boom! The sound of air rushing past your ears traveling at the speed of sound and the shock waves would entirely drown out the sound of any guitar even with the most powerful amps! ;)

And if you say well then remove the air and do the experiment in space - you will not hear any sound since it requires the air to propagate. ;) ;)

But assuming you could somehow overcome the more practical issues, the answer is you would not hear the sound at all as Bob Bills says. Even if the guitar started playing before you took off - when you reached sonic speed the guitar sound waves would be Doppler shifted to zero frequency.

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  • $\begingroup$ According to the pilots of the Concorde: You don't actually hear anything on board...But we don't hear the sonic boom or anything like that. That's rather like the wake of a ship - it's behind us So your first paragraph is apparently false. $\endgroup$ – Kyle Kanos Oct 28 '15 at 22:02
  • $\begingroup$ @KyleKanos of course if you were inside you would not hear the whoosh, the boom, nor the guitar. I assumed for this experiment to work you would need to do a bit of wing-walking in order to listen for tunes. BTW your downvoting was worth the chance to assert a bit of humor. Thanks. $\endgroup$ – docscience Oct 28 '15 at 22:18
  • $\begingroup$ I think it depends upon where the shock wave starts to form (even if you travel just under the speed of sound, some flow lines will go supersonic around you). If your ears are ahead of the shock wave, you would not hear the sonic boom. If, for instance, your nose created the leading edge of the shock then your ears probably could hear it. Note that all of this is ignoring the multiple shocks and sound waves produced by the multiple changes in cross-section of a human head and that air would be passing very quickly past the ear canal... $\endgroup$ – honeste_vivere Oct 29 '15 at 1:07

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