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Crazy title. This is out of interest sakes, and maybe a stupid question:

Theoretically, if one where to travel at the speed of sound with no air/wind (not a vacuum as there would be no sound) what would sound sound like? Imagining the sound propagates in a straight line and there are no reflections. Obviously it depends on the sound, but would one hear a sort of segment of time stuck (like a granular synth) or maybe some very slow oscillating and modulating mesh mash of frequencies?

Thinking hats on...

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up vote 11 down vote accepted

Ok, let's say, for my explanation's sake, that the sound can only travel along one plane (like you suggested in your question). So only towards you or away from you. Let's also assume this is happening at "room" temperature (68F) in dry air. Therefore, it is assumed that the speed of sound will be 767mph (or 1125ft/s or 343 m/s)

Say a sound is coming straight towards you. It will get to you at a rate of 767 mph. If you go straight towards the sound at the speed of sound, it will get to you twice as fast (343m/s + 343m/s = 686m/s), and you would get an amplified doppler effect.

Take a look at this equation to figure out how much the doppler will effect your sound:

alt text

Where: (f) is the observed frequency (what you hear), (f0) is the emitted frequency (what the original sound is), (V) is the velocity of waves in the medium (343 in this case), and (Vr) is the velocity of the receiver relative to the medium; positive if the receiver (you) is moving towards the source.

So, for a 1k tone: f = ((343 + 343)/343)1000

So f = 2000hz (or 2kHz). So, basically, if you travel at the speed of sound directly at a sound, everything will be exactly one octave higher.

However, if you travel at the speed of sound in the opposite direction of the sound coming towards you, the sound will never get to you, hence, you would never hear it, which is good, because I don't want to fight with any more equations...

But......If you were to go JUST SHY (say by 3m/s for this example) of the speed of sound in the opposite direction of the sound, it would look like this: (same equation)

Again, for the 1k tone: f = ((343 + (-340))/343)1000

So f = 8.746hz. Basically, your 1k tone would cease to be in your hearing range. Sound would sound like it's slowing down until it stops as you approach 343 m/s in the opposite direction of the sound.

So yes, in answer to your question - it would sound very strange and slow and all, because the sound would be arriving to you very slowly...

NOW, if you are currently going the speed of sound, and you make a sound, that sound will now be going 686 m/s relative to ground. So, anyone that hears you speak while you're going that speed will hear you at exactly 1 octave higher (unless they themselves are moving).

Even with reflections and all, this pretty much stays the same. However, temperature and medium (air, humidity, water, steel, etc...) would change the sound more drastically as the speed of sound would change.

Cool question! Let me know if you need me to clarify anything!

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Awesome. Totally awesome. – Matt Cavanaugh Jun 1 '10 at 6:47
    
What an answer! Thanks. – Andrew Spitz Jun 1 '10 at 13:01
    
Thanks! It was fun! I don't get to exercise that part of my brain as much as I'd like to... We should get more questions like this going! – Colin Hart Jun 1 '10 at 13:56
    
Check out the big brain on @Colin. Nice explanation! – Steve Urban Jul 29 '10 at 14:49

This is the extreme limit of the doppler effect. You hear "sound" as the changes in pressure of the sound wave. If you are travelling along with the sound wave, then you won't get any changes in pressure so you won't hear it.

One way to conceptualize it is that sound waves are like the grooves in a record. The normal situation is you (the needle) are standing still and the sound wave (record) travels past you at a fixed rate (the speed of sound in one case, the speed of the record in the other).

Now imagine moving the needle along with the record - at the "speed of sound" as it were. This is what would happen if you travel along the path of a sound wave.

Imagine the other way - move the needle in the opposite direction to the spinning record, and at the same speed. In this case, the effective speed of the record is twice as fast, and would sound an octave higher.

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really nice way of explaining. – Andrew Spitz Jun 1 '10 at 13:03

If there's no wind, then the air must be moving at the speed of sound alongside you, which would mean that you would hear in it normally, like the air carried in the cockpit of a supersonic plane.

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People do travel at the speed of sound (eg Chuck Yeager) so this really isn't an abstract question... In this article Mano Ziegler describes "transonic buffeting" which relates to the extreme forces on the plane:

http://en.wikipedia.org/wiki/Sound_barrier

But the fact you specify "no air/wind" means it isn't in a plane, in which case I think the sound you would hear, if you were truly ready to hear it would be "aum"

"the sound that is not made by two things striking together."

http://www.spiritsound.com/aum.html http://www.messagefrommasters.com/Stories/Zen/Sound_of_one_Hand_Clapping.htm

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In the special case that we are travelling with the sound source (e.g. in a plane), when we reach the speed of sound, the sound waves in the same direction will not be able to move away from the source. At this point, the sound waves will overlap which will cause accumulation of a lot of kinetic energy and resonance at every frequency in the sound. It will sound like a rumble noise and the accumulated kinetic energy and resonances should shake you like a turbulence.

In reality, it is not possible to go exactly at the speed of sound longer than a moment so every time you pass the limit upwards or downwards you should experience that rumble. If it were possible to go constantly at the speed of sound you should experience a continuous rumble and resonant vibration.

This is a special case and an addition to the other answers.

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I believe Colin Hart's very excellent explanation is false in the second-to-last paragraph, beginning "NOW, ". If I am moving at the speed of sound and make a sound, the sound does NOT move at 686 mph relative to anything. The speed of sound is a property of the medium through which it propagates. It will always propagate at 343 (in this example). If the sound source is moving through the medium at 343, then the sound energy merely piles up in the direction of travel, leading to the sonic boom phenomenon described very accurately by Guney Ozsan.

Try this thought experiment:

I am standing on the western shore of the Dead Sea, motionless relative to the sound-bearing medium, that is, the atmosphere. A fighter jet (a sound source) patrolling the border approaches me at Mach 1.5, that is, at 1.5x the speed of sound in this medium. The source approaches faster than the sound it produces, and therefore it is silent: I can see it, but not hear it, coming (true example). As described above, the sound energy "piles up" in the vicinity of the source, and in a cone-shaped wave front propagating away from and behind it. When the aircraft passes directly overhead I still don't hear anything until that wavefront hits me. Then -- BOOM! The phenomenon is aptly named; it really does sound like the word BOOM! After the BOOM wavefront passes, I am now within the "cone". I am looking at the aircraft receding toward the horizon. It produces sound that travels toward me at 343 even as the source recedes at 343 x 1.5 = 514. But the sound really does still approach me at 343. All that happens is a dramatic example of the Doppler Effect. All the frequencies of sound emanating from the aircraft are reduced by a factor of 1.5 relative to what they would be were the sound source standing still in the medium. Thus, if the jet engine is producing a whine at 12kHz, I will hear a roar at 12 / 1.5 = 8kHz.

The pilot, meanwhile, is sealed into the cockpit with a little piece of local atmosphere. He, and his MP3 player, are motionless relative to the medium, so the music he hears sounds "normal". The jet engines a few feet behind him, however, are inaudible. The sound they produce cannot catch up to him through the outside atmosphere; he is outrunning the wavefront.

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I think your right too, but what happens after the source has passed, after the boom? is anything heard? – Marc W Mar 8 at 4:16

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