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Thread: speed of sound

  1. #1 speed of sound 
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    Ok... so your in a supersonic plane... why is there a "sonic boom"? how does it form? when does it actually go "boom"?... and to make this thread even dumber: can you hear yorself talk in a supersonic plane? :P


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  3. #2  
    Universal Mind John Galt's Avatar
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    The air has to move out of the way of a moving object. It cannot move out of the way any faster than the speed of sound.
    When an object it moving through the air faster than sound it arrives at a block of air before the air can 'know' it is coming. This creates a shock wave. The sonic boom is this shock wave and is heard anywhere along the flight path of the plane that it can reach. [When I was young they flew Concorde test flights up the West coast of Scotland. We would often here its double boom as it went overhead. (Please don't ask me why it was a double boom)]
    Inside the plane the air and you are not moving relative to each other, so there is no problem in hearing.


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  4. #3  
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    so if you were "on" the plane you wouldn't hear anything, right?
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  5. #4  
    Universal Mind John Galt's Avatar
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    You wouldn't hear the sonic boom. Other than that it gets complicated. [That's another way of saying: I don't know. You've stumped me.]
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    Forum Radioactive Isotope mitchellmckain's Avatar
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    The real physics may be more complex as implied by Ophiolites post but just looking at sound waves alone you can see that something "explosive" has to happen. When the plane is near the speed of sound, the sound can't get away from the plane but must keep building up near the plane so the sound from the engine over a longer period of the flight hits us all at once. It shows up in the equation for the doppler shift of the frequency.

    frequency heard = frequecy emmited x ( vs + vo)/(vs - vn)

    where vs is the speed of sound ~ 341 m/s, vo is the velocity of the observer and vn is the velocity of the noisemaker. This equation assumes that the observer is in front of the plane. If the vn is equal to vs then you are dividing by zero which means the frequency blows up - boom.

    Now sound is composed of pressure waves and therefore it is not surprising that other pressure wave affects (there is bow wave similar to the bow wave due to a ship in the water) contribute to a sound-effect when the speed of the plane approaches or is in excess of the speed of sound.
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

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  7. #6  
    Universal Mind John Galt's Avatar
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    It is sort of off topic, but about five years ago, returning to Europe from Houston we had some really strong tail winds in the jet stream. As a consequence our ground speed was supersonic, around 720 mph. The fastest I have ever travelled.
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    Forum Junior Cuete's Avatar
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    Quote Originally Posted by F (i L i) P
    so if you were "on" the plane you wouldn't hear anything, right?
    I don't know if I got right the question, but if you were on the plane you don't hear the sonic boom at all, but you will perfectly hear what's happening inside the plain (conversations, et cetera), as nothing is going on.
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  9. #8  
    Universal Mind John Galt's Avatar
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    I think he meant on, that is outside, as opposed to in, that is inside. That was how I took it. Obviously a thought experiment rather than a practical one.
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  10. #9  
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    Quote Originally Posted by Ophiolite
    I think he meant on, that is outside, as opposed to in, that is inside. That was how I took it. Obviously a thought experiment rather than a practical one.
    yeah ... that's what I meant... just like YOU could fly (on your own) at supersonic speed...
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