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How can shock waves travel faster than sound?

  1. Jul 9, 2015 #1

    kelvin490

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    Shock wave is caused by the disturbance of air by the airplane. When it propagate the mechanism should be the same as that of longitudinal sound wave. Why sometimes it can travel faster than sound?
    (also see: http://physics.info/shock/ )
     
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  3. Jul 9, 2015 #2

    HallsofIvy

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    Where did you get the idea that shock waves, in air, can move faster than the speed of sound?
     
  4. Jul 9, 2015 #3
    A shockwave always travels faster than the speed of sound, when it slows to the speed of sound it is then normal sound waves. I am not definite about why it is faster than sound but I imagine there is increased energy density "pushing" the sound waves faster than they travel normally.
     
  5. Jul 9, 2015 #4

    CWatters

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    If you have an aircraft travelling at twice the speed of sound the point at which sound from the aircraft reaches the ground also moves at twice the speed of sound along the ground. But that doesn't mean the sound waves from the aircraft move that fast downwards.

    PS: Waves hitting a beach at an angle also appear to travel along the beach faster than the original wave
     
  6. Jul 9, 2015 #5

    davenn

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    why seems to be hard to answer
    its just is

    this page states they always are and their speed varies with amplitude. Also states that the speed drops off quickly and its speed will lower to that of sound and the sound wave and shock wave will merge

    http://physics.info/shock/
     
    Last edited: Jul 11, 2015
  7. Jul 9, 2015 #6

    sophiecentaur

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    This is just a matter of definitions. My understanding is that the shock wave (say in front of a fast aeroplane) is only present very close to the front of the plane because the plane is actually pushing molecules forward, faster than the speed of sound. Once the shock wave has been 'shed' from the plane, you just have an impulsive disturbance (sound) wave, travelling at the speed of sound.
     
  8. Jul 9, 2015 #7

    anorlunda

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    Wikipedia has a pretty good description of shock waves.
     
  9. Jul 11, 2015 #8

    kelvin490

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    Can we say that because wave travel at the speed of sound relative to the average velocity of the medium. If the medium travels at supersonic speed the shock wave also travel that fast relative to a stationary observer?
     
  10. Jul 11, 2015 #9

    Drakkith

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    I'm not sure I understand what you're asking. Are you aware that a shock wave is, by definition, a disturbance that travels faster than sound through a medium?
     
  11. Jul 11, 2015 #10

    kelvin490

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    when I first ask this question I assume the medium doesn't have net movement and just wonder how shock waves travels faster than sound. There are different replies here and now I am not sure whether the medium itself is pushed by airplane and moves in high speed.
     
  12. Jul 11, 2015 #11

    Drakkith

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    The shock wave is the molecules of the medium (air molecules) traveling faster than the speed of sound relative to the surrounding air. When we refer to the medium, we usually mean the bulk of the material that an object or disturbance travels through. While the shock wave is composed of fast-moving air molecules, I wouldn't say that the medium is moving.
     
  13. Jul 11, 2015 #12

    sophiecentaur

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    It's just a way of looking at things, in the end but I would say that, because it is being moved / pushed along in front of the plane, it is moving faster than a sound wave would. I guess you could argue that the air that any plane encounters will return to the 'hole' left behind, as it travels through it. Drag will always cause turbulence, so the air won't return to the same place that it was originally, of course. When there is a shock wave, however, I would suggest that some air is actually being carried forward on the front of the plane because it can't get out of the way in time (as in sub-sonic flight).
    But, as in so many PF threads, we tend to argue about these things when they aren't really as relevant or useful as the description that good ol'Maths gives us.
     
  14. Jul 11, 2015 #13
  15. Jul 11, 2015 #14

    sophiecentaur

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    Well, in that case, there is a massive increase in volume and then the surrounding air comes in from the side - giving the mushroom. So the shock wave definitely does 'move' volume of air before equilibrium is established again.
    In a situation where shock waves are produced, the medium (air) is not behaving linearly so the simple speed of sound calculation wouldn't be expected to apply. When the positive pressure is greater, or close to 1Bar, the least 'negative' pressure can only be zero Bar. The tables in the reference are interesting.
     
  16. Jul 11, 2015 #15
    I can not find a good link to it anymore but what I had read before was that the supersonic overpressure was specific to a ground force effect of surface level detonations. Not certain if this might have been predated to before the H-bomb though. Imagine the explosion being a bubble, where the bottom half hits the ground and is reflected back up into the top half of the bubble. From this you have a second shock wave pushing behind the first shock wave.
     
  17. Jul 11, 2015 #16

    Drakkith

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    Saying the medium is moving, to me, implies that the air the plane is flying through is moving relative to the ground.

    As I understand it, that is exactly what is happening. The plane is flying faster than sound so the air literally cannot get out of the way fast enough and ends up being accelerated and compressed, forming a shock wave.
     
  18. Jul 11, 2015 #17

    sophiecentaur

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    . . . . and carried forward. ??
     
  19. Jul 11, 2015 #18

    Drakkith

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    Carried forward by the plane? Sure.
     
  20. Jul 11, 2015 #19

    davenn

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    not just nuclear blasts, but any blast .... its seen so often even in relatively small blasts done on programs like Mythbusters
    They often slow down the high speed cam so you can see the shockwave moving out
     
  21. Jul 11, 2015 #20

    anorlunda

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    This photo make Drakkith's words seem graphic.

    shock_wave.gif



    But photos like this make me wonder about the shock wave rapidly slowing to sonic speed. If it did so, I think the trailing front would be parabolic, not straight lines. Every photo I've seen shows the shock wave front propagating as a straight line until it intercepts the ground.

    Schlierenfoto_Mach_1-2_Pfeilflügel_-_NASA.jpg

    This drawing also helps.

    shock-waves.jpg
     
    Last edited: Jul 11, 2015
  22. Jul 11, 2015 #21

    Drakkith

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    From wiki:https://en.wikipedia.org/wiki/Shock_wave#In_supersonic_flows

    Shock waves are not conventional sound waves; a shock wave takes the form of a very sharp change in the gas properties. Shock waves in air are heard as a loud "crack" or "snap" noise. Over longer distances, a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft.

    From your picture, it looks like the overlapping of many different sound waves creates the straight lines. This would mean that the line is not a shock wave, it is just the constructive interference of many different sound waves created by the dissipating shock wave over time.

    Consider the following. As a plane travels past mach 1, the sound waves generated by the plane can no longer propagate forwards faster than the plane. So each moment in time a new sound wave is created ahead of the existing wavefronts. Each new sound wave would then overlap and interfere with the existing sound waves and create a new wavefront that travels outwards from the source. Since the source is moving, this wavefront takes the shape of a cone. And what does the point of the cone look like as a 2d shape? A triangle with straight sides.

    In other words, while a sound wave is circular in shape (spherical), the outermost edge of the overlapping pattern of many soundwaves created by an object traveling faster than sound in a medium is a straight line (aka a cone, not a sphere).
     
  23. Jul 11, 2015 #22

    nsaspook

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    The F-14 was famous for low level supersonic passes over water.
    Shock wave below the plane:


    Water vapor image of shock wave.
     
    Last edited: Jul 11, 2015
  24. Jul 12, 2015 #23

    boneh3ad

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    Shock waves can persist for a great distance coming off of an airplane depending on their strength and the ambient conditions. In the example of the second schlieren/shadowgraph, a my of those lines are definitely shocks, though some may be other types of waves that are strong enough to be seen.

    In reality, though, you have to think about this kind of differently from how some people here have been trying to think about it. The study of nonlinear wave phenomena is very interesting but it boils down to this: a shock travels into a medium faster than the speed of sound in that medium. However, the shock accelerates the gas as it passes through, and therefore the gas through which it is propagating has a nonzero velocity. Relative to the gas from which the shock is advancing, the speed is not supersonic.
     
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