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I Maximum Amplitude of Sound Wave?

  1. Jun 19, 2017 #1
    This animation demonstrates a longitudinal wave by means of moving bars.



    I realized that if we increase the amplitude of the wave, the bars will eventually start passing through each other, which sounds (no pun intended) like an unphysical scenario.

    Does this mean that there is a cap, a maximum amplitude to this simulation? Is there a real world equivalent to this, a maximum loudness?
     
  2. jcsd
  3. Jun 19, 2017 #2

    mfb

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    The bars are just for illustration, they don't have a width.

    There is no relevant upper limit for air pressure, but there is a lower limit - zero. Once the sound gets too loud, you don't get classical sound (with symmetric pressure changes) any more, but you can still have shockwaves (very large pressure). This limit for normal sound would roughly correspond to 190 dB.
     
  4. Jun 19, 2017 #3
    Are you referring to the width of each bar or the spacings between the bars?
     
  5. Jun 19, 2017 #4

    mfb

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    The widths of the bars.
     
  6. Jun 19, 2017 #5
    Ok, I'd assumed that the bars were just lines of zero thickness.

    But when they oscillate at a large enough amplitude, in the simulation, they pass through one another.
     
  7. Jun 19, 2017 #6

    mfb

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    No they do not. At least not if the animation is good. In the video they don't pass each other.
     
  8. Jun 19, 2017 #7
    When there are no waves, the lines are evenly spaced, with a default spacing width.

    Now a longitudinal wave starts. The frequency and wavelength remain fixed.

    But if we keep increasing the amplitude of each bar's oscillation around its default position, wouldn't they collide with each other eventually?
     
  9. Jun 19, 2017 #8

    mfb

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    Well, eventually you'll run into the same problem as with air. You would need regions of 0 bar density, and that you cannot have with this bar model.
    As long as the bars properly show the motion of air, they don't collide.
     
  10. Jun 19, 2017 #9
    What happens when we reach that situation with air in real life?
     
  11. Jun 19, 2017 #10

    mfb

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    See above, you get shockwaves.
     
  12. Jun 19, 2017 #11

    Tom.G

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    Those bars in the animation represent air density, not necessarily air movement. When the amplitude passes about 190db, as @mfb pointed out, the simulation would/should show NO bars at the troughs. As the amplitude increased above 190db, the width of the vacuum region (region with no bars) would widen, approaching 1/2 wavelength at the limit. Of course the real-world details may vary a bit, it is hard to get a perfect vacuum.:rolleyes:

    See, starting around the 1 min. mark:
     
  13. Jun 20, 2017 #12
    In that case, is it correct to say that while the pressure increases by large increments, the displacement amplitude (units of length) only increases by tiny increments?
     
  14. Jun 20, 2017 #13
  15. Jun 20, 2017 #14

    Tom.G

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    Okay, I would call that a valid conclusion. For a different physical example see the below video. The central particles barely move but the pressure (force) on the central ones obviously has to increase substantially at certain moments.

     
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