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I How do standing waves continue propagating?

  1. May 23, 2017 #1
    In a sound wave, air is being compressed and decompressed. If sound is reflected at the end, then to create a standing wave the compressed layer of air coming back must coincide with another compressed layer of air going forward.

    If that's the case, how can the compressed airs continue to travel in their respective directions after they hit one another? Wouldn't they just stop moving once they hit each other?

    Same for the string. If the original wave starts pointing up, then the reflected wave points down. But wouldn't each point at which the original and reflected wave coincide pull on the same part of the string just as strongly but in opposite directions, and therefore the string would just be flat?

    In this image: http://imgur.com/a/1d2f3

    The wave going to the right will meet the wave going to left. The right side of the right-traveling wave will go up, and the left side of the left-traveling wave will go down. When they meet, they will be pulling on the same parts of the string. So wouldn't the up and down forces cancel to give a string that's flat everywhere?
    Last edited: May 23, 2017
  2. jcsd
  3. May 23, 2017 #2


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    Good question because it doesn't make sense at first sight. However, sound waves have varying pressure and they have displacement at any point. The phase of the pressure is 90° out of phase with the displacement. So, although the pressures may cancel, the displacements add together and both waves 'exist'.
  4. May 23, 2017 #3
    But won't the displacements cancel each other out since they meet together with the same force?

    Is it possible that the reflected compressed air molecules only coincide with MANY of the original compressed air molecules, but not all? So some of the reflected molecules are able to pass through the original sound by not being hit, then hitting the still air molecules on the other side?
  5. May 24, 2017 #4


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    some weird comments in there, I suspect you don't really understand how waves interact
    have a watch of this classic AT&T - Bell Labs video, it covers many wave properties including standing waves

  6. May 24, 2017 #5


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    Yes, I said they can in my earlier post. But I also made the point that there is displace AND pressure involved (as in all waves). "Meeting together with the same force" is a bit of a meaningless statement. The pressures can cancel out but the displacements do not. There is still Energy travelling in both directions.
    You need to understand more about waves before you can come to valid conclusions about how they should behave. No kind of wave has only one variable involved. Strings have stored energy in the tension and kinetic energy of motion, sound / pressure waves have pressure and displacement and EM waves have magnetic and electric energy. You can reduce one variable to near zero but the energy still exists due to a larger value for the other variable quantity.
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