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Homework Help: Standing waves in stringed instruments

  1. May 14, 2010 #1
    1. The problem statement, all variables and given/known data
    Hi, I'm currently doing a project on the physics of music, more specifically physics of stringed instruments. I don't really understand how are standing waves generated and used in stringed instruments. Are the standing waves present in the string, in the sound wave, or both?


    2. Relevant equations



    3. The attempt at a solution
    Just to confirm, the wave travelling along the string is a transverse wave right? And isit right to say that the standing wave in the string causes the string to vibrate? I haven't been able to find much information on this, hope someone can enlighten me:) thanks.
     
  2. jcsd
  3. May 15, 2010 #2

    diazona

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    I assume you've seen a string instrument play, right? The pattern of motion that you see the string undergoing when it's producing sound is a standing wave. You're correct that it is a transverse wave, since the motion of the string is perpendicular (transverse) to the string itself.

    Basically the way string instruments work is that when the player draws the bow across the string, the bow exerts a frictional force on the string. This pulls the part of the string that the bow is touching sideways a little bit. Then that part of the string pulls the parts adjacent to it sideways, and those parts pull the parts adjacent to them sideways, etc. etc. This is a traveling wave. (For another example, imagine - or better yet, actually try this - holding a string a few feet long and shaking it slightly side to side. You'll see a wave traveling down the string, although unless you have the string pulled tight, it won't travel very far) Now, when the wave gets to the end of the string, it can't go any further, so it bounces back. Of course, the bow is still pulling on the string this whole time, so when the wave bounces back, it meets another wave that's on its way up from where the bow is pulling the string. The two traveling waves going in opposite directions make a standing wave. For some more explanation of this, you can look at Wikipedia.

    Anyway, the net result of all this is that when you play a string instrument, it doesn't take very long at all to create a standing wave on the string. Remember, this means the string is vibrating side-to-side all the way along its length. (unless there are nodes at certain points on the string - that's something for you to look up) These vibrations cause the string to bump into air molecules, which creates pressure waves, a.k.a. sound waves, that travel through the air much the same way that the waves of vibration travel along the string (although the waves in air are longitudinal, not transverse). That's how the sound reaches people's ears.
     
  4. May 15, 2010 #3
    Thanks for the explanation. However, I forgot to say that the instrument I am working on is something like a guitar, and everytime I pluck it it only produces one wave, and there is no wave to meet with the reflected wave, so how does the standing wave come about? Is it because λ=2L, so the standing wave consists of the first half of wave (reflected) and the second half of the wave?
    I still have some other queries:
    -Does a standing wave cause the wave to "last longer", hence disturbing the air molecules around for a longer period of time, sustaining the sound? If so, is this what resonance is all about?
    -I have read that in musical instruments, instead of cancelling each other, the superposition of the two waves are supposed to increase the amplitude. Will the cancellation of waves ever happen in a string where both ends are held?
    -Does changing the tension of the string have anything to do with changing the number of nodes in the standing wave, or harmonic?
    -Is it true that the more nodes there are, the more waves the are hence having a higher pitch?
    Thank you very much:)
    (By the way, I am new here, so am I posting in the right forum?)
     
  5. May 15, 2010 #4
    whoops sorry, ignore what I asked about resonance.
     
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