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Wine glass sound physics behind

  1. Jul 2, 2014 #1
    when you slide your thinger around the rim of glass it produces resonance

    the matter which vibrates is a glass, not air, right?

    i understand that when ou hit a glass once it produces sound as by hitting you displace mass of glass from its equilibrium.

    but why sliding the rim produces sound? if it is due to displacement caused by weight of the finger placed on the rim, then why sliding the wall of the glass does not produce sound?

    i have read previous threads, but the topic is still unclear.

    thank you
     
  2. jcsd
  3. Jul 2, 2014 #2

    A.T.

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    Both. If you can hear the sound the air vibrates. The glass vibrates as seen here:

    https://www.youtube.com/watch?v=iak1VuJyyoM

    It is the friction between the surfaces.

    It does. But putting the finger on the wall it might dampen the oscillation more than on the rim.
     
  4. Jul 2, 2014 #3

    olivermsun

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    An important feature of a lot of these musical "instruments" is actually a kind of stick-slip motion (typically aided by, e.g., a wetted finger) that needs to be "synchronized" with the natural resonance in the system.
     
  5. Jul 2, 2014 #4

    A.T.

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    Yes, you need a regularly oscillating friction force, which happens when static and sliding friction alternate due to elastic interaction. And then you have to move the finger at the right speed to get the frequency right.
     
  6. Jul 2, 2014 #5
    very useful answer, A.T.

    However, if it is a matter of friction, does this imply that rubbing one single spot on the rim will also produce resonance? or it is due to the fact that keeping the finger at one spot will damp produced by this spot oscillations the finger should move?

    upd. i guess it is answered in the previous post, did not see it before posting

    if oscillations produced by alternating static an moving friction, then as far as i know static friction occur until object is displaced, therefore in a moving finger case, alternation between static and moving friction should not occur as once finger is displaced there is only moving friction afterwards?
     
    Last edited: Jul 2, 2014
  7. Jul 2, 2014 #6

    A.T.

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    The finger is not a rigid body. The skin at the contact patch deforms.
     
  8. Jul 2, 2014 #7

    olivermsun

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    In studies of bowed instruments, what you find is that the string gets deformed by the moving bow during the "sticking" phase — until a point where the string "slips" and snaps back — and then it sticks once more and the cycle repeats. This is how the string "oscillates" even though the bow is moving in a single direction, and it results in a pattern of string displacement that's actually more of a sawtooth than a smooth wave. Interestingly, the speed of the bow doesn't change the resonant frequency of the system — it only affects the amplitude of the disturbance.

    So I imagine something analogous is happening with the wineglass. The wineglass itself is deforming and snapping back rapidly as your finger moves along the rim!
     
    Last edited: Jul 2, 2014
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