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Frequence of resonating wine glass

  1. Dec 10, 2006 #1
    Rubbing your finger around the rim of a wine glass produces a fine tone.It seems that my litle kitchen-table experiment produced 1000 Hz at a higt amplitude and the first overtone 2000 Hz had a small amplitude, 3000 Hz was higher and so it seems wiht 5000 Hz.
    So the even overtones are suppresed just as in a half-open resonant tube (a pan flute)

    Here is my question :
    Why is the frequency lowered (to about 500 Hz with the glass almost full of water) instead of increased when I poor water in the glass. It should be increased as the air column is shortend ? or ?
    Can anyone help ?

    -ZeroGravity
     
  2. jcsd
  3. Dec 10, 2006 #2

    Gokul43201

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    The water plays another dominant role: it affects the mass. How does the natural frequency of a harmonic oscillator depend on its mass?
     
  4. Dec 10, 2006 #3

    marcusl

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    Hmm, this differs from blowing over a coke bottle, where the height of the air column determines the resonance. I think that the glass walls actually vibrate when you rub the rim, so adding water increases the mass and lowers the tone just as rubbing a glass with thicker walls.
     
  5. Dec 10, 2006 #4
    Does this work only with crystal wine glasses, or does normal glass work as well?
     
  6. Dec 11, 2006 #5

    Meir Achuz

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    In a coke bottle with fat walls, it is the air column inside that resonates.
    With a wine glass, it is the glass that resonates. Water damps the oscillation of the glass. A cheap wine glass with thick walls would be morre like a coke bottle. Of course, the instrument, called the "harmonica" was invented by Ben Franklin. Mozart composed for it.
     
  7. Dec 11, 2006 #6
    The mass of a harmonic oscilator affects the frequency as 1/SQRT(m), this would mean that a frequency of 1000 Hz with 1 part (of say water) in the glass should be lowered to 500 Hz with 4 parts (of water) in the glass, but in my wine glass (of a typicas wine glass shape) it is only lowered to about 750 Hz. the shape og the glass might affect the effective mass, so maybe I should try disserent shapes. - other ideas ?

    - ZeroGravity
     
  8. Dec 11, 2006 #7

    marcusl

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    It works with regular glass too, although the sound is different because the material is different. "Crystal" is actually glass with up to 25% lead content. The lead makes the glass harder and less lossy; you can hear the sweet tone and long ring time (high "quality factor") of a crystal glass compared to the short tinny tone of regular glass if you rap them with a spoon.
     
  9. Dec 11, 2006 #8
    It seems to follow this behavior for cylindrical glasses of same material and thickness.
    Frequency in Hz
    Any ideas ?
    (i have tried to attach 3 graphs...first time- hope it works)

    - ZeroGravity
     

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  10. Dec 9, 2009 #9
    The glass is what is vibrating, I don't know the modes but they're probably very complicated. My guess is the water level is creating a boundary condition on the glass, below this level the oscillations are damped, and above they are undamped, so maybe you are imposing a node at the water level, and therefore changing the maximum wavelength of oscillations?

    I suggest trying a simpler case, eg. a string from x=0 to x=l where for some a,0<a<l, in the region [0,a] you have undamped motion and for [a,l] you have damped motion, then see what the conditions are at x=a.
     
  11. Dec 9, 2009 #10
    As others have said, it is the glass that is vibrating and producing the tone, not the air inside.
    I did a study on something similar to this a while back and it seems the fundamental vibration occurs when the rim of the glass vibrates such that there are 4 nodes and 4 antinodes spaced equally around it.
    The glass needs to vibrate in and out, and as such, placing water in it means that there is a greater amount of mass to be moved than if you had air in it. This results in the fundamental frequency being reduced. The more water, the more mass needs to be moved.
     
  12. Aug 25, 2011 #11
    hi im doing about this in my physics experiment for college. if you look at a bottle when you blow across the top it is the air inside the bottle that resonates - so less air causes a higher note like a shorter string on an instrument. well when you rub your finger around a wine glass it is the glass itself which is resonating so there is a lower note when the glass is full. i think its to do with the speed in which the glass resonates - this causes the frequency to change. this is because the water or liquid inside the glass slows down the speed in which the glass is able to resonate. Hope this helps!
     
    Last edited by a moderator: Aug 26, 2011
  13. Aug 26, 2011 #12

    Philip Wood

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    Suspect this involves a different mode of vibration, but it's fun, and not hard to explain...

    Tap the rim of an empty cup or mug (which must have a handle). Tap it first at the point on the rim (call it 12 o'clock) just above the handle, then at 1.30, then at 3.00, then at 4.30 and so on. In other words, advance 45° each time. No need to be precise. What do you observe about the notes sounded?
     
  14. Aug 26, 2011 #13

    Philip Wood

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    Suspect this involves a different mode of vibration, but it's fun, and not hard to explain...

    Tap the rim of an empty cup or mug (which must have a handle), with a radially-directed hit from, say, a pencil. Tap it first at the point on the rim (call it 12 o'clock) just above the handle, then at 1.30, then at 3.00, then at 4.30 and so on. In other words, advance 45° each time. No need to be precise. What do you observe about the notes sounded?
     
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