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Wineglass singing vs. pinging

  1. Jun 25, 2015 #1
    I recorded the sound spectra of a singing wineglass (rubbed the rim with a wet finger) and a pinging wineglass (tapped anywhere at its wall) with a smartphone. Evidently the fundamental tones of both vibrations are identical, but the overtones are very different (harmonic vs. anharmonic). As the fundamental tones are identical it seems fair to assume that the singing and the pinging vibrations have a lot in common. Then why is pinging unable to excite the singing-overtones, and vice versa?

    Last edited: Jun 25, 2015
  2. jcsd
  3. Jun 25, 2015 #2
    'cause you have your finger on it? I am just speculating here as I am no vibrations guy...but when you are singing, not only do you have continuous excitation, but also have your finger right on the glass itself changing the system a bit one way or another (stiffness?), maybe your finger acts a bit as some kind of anchor....when you ping, you do not have your finger on the glass and vibrates "freely".
  4. Jun 25, 2015 #3


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    Rubbing and tapping are orthogonal (One along the rim and one right angles to it). Perhaps they don't excite the same modes?
  5. Jun 25, 2015 #4


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    One could check the singing, just after taking off the finger.
  6. Jun 25, 2015 #5
  7. Jun 25, 2015 #6


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    I notice that for "singing", every harmonic is present. This seems correct for flexural vibration of a ring. It is similar to the electrical resonance of a metal ring, where the lowest frequency corresponds to one wavelength circumference. On the other hand, the striking method has only odd harmonics, so it looks as if an antinode is forced on the system, like a shorted transmission line. But the fundamental frequencies are similar in both cases, so I am thinking that the vibrations in the two cases have different modes.
  8. Jun 26, 2015 #7
    The harmonic singing overtones persist for a second or so (the fundamental tone lasts longer), so my finger is required for the creation of the singing overtones, but not for their persistence.
    Remarkably, pinging overtones grow during the first second after hitting the glass.

    Then why do they excite the same fundamental tone?

    Flexural vibrations are dispersive, high frequency waves travel faster. In a straight bar the velocity is proportional to the square root of the frequency. As a result the overtones of a straight bar would be proportional to n2. The pinging overtone frequencies of the wineglass are not odd harmonics, they are approximately proportional to n2. The harmonic spectrum of the singing overtones is harder to explain.
  9. Jun 26, 2015 #8


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    I guess the energy, initially stored as a certain deformation, is distributed across different modes of vibration.
  10. Jun 26, 2015 #9


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    In music terminology, I would say that it is a result of a different attack (much lIke different plucks elicit different tones from a guitar string vs. bowing a string). That is, the shape of the initial perturbation for a ping is one large impulse, while the driving force for a ping is constantly applied (probably a series of smaller impulse as each element of glass your finger passes over is released).
  11. Jun 26, 2015 #10
    You are comparing it to a string which is fixed at both ends. The different tones of a string are harmonics of the fundamental, irrespective of the attack. But flexural waves in a string are something else than flexural waves in a rod (nondispersive vs. dispersive).
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