Hello Stackexchange people,

I have question concerning the singing glass:

Is the audible sound that is heard the same as that of sound in air (around 340 m/s) or is the same as sound in glass?

To make my question sound clearer, after sliding the finger, the resulting wavelength is that of sound wave in air or glass?


The speed of sound in the glass doesn't play any direct role in this case.

The glass is producing sound by means of the standing wave that forms in its rim, and these waves' properties are a function not only of the material (with its elasticity and sound speed), but also of its geometry: thickness, shape, size, perhaps amount of liquid in the container, etc.

So, from the recorded frequency $f$, you obtain neither the speed of sound in the glass, nor in air, but the speed of the wave in the glass rim. And even for that, first you need to find out which mode has been excited by the sliding finger. Oversimplifying a bit, that means finding what's the number $n$ of wavelengths in that rim: then $\lambda=\pi r/n$, and $v= \pi r f/n$, but probably exciting the fundamental mode ($n=1$) is the easiest.

For more details, there are of couple of easily found papers with measurements and more realistic modeling, such as Jundt et al. Vibrational modes of partly filled wine glasses (e-print).

  • $\begingroup$ Maybe one can use the patterns formed on the water inside the glass or, more doable, if you have access to a high-frame-rate camera, analyze a slow motion video of the glass. As for the second question, that's the main point of the answer: no. Consider waves in a rope: their speed varies with the tension on the rope and linear mass density, while the speed of sound is little affected by it $-$ besides, the waves are transversal, while sound waves are longitudinal, so they're certainly not the same thing. $\endgroup$ – stafusa Dec 19 '17 at 8:58
  • $\begingroup$ Good question, I'm not sure it can be answered without more detailed knowledge of the system, but my guess is that it isn't so simple: the first peak gives you the fundamental frequency of the sound, which doesn't mean the glass is vibrating in its fundamental frequency. Besides, given that's a nonlinear system, I think the existence of subharmonic resonances cannot be discarded. $\endgroup$ – stafusa Dec 19 '17 at 9:10
  • $\begingroup$ BTW, the equations become much easier to read, search and edit when mathjax is used. It'd be great if you could use it in your next posts. $\endgroup$ – stafusa Dec 19 '17 at 9:12
  • $\begingroup$ Oh this is more complicated than I thought. Then the best solution for this would be the high speed camera, right? $\endgroup$ – Refu1 Dec 19 '17 at 9:12
  • $\begingroup$ Yes, I'd say so. $\endgroup$ – stafusa Dec 19 '17 at 9:12

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