Pub trick -- musical wineglass (do you need a gift idea for the season?)
Jearl Walker www.flyingcircusofphysics.com
December 2014 For the holidays, I gave myself a gift, a musical wineglass from Uncommon Goods.
The wineglass has thin walls so that when I rub a wet finger around the rim, I can easily cause the wall to oscillate, which sends out a sound wave. The frequency of the oscillations (and thus of the sound waves) depends on the depth of wine or any other liquid in the glass. In general, adding more wine makes the oscillations more sluggish and thus lowers the frequency.
Of course, other thin-walled wineglasses will do the same but the delight of this particular wineglass is that marks have been put on the wall showing me what the wine level should be in order to get a particular note. For example, in my photograph, I have added enough liquid (stout, not wine, for me) to the level marked G# so that the oscillations produce a G-sharp note.
Here is a link to a video produced by Uncommon Goods to demonstrate their wineglass musical capabilities.
https://www.youtube.com/watch?v=BGrEjjNi8Pg Jingle Bells
From The Flying Circus of Physics book, here is my explanation for the physics of a musical wineglass. As your finger rubs against the rim, the finger and rim are continually undergoing sticking and slipping. During the sticking phase, the rim is pulled very slightly in the direction of your finger’s motion, distorting the rim’s shape. During the slipping phase, the rim breaks free of your finger and attempts to regain its original shape, but it ends up oscillating. The strongest oscillation is said to be resonance, in which the rim oscillates as shown in the overhead view of this figure:
Here is a slow-motion video showing the rim oscillations being driven acoustically by intense sound waves instead of a rubbing finger. Eventually the oscillations are large enough to break the glass.
In playing a wine glass, the oscillation pattern follows your finger around the rim, producing a pulsation to the sound (it comes and goes with a frequency of a few hertz, depending on the speed of the finger on the rim). The frequency at which the rim pushes on the air and the frequency that you hear are roughly proportional to the rim thickness and inversely proportional to the square of the glass’s radius at the open end. Thus, generally the frequency is higher for a thicker rim and smaller radius. If you add liquid to the glass, you lower the resonant frequency because the liquid’s mass decreases the rate at which the glass wall can oscillate.
Some musicians are skilled at playing music on an array of glasses containing various levels of liquid. Here is my favorite example:
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
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