Saturday morning cartoons would have us believe that anyone in a dress and Viking helmet can shatter wine glasses, monocles and even the opera house chandelier with one powerful high note. Is this one of those skills that only cartoon characters possess, or can we mortals do it too?
The physics seem pretty straightforward. Every object has a resonant frequency "“ the frequency at which it naturally vibrates. Finding the resonant frequency of a wine glass, which is especially resonant because it's tubular and hollow, is easy. Running a wet finger around the rim and making it "sing" or simply tapping the glass will cause it to vibrate at its resonant frequency (which varies from glass to glass) and push out waves of air pressure. Our ears and brain interpret these waves as sound, with the pitch of the sound determined by the frequency of the waves.
If you've got a good ear or the appropriate computer program, you can find out what that note is and match the pitch of your voice to the to the glass's resonant frequency. Singing that tone will get the air around the glass, and then the glass itself, vibrating. And if you can sing loud enough and long enough, the glass will basically vibrate itself to death.
Loud and long are important for glassware destruction. The volume of a sound is related to the amplitude of the sound wave and the extent that it displaces air. The louder the tone, the harder you're pushing air at the glass. Sustaining the note allows the vibrations to build up enough to cause the glass to fracture. Plain old luck also factors in, since the size and location of microscopic defects in glasses vary and some glasses can withstand more tonal punishment than others.
In 2005, the guys on MythBusters brought us the first recorded and confirmed proof that an unamplified voice can break glass. Rock singer and vocal coach Jamie Vendera had the right frequency (556 Hz) and, after 20 attempts, worked up the volume (105 decibels) needed to needed to shatter a glass. (Here's a link to the MythBusters segment.)
And here's a Popular Science video that has a wonderful strobe-light lit shot of the glass shucking and jiving to the resonant frequency.
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