A few weeks back we looked at the phenomenon of resonance with oscillating metronomes. As a follow-up to that meditative and Zen-like video, we've included a crystal-clear demonstration of that favorite old opera singer's trick: shattering a wine glass with resonance.
You can't just sing any old note if you want some breakage. As the video illustrates, you have to find the resonant frequency of the glass: the frequency at which the glass naturally vibrates. That's easy to do: just tap the glass and listen to (and/or record) the pitch at which it rings. The pitch that you hear corresponds to the vibration frequency -- the higher the pitch, the higher the frequency. The volume that you hear, contrary to a common misconception, is not related to the frequency; rather, it is due to the amplitude of the sound wave.
To get our wine glass to break, we want to create the largest possible amplitude. Now, if you force a glass to oscillate continuously by allowing it to be affected by an external vibration, waves will be generated that travel around the perimeter. Waves of most frequencies will constructively and destructively interfere with each other somewhat randomly. But if you generate waves at the resonant frequency of the glass, the wave peaks will line up and augment each other so that the glass will start to oscillate wildly. All you need to do to get it shaking and breaking is find an external source of sound at the correct frequency and play it, sing it, strum it, or blow it -- with a sufficiently large amplitude.
Notice the beautiful large wobbling standing waves in the video. The points in the glass which are oscillating the most are called the antinodes of the standing waves -- where constructive interference is at a maximum. The locations that seem to be stationary are called nodes. They are experiencing continuous destructive interference. To shatter the glass, just turn up the volume until the amplitude of vibration exceeds the tensile strength of the glass. Most people don't have the lung power to do this, so if you really want to break some crockery, either use an amplifier or hire an opera singer.
Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.