What is the physics involved with breaking glass with your voice?

Asked by: Grace Jackson


Ella Fitzgerald breaking a crystal glass with her voiceWhile breaking glass with sound is an interesting trick, it really takes a lot of effort to get the job done. First off, the glass in question needs to be crystal, not just any glass. Crystal, resonates all at one particular harmonic frequency. Try getting a crystal wineglass, dipping your finger in water, and rubbing it gently around the rim of the glass. Your finger will vibrate on the glass, and once you get it right and consistent, the glass will 'hum'.

Pitchfork mounted on a resonator boxWaves of any sort set up sympathetic vibrations in the materials they impinge upon, which is the principle behind many many things, including telephones and radar. The glass hums (and doesn't shatter) because the amplitude of the waves (the actual physical displacement that creates the sound) is not sufficient to surpass the strength of the glass. If you generate a sound that is the same frequency the glass resonates at, after you turn off the sound, the glass will hum a little, much like a pitchfork. To break the glass, you need to broadcast not only a sound that is *just* the right frequency, but also has a high enough amplitude (loudness) to exceed the strength of the glass to resist those vibrations. When the sound gets too loud for the glass to vibrate, it shatters the glass. I HIGHLY recommend asking a teacher about this, since doing experiments with crystalware at home is not likely to win much support whoever owns the crystal.

Further reading: Breaking a Wine Glass With Sound

Answered by: Frank DiBonaventuro, B.S., Air Force officer, Tinker AFB, OK.

Any physical object has frequencies at which they naturally vibrate, known as resonance frequencies. If you flick a crystal wine glass with your finger, you will hear a fairly clear tone as the glass vibrates, causing waves of air pressure to emanate out from it, which your ears and brain interpret as sound. The sound gradually gets quitter and dies out as the amplitude of the vibrations diminishes due to energy being carried away by the sound waves.

Your voice is also a series of air pressure waves, with the pitch related to the frequency of the waves, and the volume related to the amplitude of the waves.

Now to fit these two together. If you can match the pitch of your voice to the resonant frequency of the glass the vibrating air will start the glass vibrating too. If you can do this with sufficient volume, the glass will try to move in its vibration farther and faster than the material in the glass is able to move, and the glass will break under the strain.

This is an example of a driven oscillation. Imagine pushing a friend on a swing: after one big push, the swing slows down, but continues oscillating for a while with a given frequency. If you impart randomly timed pushes to your friend, you are unlikely to get her moving very much. But if you carefully time your efforts so you administer a small push at the same time in each cycle, your efforts can add up and your friends amplitude (height) will increase a bit each time. Likewise, the sound of your voice administers hundreds of tiny 'pushes' each second to everything around you. If the timing is right, the energy of the pushes can add up, leading to a large stress on the glass.
Answered by: Rob Landolfi, Science Teacher, Washington, DC