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Shattering a Glass with Sound

Quicktime movie of the glass shattering demonstration (1 MB)

The "resonant" or "natural" frequency of the goblet can be determined by "pinging" it and listening to the pitch of the "ringing". By using a microphone and oscilloscope we can determine this resonant frequency quite accurately. The goblet shown in the movie had a resonant frequency of about 800 Hz (that is, 800 "cycles per second"). It is a rather ordinary goblet, costing about $6.

We placed the goblet near a high quality loudspeaker that was playing a note that very nearly matched the goblet's resonant frequency. The volume on our audio amplifier was turned up rather high. (The sound is annoyingly loud, but not painfully loud.)

The movie is not recorded in slow motion; it is a "real-time view" of the demonstration. We used stroboscopic illumination (a very short pulse of light that flashes approximately one per cycle) to slow down the apparent motion. The apparent frequency of the goblet's vibration is equal to the difference between the goblet's actual frequency of vibration (which is about 800 Hz) and the frequency at which the strobe is flashing. In the sequence shown in the movie, the actual vibration frequency of the goblet is changing slightly because the note being played by the loudspeaker is gradually increasing in pitch throughout the movie. This explains why the apparent frequency of the goblet's vibration is decreasing during the movie. As the note becomes closer in pitch to the goblet's resonant frequency, its amplitude of vibration increases, much the way the amplitude of a swing's motion increases when you pump your legs in synchrony with the swings's resonant frequency. When we have tuned the note that the loud speaker is playing to be sufficiently close to the goblet's resonant frequency (within about 0.5 Hz), the amplitude becomes big enough to cause the glass to shatter.

As you watch the movie it may amaze you to consider that a "rigid" piece of glass is able to undergo the motion you see 800 times a second for several seconds before shattering!

Viewing tip: Try moving the slider slowly back and forth so that you can see a "frame by frame" view of the goblet repeatedly shattering and reconstructing.

Robbie Berg / Wellesley Physics Department / / March 1, 1998

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