C66-BREAKING GLASS WITH SOUND

C66. Breaking Glass with Sound

OSCILLATIONS - RESONANCE - Audio Resonance

Materials used:

scope (HP 54600A oscilliscope)
counter/timer (Tektronix DC504A)
function generator (Tektronix FG501A)
[The counter/timer and function generator together are housed in the Tektronix TM503A unit.]
amplifier (Adcom GFA-545 II)
speaker (Scorpion S-1025 8Ω Utility Driver) (Peavey)
microphone
strobe light
glass (e.g. Crate & Barrel wine goblet, with wide mouth $3.25)

The set-up:

The glass is taped on a block.
The tube extension of the speaker is within a few mm of the glass, and a little lower than the lip of the glass.
The microphone is placed at about the same height, 135° around the glass.
The trigger output of the function generator is connected to the input of the counter/timer.
The (sinusoidal) output of the function generator is connected to the amplifier, which is in turn connected to the speaker. The -20dB button on the function generator should be pressed so the maximium input of the amplifier is not exceeded. *NEVER set function generator at -0dB; speaker and amp will blow!
The output of the microphone is connected to the scope.
The strobe light is about 6" from the glass, pointing down on it.

To determine resonant frequency of the glass:

Wet your finger, and rub it around the rim of the glass, while pressing down firmly on the base of the glass. This takes a little practice, and you might have to wet your finger several times. In the meantime, the scope should be continually measuring frequency. These values can vary quite a bit, but they generally average with 2-3Hz of the resonant frequency. (Most glasses will probably exhibit resonance around 500-1000Hz.)
Next, set the frequency of the function generator (using the counter/timer to measure frequency, not the knob on the function generator.) The amplitude of the sound vibrations at the mike are greatest when the glass is at resonance. (This is most pronounced when the mike is suspended facing down, a few inches from the center of the glass.) Adjust the fine frequency control of the function generator until the scope indicates you are in resonance (you should be within about 0.5-1.0Hz now).
Finally, you can use the strobe, set to about 1500rpm (which is considerably less than the resonant frequency), and play with its frequency control until the vibrational mode is clearly seen.

Miscellaneous comments:

Wear earmuffs or earplugs when the speaker volume is significant.
We did not investigate higher vibrational modes. It should be possible to get a good guess for the resonance frequency by tapping the glass, and- supposedly- with practice you can excite preferentially higher vibrational modes. In any case, given the higher resonance frequency, it should be trivial to show it with the scope, or break the glass in that mode.
It is possible to show the exponential decay of the amplitude of vibration after tapping the glass. Be aware that initially there are relatively rapid transients, but the fundamental mode died out with a τ_1/2~0.5-1.0 sec. on the cheap glasses we tried.
Finally, better glasses, with higher Q, might break at lower volumes.

Location: Room 26-001, Loft