PURPOSE: To break a glass beaker with a sound wave.
DESCRIPTION: An audio oscillator and 100 Watt power amplifier are used to drive a heavy-duty horn driver which is mounted in the back of the plastic beaker cavity with the sound emerging through a hole, which can be seen in the photograph. The beaker is positioned on a foam pedestal in front of the speaker hole. A microphone is mounted at 90 degrees from the position of the speaker.
With the sound at some intermediate level, the resonant frequency is found by sweeping the frequency of the oscillator very slowly past the resonant frequency of the beaker. The resonant frequency of the beaker, typically about 900 Hz, can be found by gently tapping the rim of the beaker. Because the oscilloscope displays the sound sensed by the microphone, but is triggered by the sound going to the speaker, both the increase in amplitude and the phase change at resonance are easily observable. The motion of the beaker can be nicely seen using a stroboscope, and may be displayed for a large group using the TV camera mounted directly above the beaker.
After the resonant frequency is found the amplitude can be turned up, causing the oscillation of the beaker to exceed its elastic limit and thus to shatter.
SUGGESTIONS: This is a very dramatic demonstration, and may be used at a number of levels.
REFERENCES: (PIRA 3D40.55) See Demonstration Reference File for information on the theory of this demonstration as well as information on how our device was constructed.
EQUIPMENT: Beaker with oscillator, 100 watt monaural power amplifier, JBL2445 120 Watt 16 ohm horn driver, microphone, stroboscope, TV with monitor, and oscilloscope.
SETUP TIME: None.
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