Additional Info

• ID Code: H3-61
• Purpose: Breaks a glass beaker with sound
• 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.

  The beaker is marked with its primary resonant frequency, found in advance using digital spectrum analysis of a recording of the beaker ringing after being tapped. Most beakers have two possible resonant modes 45 degrees apart, due to the weight of the spout; the most effective technique is to drive the resonance with the spout facing directly away from the speaker. Set the frequency of the oscillator as shown on the beaker, with an amplitude of around 140mVpp. The oscilloscope will show two waveforms, the input signal and the signal picked up by the microphone. You may need to adjust the frequency slightly to account for changes in temperature or age since the beaker was tested; slowly shift the frequency by tenths or hundredths of a Hertz to find the amplitude peak (do not try to tune by watching for a displacement in the phase relationship, as there is a time delay between the signals introduced by the hardware). This done, set the strobe around 3000 cycles per minute, and adjust it until you can see the sides of the beaker flexing.

  This can be used to show the resonance of the beaker. You can also, optionally, shatter it, by increasing the input voltage at resonance. Be careful not to exceed 1Vpp.

  After the resonant frequency is found and the amplitude turned up, the oscillation of the beaker can be caused to exceed its elastic limit and thus to shatter. See the video links below to view a slow-motion video of the beaker at the moment it breaks.

  Engagement Suggestion

  • Show the students that there are two different resonant frequencies, and challenge them to develop theories of why this is.
  • Consider using this in conjunction with H3-62 to illustrate the effects of the beaker's spout in a more obvious (and quieter) manner.
  

  Background

  This process of driven resonance potentially leading to mechanical failure can be related to many engineering problems. This is an excellent opportunity to discuss how physics applies to real-world problems, like the Tacoma Narrows Bridge collapse.
  Also, be sure to explore our directory of oscillations and waves simulations to show other examples of complex mechanical oscillations.
• Availability: Temporarily Unavailable, undergoing maintenance
• References: REFERENCES: (PIRA 3D40.55)
• Loc codes: FS1, LS2, SU5
• Videos-1: Slow Motion Video