An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

Engagement Suggestion

Background

For small groups, also consider H1-04, a more portable version of this demonstration.

With a bit of exploration, one can find resonances in the system that produce the most dramatic flame displacement. Consider having students make predictions about how different waveforms will make the flame respond differently

A variety of sound sources can be requested separately.

For comparison, a half-length tube is also available. Invite students to predict how this one will compare to the open and closed tubes of twice its length

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

Background

According to the theory of the compact disc, the Cross Interleave Reed-Solomon Code (CIRC), in which the data are impressed on the disc, is coded so as to override certain types of localized damage to the disc, correcting for so-called burst errors. This is done so as to avoid playback problems as the disc ages and gets a bit scratched up.

Being scientists, we decided to check this by drilling holes in a compact disc to see if it would still play. It did not take long to find a CD in which it was imminently worth drilling a hole: a bunch of hackneyed overtures by Rossini. Did it work?

It sure did!! The CD played right past holes of 0.0083" (0.2mm), 0.0135" (0.34mm), 0.021" (0.53mm), and 0.032" (0.8mm), and only sometimes gave small clicks for a hole of 0.062" (1.55mm). A 0.118" (2.95mm) hole really did the job, causing large clicks and skipping. This can be heard on the CD, Track 2 (William Tell Overture) as repeated brief skips beginning at about 0:48, clicks at about 3:25, and major, repeated skipping just after 4 minutes.

Engagement Suggestion

This experiment demands the proper explanation in terms of blackbody radiation emitted by all objects, not just "hot" objects. The historical struggle of physicists to deal with this is documented in an interesting article by Hasok Chang, Lecturer in Philosophy of Science at University College, University of London, entitled Rumford and the Reflection of Radiant Cold: Historical Reflections and Metaphysical Reflexes, in Physics in Perspective Volume 4 Issue 2 (2002), pp 127-169.

Note that this experiment uses materials from I5-51 and L3-16. If you want to use those demonstrations in the same class, be sure to discuss logistics with Lecture-Demonstration staff in advance.