Activity/Demo overview:
A Rubens tube is used to visualize standing sound waves. When filled with flammable gas, holes in the top of the tube allow gas to escape and can be lit on fire. Playing a tone at one of the harmonic frequencies of the tube will create a standing wave, which is reflected in the height of the flames across the length of the tube.
[WARNING: caution should be used for this demo as it involves flammable gas and open flames. DO NOT try this at home or in any crowded space]
Equipment:
- Rubens tube
- Speaker control box
- Banana cables
- Lighter or matches
- Gas supply
Setup:
-
- Connect tube to gas supply
- Contact Steve Narf for assistance if necessary
- Connect tube to gas supply
- Ensure all valves are properly closed when the tube is not in active use.
- Plug in control box, connect to speaker using banana cables, turn the box on
- Set signal to sine wave
- Test the tube and find a good-looking harmonic frequency (~600 Hz)
- Set the amplitude to 0 once you have found a good looking wave. The frequency will default to 1kHz if the control box is turned off, so leave it on or make note of the frequency you chose when testing.
Performing the demo:
- Before lighting the tube, explain that sound is a wave travelling through the air.
- Depending on the level of the audience, you can get into longitudinal vs. transverse waves (compare doing the wave at a sporting event or a boat floating on the ocean to a row of dominos), speed of sound, needing a medium for some types of waves, etc.
- Open the valve to fill the tube with gas and light the tube. It will take a few moments for the tube to pressurize and begin forcing gas out of the holes, so do this near the end of your explanation of waves.
- If the pressure has built up enough, the fuel jet from each hole will light adjacent holes and the whole tube will light itself. You can manually light any holes that don’t light right away.
- Explain the physics of standing waves (when waves overlap, they interfere, sound waves reflect off the closed end of the tube, etc.).
- Turn the speaker amplitude up to maximum to show the audience the standing wave
- The tube will expand as it heats up, so the harmonic frequencies may change slightly during and between uses. You may need to hunt around for a good-looking frequency.
- Use the up and down buttons on the box to quickly change the frequency. The knob is useful for precise adjustments, you can mostly ignore it.
- Re-emphasize or restate physics of standing waves, reference the height of the flames at different points.
- Search for another harmonic frequency, point out that there are more (fewer) visible nodes at a higher (lower) frequency and that this is related to the wavelength of the wave.
- Optional: roast smores over the fire to end the demo! You will want to turn the speaker off, as the sound can be quite irritating if left at a high volume for an extended period of time.