Sonic Flame Tube

Introduction

Use this demonstration to ignite your students’ interest in standing waves! Connect a long metal tube with evenly spaced perforations across the top to a gas source, and light the gas. When a resonant frequency is played in the tube, a standing wave is created. The node and antinodes created inside the tube will result in the flames from each perforation reaching different heights, beautifully demonstrating a standing wave.

Concepts

• Sound
• Resonance
• Compression vs. rarefaction
• Standing wave

Materials

Safety Precautions

If your school uses a gas source other than a piped municipal service, then check to ensure the gas supply has a flashback arrestor inherent in the system, ensuring the flames will not travel back to the gas source. If you choose to use a propane tank gas supply, you must add a flashback arrestor. Inspect the rubber latex sheeting for signs of wear and tear. Remove all flammable objects from the vicinity of the flame tube. Work in a fume hood or a well-ventilated classroom. To prevent overheating, never keep the flame tube lit for more than 5 minutes at a time. Practice this demonstration before performing it in front of a class. Wear safety glasses when performing this demonstration, and tie back long hair. Follow all laboratory safety guidelines.

Disposal

All materials in this demonstration may be saved and stored for future use. Allow the flame tube adequate time to cool before storing.

Prelab Preparation

1. Find an open, well-ventilated area to set up the flame tube. Clear the area of all flammable materials.
2. Set up the tube support stands about two feet apart.
3. Secure the rubber tubing to the protruding end of the connector in the rubber stopper.
4. Secure the rubber stopper in one end of the flame tube, ensuring a tight seal. Note: An incomplete seal may result in gas leaking from the tube, which may then ignite when the tube is lit.
5. Connect the rubber tubing to the gas source.
6. Obtain the latex sheet and secure it to the metal tube with a rubber band. Note: It is not necessary to pull the membrane as tight as possible. Overstretching may result in the latex membrane tearing.
7. Place the speaker over the end of the tube with the latex sheet. Note: It may be necessary to remove the wire grating to ensure the speaker is flush with the latex membrane.
8. Set up the tube in the stands with the row of holes pointing straight up (see Figure 1).

9. Obtain the frequency generator and connect it to your speaker. Note: For tips on where and how to obtain a frequency generator, see the Tips section.

Procedure

1. Make sure all flammable materials have been removed from the area around the metal tube. Turn on the gas source. Allow the tube to fill with gas before lighting it. The smell of gas may be detected once the tube is nearly full.
2. Starting from the end nearest the gas source, light each hole, sweeping the lighter along the top of the tube. Caution: If the gas near the rubber stopper ignites, turn off the gas, refit the stopper by pressing firmly and twisting it into place and try again.
3. Adjust the gas pressure so the flames are 1"–2.5" high. Note: At this point, tap lightly on the rubber membrane to show the flames uniformly change height with the increase/decrease in pressure.
4. Turn on the frequency generator and cycle through the frequencies until a wave shape appears. Note: If the option is available, once the waveform is found, vary the frequency until the waveform in the flames becomes the most distinct.
5. If the actual frequency of the sound is known, list this number for the students. If it is a note being played on an instrument, let students know which note it is.
6. Increase the frequency to find a new wave form and repeat steps 4–5. Continue to demonstrate various frequencies for up to five minutes.
7. After no more than five minutes, turn off the gas source. Note: If excess gas is detected, turn on the hood or open the windows to ventilate the room.
8. Inspect the stands and any nearby objects for “hot spots”—that is, areas that have experienced an excessive increase in temperature. Remove these objects (if possible) before repeating the demonstration.
9. When the flame tube is once again cool to the touch, you may repeat.

Student Worksheet PDF

11968_Student1.pdf

Teacher Tips

• All materials provided in this kit are reusable; the latex sheet will need to be periodically replaced.
• Over time the latex sheeting will become oxidized and develop cracks. It should be replaced with a fresh sheet with no signs of wear and tear. Replacement sheets are available from Flinn Scientific, both in 12" x 12" size (Catalog No. AP4573) and 48" x 48" size (Catalog No. AP6349). Alternatively, cut down the center of a balloon and cut off the ends to fashion a rubber square.
• Many computer programs that produce sound waves at desired and variable frequencies can be found online. Additionally, free programs for the iPod^® such as FreqGen are available in the Apps store.
• Playing music will invoke a flickering effect and, on occasion, show brief wave forms, although it will not create sustained resonance in the tube. It is still entertaining to watch and serves as a discussion point on the difference when sound is played in the tube compared to when resonance is actually achieved by using a pure tone.
• For some frequency suggestions, see the sample data table below. Note: Since the speed of sound in a medium depends on temperature, the observed waveforms may vary depending on how long the tube has been lit.
• If you are having difficulty forming a standing wave pattern, try lowering the gas pressure and increasing the sound volume while searching for a correct frequency. Once the right frequency is found, decrease the sound volume.
• If your speakers do not produce a loud enough sound, consider using an amplifier, such as RadioShack’s mini Audio Amplifier, RadioShack Cat. No. #277-1008.
• Do not use the flame tube for longer than 5 minutes at a time to prevent damage to the stands and the rubber sheet. The aluminum pipe will cool quickly—be sure to give it enough time to cool between demonstrations.
• When Rubens published the first paper on the sonic flame tube effect, he recognized there were sometimes two modes of operation. The flames may be at their maximum at either the displacement antinode and pressure node, or at the displacement node and pressure antinode. The latter case is called “reversal” by some. The reasons for this are complex and under scrutiny, and are related to the gas pressure and sound amplitude. For further information, see the article by Ficken and Stephenson in the References section.
• For further discussion regarding standing waves, see Open-ended Resonance Tube Set, Flinn Scientific Catalog No. AP4616, and Waves and Sounds—Student Laboratory Kit, Catalog No. AP7014.

Sample Data

Answers to Questions

1. What is a standing wave?
   

   A standing wave is a wave that is reinforced by itself; when the wave from the speaker is reflected off of a surface, it will travel back to the source. If that wave takes the same shape as the wave leaving the speaker, the two waves will reinforce each other constructively and produce a standing wave.

2. Based on the data above, what conclusion may be drawn about the relationship between pitch and wavelength?
   

   The higher tones corresponded with shorter wavelengths, and the lower tones corresponded to longer wavelengths.

3. What are nodes? What are antinodes? What do they correspond to in the shape of the flames?
   

   Nodes are areas where the sound wave is at zero amplitude, antinodes are areas of either maximum or minimum amplitude. The nodes correspond to the lowest flames and the antinodes correspond to the highest.

4. Why do only some notes produce a waveform?
   

   Only some notes produce a waveform because the reflected wave needs to match the incoming wave, which means only a node or an antinode (open end) may be at the barrier.

Discussion

The Sonic Flame Tube is commonly called a Rubens tube, named for Heinrich Rubens (1865–1922), who created the first one. Rubens combined the work of many others, including Lord Rayleigh (1842–1919), who originally proposed the concept. Rubens first published his discovery in the 1905 issue of Annalen de Physik—right next to Einstein’s famous special relativity article. The tube, when filled with a flammable gas and then lit, will show a series of flames rising to nearly equal heights. When a standing wave is generated inside the tube, the flame heights change to reflect the wave.

A standing wave is a wave pattern that results when two waves having the same frequency, wavelength, and amplitude travel in opposite directions and interfere constructively with each other. The standing wave may be produced by a frequency generator, a musical instrument or a tone generated by a computer or portable music player. This wave consists of alternating high- and low-pressure variations as it moves through the tube. Sound waves are often depicted as a sine wave as shown in Figure 2. When a sound is played in the tube, the wave is reflected back off the closed end of the column, which is the gas source. The sound wave is ultimately reflected back toward the speaker—the vibrational source. If the reflected wave reaches the vibrational source at the same moment another wave is produced, then the leaving and returning waves reinforce each other. This reinforcement, called resonance, is achieved and a standing wave is produced. A node is a point in a standing wave that always undergoes complete destructive interference and therefore is stationary. An antinode is a point in the standing wave, halfway between two nodes, at which the largest amplitude occurs (see Figure 2).

In the flame tube, the closed end of the tube is the displacement antinode. The flexible rubber membrane can be either a displacement node or antinode. A displacement node will be a pressure antinode, and conversely, a displacement antinode will be a pressure node (see Figure 3). This means that the fixed end of the tube will be a pressure node, while the free end will be a pressure antinode. However, the flame maxima may correspond to either the pressure antinode or the displacement antinode, and the reasons why are still debated today. Regardless, the wavelength can be calculated as the distance between three flame maxima (see Figure 4).


Standing wave patterns are only created at the tube’s natural frequencies, also known as harmonic frequencies. The possible frequencies that will generate a standing wave in the tube can be found by using Equation 1.

where

f is the frequency 
v is the speed of sound in the medium
L is the length of the tube
n is the vibrational mode

The harmonic mode “n” must be a positive integer and describes the shape of the wave, corresponding to the number of antinodes. The harmonic mode is related to the wavelength of the sound by the equation

where λ is the wavelength of the sound.

The fundamental frequency is n = 1, and corresponds to one antinode. Two antinodes, which appear as two flame peaks, correspond to a wavelength the same as the length of the tube.

References

Bilash, B. & Maiullo, D. A Demo a Day™—A Year of Physics Demonstrations; Flinn Scientific: Batavia, IL, 2009; pp 212–13.

Dornbrush, A. “Rubens’ Flame Tube,” Experimental Physics, Wheaton College, IL, 2008. (Unpublished)

Ficken, G. W. and Stephenson, F. C. “Rubens’ Flame-tube Demonstration,” The Physics Teacher 17, 306 (1979).

Hyperphysics: Georgia State University http://hyperphysics.phy-astr.gov.edu/hbase/Hframe.html (accessed October 2009)