A Rubens’ Tube is a long hollow tube with many small, evenly spaced flames on the top of it that can be used to visualize sound waves. We’ll get into the specifics later, but for now the general idea is that sound creates pressure waves, which concentrate a flammable gas at some points and push it away from others, creating tall flames at sound wave peaks and short flames at sound wave valleys.

Here’s a rough video of it in action, using a theremin creating the sound. We’re working on a more polished video with better lighting and more clear demonstrations that will be published shortly. In case you couldn’t tell from the video, we’re definitely engineers, not videographers.

Disclaimer: one camera may have been slightly harmed in the making of this video.

Other Disclaimer: We apologize that most of the steps in here don't have photos. If you just scroll through and look at the pictures, we don't blame you.

Step 1: Motivation and Goals

Why do this?

Certainly not because it got us out of writing a research paper for a graduate combustion class. That would be a silly reason. Research papers are way more fun than Instructables! Don’t let anyone tell you otherwise.

Besides that hypothetical reason, a Rubens’ Tube is a neat way to visualize something rather abstract: sound waves. Like electricity, you just have to take someone’s word that sound waves exist and behave in these loopy sinusoidal patterns. UNTIL NOW. The Ruben’s Tube shows about 100 small candle-like flames when there is no noise applied. When some sound is made near the tube with some pitch (with an associated frequency and wavelength), the flames change height to reflect the exact shape and size of that sound wave. How neat is that?

Haven't others done this already?

Plenty of other internet-goers have built Rubens’ Tubes, but ours has an extra twist: we incorporated a theremin to generate sound. A theremin is an electronic instrument that can be controlled by the position of your two hands relative to two metal antennas. Typically, as your left hand gets further away from the left antenna, the volume increases. As your right hand gets further away from the right antenna, the pitch decreases. Here’s the wikipedia page on theremins if you want to learn more.

Goals
Our primary goal was pretty straightforward: build a Rubens’ Tube that can be controlled by a theremin or computer based sound generator. Secondary goals were to make it reasonably portable and robust so that it can be used for future classroom demonstrations.

Step 2: Background (aka SCIENCE)

How does it work?
A Ruben’s Tube is a long, hollow tube with a valve where gas (typically propane) can be injected. There are many small holes evenly spaced along the top where the gas can escape, creating small flames. One end of the tube has a rigid cap and the other is capped with a flexible membrane (think rubber glove material). On the side with the flexible membrane, a speaker is mounted. The sound waves coming out of the speaker vibrate air, which vibrates the membrane, creating pressure waves inside the tube. These pressure waves concentrate the gas near the peaks (creating taller flames) and displace it near the valleys (creating shorter flames). See the attached image (photo credit: Sciencefacts.net).

How did you make it?

Here are some specific details from our build. They may not be the best parameters, but are at least a starting point for something functional:

Tube length: 6 feet
Tube outer diameter: 2 inches
Tube wall thickness: 0.065 inches
Tube material: mild steel
Flame hole diameter: #42 drill bit (0.0935 inches)
Hole spacing: 1 inch on center, starting 4 inches from both edges.

Step 3: Tools and Materials

On to the good stuff! What did we actually use to make this?

Materials

Steel pipe, 6 ft long, 2in OD, 0.065in wall thickness
Ball valve
Steel manifold
Assorted steel pipe fittings for tubing
Heavy duty rubber gloves
Rubber traffic cone
Rubber hose rated for propane gas
Hose clamps for propane hose
20 lb propane bottle
Scrap steel for brackets and legs

Tools

We relied pretty heavily on welding metal parts together. This could easily be worked around, but we had a welder and took advantage of it. Tools included:

MIG welder
Angle grinder with cut-off wheel and grinding wheel
Drill with high speed steel bits
Flat head screwdriver
Adjustable wrench
Hammer and center punch
Benchtop vise

Step 4: Time and Budget

Time
This project took roughly 28 hours total, broken down into:

Planning / Research: 4 hours
Materials procurement: 3 hours
Assembly: 10 hours
Testing: 2 hours
Write-Up: 6 hours
Video editing: 3 hours

If you’re just looking to make some cool flames, this project could easily be knocked out in a day.

Budget

The total cost of this project was about $350. As with our time budgeting, this project could be done for significantly less cost. The cost was roughly broken down into the following:

20 lb propane tank: $50
Steel pipe: $60
Propane-compatible tubing and clamps: $70
Steel pipe fittings / manifold / valve: $100
Speaker: $40
Fire Extinguisher: $15
Misc: $15
Watching flames form perfect sine waves: priceless

Step 5: Assembly

If you’ve been following along so far, assembly should be pretty self-explanatory. If not, there are a whole bunch of pictures to walk you through it. Here are the steps that we roughly followed (doesn’t include several dumb mistakes):

Mark and drill the small gas holes in the main pipe
Attach all needed fittings to the manifold
Drill large holes in main pipe, attach fittings, and weld to seal them
Weld the manifold to the main pipe
Measure and cut rubber tubing
Attach rubber tubing to fittings with hose clamps
Cut out and weld on steel end cap for main tube
Weld on some dinky little legs onto the main tube (made from scrap steel)
Attach flexible membrane to other end of main tube, secure with too much duct tape
Cut traffic cone to fit over membrane end of tube
Attach traffic cone to main tube with large hose clamp
Support traffic cone with metal bracket
Move on to testing!

The purpose of the manifold is to distribute the gas more evenly throughout the pipe: we had four points where the gas entered instead of just one in the center. The purpose of the traffic cone is to concentrate the audio input towards the flexible membrane.

On to testing!

Step 6: Testing

Our first test was without sound, just to make sure that the flames would sustain themselves and be an appropriate height. We did this outside just in case. It only took about two minutes to get the flames going the first time, so there’s not too much to say here. One thing to note is that 20 lb propane tanks have a leak detection valve that can be tricky to work with. Basically, if propane starts coming out of the tank too quickly, the valve will shut off the flow to prevent an explosion. Ideally we would have bypassed this, but we got around it by opening the tank very slowly and opening the ball valve very slowly. Later on, we tried to see how high we could make the flames, but after about 4 inch tall flames, the leak detection valve closed off the tank. Bummer.

Testing was a lot of fun with the theremin. There’s no perceptible technology barrier with everything set up: you just move your hands around, listen to the sound coming out of the speaker, and watch the flames. On the other hand (ha!), just playing around with the theremin didn’t make for a great video.

In the next few days, we’ll be doing some more testing and a better, more scripted video. More on that in the “Future Work” step.

Step 7: Discussion

Step 8: Future Work

This was a great project to learn firsthand about sound waves, flame stability, pipe fittings, welding, and theremins. We came up with a few things we still want to do and others to do differently if we were to do this project again.

To Do on Current Setup

In the next week or so, we plan to:

Film a scripted video with better lighting, helpful dialogue, and a guided plan of what we want to demonstrate.
Try out a few different speakers with the theremin and some tone generation software to see what effect those have on the flames.
Polish the finished setup, including painting, grinding down rough edges, and mounting a permanent speaker with cable.

Notes for Next Time

If we were going to build this again, we would have:

Drilled smaller holes. Probably something like 0.040” diameter holes would have given thinner, more clean looking flames.
Drilled more holes. We spaced ours 1 inch on center, but ¾ or even ½ inch would have allowed us to see more smooth looking waves.
Try out different flexible membrane materials and thicknesses. We only tried the heavy duty rubber glove material, so can’t comment directly on how other materials might work. We suspect that getting the right thickness, material, and tautness over the pipe could make a big difference.
Related to the previous, a larger diameter main tube could have potentially shown more exaggerated sound waves. Ours at 2 inch outer diameter was about the smallest we found; something closer to 4 inches may have been more appropriate.

Step 9: References and Acknowledgements

Thanks to our hypothetical professor for letting us skip a research paper to build something cool. Here are some of the papers and sources we used in our design and build. IS THIS MLA? Noooo, this is Patrick!

Ficken, G.W., Stephenson, F.C., Rubens flame-tube demonstration. The Physics Teacher, 17, pp. 306-310 (1979).
Gardner, M.D., Gee, K.L., An investigation of Rubens flame tube resonances. Journal of Acoustical Society of America, 125 (3), pp. 1285-1292 (2009).
“The Rubens Tube by Kent L. Gee” https://asa.scitation.org/doi/pdf/10.1121/1.36360...
Sandoval, Christopher. (2013). The Rubens Tube: A Flaming Good Way to Teach Waves. Teaching Science, 59(1), 45-47.
Spanga, G.F., Rubens flame tube demonstration: A closer look at the flames. American Journal of Physics, 51 (9), pp. 848-850 (1983).
Szwaja, & Naber. (2013). Dual nature of hydrogen combustion knock. International Journal of Hydrogen Energy, 38(28), 12489-12496.