What could be more entertaining than fire and good music? How about fire that will dance to that music? The following will not only cause flames to pulse to a beat, but it will also allow you to see different wavelengths of audio frequency. The following is my first Instructable, and my entry to the Instructables Science Fair contest.

In addition to documenting the build steps to create something called a Rubens' Tube, I'll also go over some of the basic concepts and science involved with sound waves.

From the moment I discovered what a Ruben's Tube was, I wanted to build one... and considering I was in a physics class at the time, there couldn't have been a better excuse than extra credit.

Update: I'm thrilled to see that this Instructable's become popular, and I'm more than happy to have people build on the project, but please respect the Creative Commons BY-NC-SA license.

Step 1: What Is a Rubens' Tube?

Heinrich Rubens was a German physicists born in 1865. Though he allegedly worked with better remembered physicists such as Max Planck at the University of Berlin on some of the ground work for quantum physicists, he is best known for his flame tube, which was demonstrated in 1905.

This original Rubens' Tube was a four meter section of pipe with 200 holes spaced evenly along its length. When the the ends are sealed and a flammable gas is pumped into the device the building pressure will have only one route to equalize. The escaping gas can be lit to form a row of roughly even flames. Upon introduction of a loud speaker to one of the sealed ends, standing waveforms can be seen in the flames.

Within the Rubens' tube, as soon as gas is ignited generally uniform flames will be seen. This is because there is very little pressure differential between any given area of the space inside the tube. Once sound is applied from one end, pressure will change within the tube. Should the sound be an easily measurable frequency, the wavelength will be visible in the series of flames, with the highest flames being where condensation is occurring and the lowest where rarefaction is occurring.

They can also be used as an incredible visualizer for music.
(Note that I've muted that audio intentionally, as I don't own the copyrights to the music being used in the clip, and I'm not sure what the Instructables policy on "fair use" is. Searching on Youtube or Metacafe for "Rubens Tube" will show you several videos with music.)

Step 2: Nerdy Stuff - a Bit on Waves.

Visualizing sound as a wave can be a bit misleading. Sound is often a often portrayed as a sine waveform because it's easier to illustrate than the longitudinal compression wave that it actually is.
The first image shows the typical way a sound wave, or just about any wave, is portrayed visually. Because sound is basically a vibration created by changes in air pressure, the peaks on the waveform correspond to the highest pressure and the troughs correspond to the lowest pressure. The wavelength is measured by the distance between two peaks, or two troughs.

The second image is more accurate way to visualize compressional waves, including sound waves. Each dot could represent a molecule of the matter (such as air) that the wave is traveling through. At the points where the pressure is highest you can see that the density of the material is relatively higher than the areas with lower pressure. These are called condensation and rarefaction, respectively.

The third image shows the previous two stacked, it's easy to see how the peaks of the wave correspond to condensation, and how the troughs correspond to rarefaction

Step 3: Nerdy Stuff - the Application.

In step two we were able to grasp some of the basics on sound waves, and although the concept isn't all that abstract, I thought it might be nice to take it from the theory level, to the application level as it applies to what's going on inside a Rubens' Tube. I've made a few diagrams to help illustrate the concept.

The first image represents a functioning Rubens' Tube under normal conditions. We can assume that in this image an arbitrary, constant tone is being played into one end of the tube. But what might we witness if we were able to peer inside the tube, and see the sound waves?

The next image, gives us an idea what's going on if we were able to see the sine wave of the tone. But what's critical to remember is that what's actually occurring is a change in pressure between different amplitudes of the waveform.

The final image illustrates these pressure changes as a compressional wave. Understanding sound in the way makes it clear as to why we have the varying flame height. The taller flames correspond directly with the higher pressure. This higher pressure in the compressional wave is what's pushing the gas out of the holes with more force than in the areas with lower pressure.

Step 4: Nerdy Stuff - Measuring Sound.

The speed of sound is roughly 340 meters per second at sea level, but this is when air is the medium through which the sound waves travel. But because propane is of a different density than air, the velocity of sound is also different, and like all gases, the density changes with heat or pressure changes. For our purposes, we can work with the a velocity of 257 meters per second.

As mentioned in the last step, sound is a vibration, we measure the frequency of this vibration in hertz (Hz), which is the number of cycles of the vibration per second. Wikipedia tells us that "The frequency (f )is equal to the speed (v) of the wave divided by the wavelength (lambda) of the wave".

So in other words - frequency = speed / wavelength or:
f = v / lambda

To find the wavelength, we use basic algebra - multiply by lambda and divide by f to get.
lambda = v / f

To test this we can take the sound wave used to demonstrate the device in the video as an example (360Hz), and use or rough speed of sound for v.

lambda = 257(m/s) / 360Hz
This gives us a value for lambda of about 0.71 meters. Which should be close to the distance between the peaks of the flames. Though the actual measured value may differ from what is calculated given the above mentioned scenarios.

Note - for some reason the lambda symbol keeps turning into this when I save "Ã�Â»". So I've replaced the symbol with the word "lambda". I apologize for any confusion.

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Special thanks to user cposparks, who found an error on this page when it was originally published, I've since made best efforts to correct it.
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Step 5: Materials and Tools

The materials required to build this Rubens' Tube are fairly inexpensive, most of which can be found on Amazon.com, or your local hardware store. Note that some of the recommended are not identical to the parts that I've used, they will however, serve the same, or superior function to what I've used. That being said, the materials you use, and the build process is extremely flexible and forgiving.

Building Materials:
Ventilation DuctingAmazon
Ducting used for HVAC, though any thin-walled metal pipe should work. My tube uses 4 inch ducting, I'd imagine success using anything from 2, to 6 inch material.
Amazon has a number of other products that should work. Something to consider is that if you use a large tube (in both length and diameter), you'll be dealing with more volume, which will result in taking more time to "prime" your tube. See step 13 for details.

Brackets x2 Amazon
These are brackets used for the supporting the ducting. Obviously the same size as the above ducting. (I used brackets which were an inch larger in diameter for buffering, however, in retrospect, I do not believe this is necessary)

PropaneAmazon
This is the fuel source. Anything with a valve so you can turn it off and on safely should work.

TubingAmazon
Nothing too insightful here, 1/4 inch inner diameter, or whatever fits your fuel source.

Hose T ConnectorAmazon
I used brass, but something like this would work fine.

Hose Splicers x2 Amazon
Used for fuel delivery. This will work much better than the small piece of copper tubing I used.

Latex SheetsAmazon
This will serve as our diaphragm. I used polyethylene sheeting, however, latex should perform much better as it's thinner and more pliable, but either will work and it just goes to demonstrate the flexibility of the build.

Scrap Wood

Tools and Supplies:
DrillAmazon
You'll also need a 1/4 inch bit and a bit between 1/16 and 1/32 Amazon

HammerAmazon

Tape MeasureAmazon

Knife or ScissorsAmazon

Silicone SealantAmazon

Epoxy PuttyAmazon

Duct TapeAmazon

Masking TapeAmazon

Teflon TapeAmazon

Misc.
Various hardware for attaching the brackets to your wooden base. Simple wood screws should work fine. A nail or similar may be needed for pre-drilling. And finally some hot water for working with the tubing.

Step 6: A Note on the Sound Source

For this project, any sound source should work. I used my iPod with the driver from old 3 computer speakers I got from Goodwill. But nearly anything should work, from a do-it-yourself setup, to an old radio, to a hi-fi sound system, to a boom-box. In keeping with the Amazon spirit - I'd imagine most of these would get the job done, provided they require an external power source.

I also, have three recommendations.
1 - Whatever speaker you use should be able to reach volumes to move the diaphragm.
2 - The speaker should be placed or mounted close enough to the diaphragm to easily move it, without making physical contact
3 - The speaker's diameter should be as close as possible to the tube's diameter to ensure maximum diaphragm movement.

We're even lucky enough here in the Instructables community to have projects that would work wonderfully as a source of sound with little to no modification.

If you're interested in getting your hands on sine wave audio tones, they can be had by Googling for "Audio Test CD" or "Audio Test Tones". The one I used for this project was found here.

The image below shows the basic components of the business end of my setup.

Obviously, depending upon what you use to push sound into your tube, your mounting methods will vary, if you use a speaker that's still in it's housing, in all likelihood, you should be able to simply move the speaker close to the diaphragm. I've attached an image of my setup for reference, clearly not the most elegant solution, but it got the job done.

Step 7: Construction 1 - Marking and Drilling

First - determine what will be the top side of the tube. If you're using ducting like I did, there's a seam on where the ends of the tube connect, this will be the bottom.

Second - Run a piece of masking tap along the top of the ducting. Using a tape measure mark off every half inch running down the center of the tape. I came 14 inches out from the center in both directions - this left 4 inches on each end of the tube without markings.

Third - Using a nail, or other object carefully tap a point in each marking with a hammer. The goal is to create a small depression at each location to facilitate drilling.

Fourth - Remove the tape and carefully drill through the ducting at each depression.

Step 8: Construction 2 - Sealing and Diaphragm

Obviously, in order to get this thing to function the way we want it to, we're going to have to seal up the tube and create a diaphragm.

First - Use duct tape to cover the potentially sharp edges of the tube - this will prevent the ducting from ripping through the diaphragm. You'll probably want to use more than one layer, but again, this is coming from my experience with the ventilation ducting. Depending upon the type of material you're using, it may not be necessary.

Second - Cut a latex sheet to serve as your diaphragm. You'll definitely want to cut it large enough to completely cover the end of the tube with enough extra to tape down and pull taut. You can see from the image that the size I selected was roughly 6 inches square. It's also worth noting that my picture reflects the use of polyethylene sheeting. This will work, however latex will provide additional flexibility. It's also worth noting that latex will slowly oxidize, and may need replacing over time.

Third - Start taping the sheeting down to create the diaphragm. The concept here is to basically emulate the head of a drum. It needs to be tight enough to allow it to easily vibrate, but not so tight that it will tear. Once satisfied tape down, or trim the excess, and tape around the circumference of the tube in order to create an airtight seal.

Fourth - On the other end, repeating the processes will allow for later experiments using a "stereo" Rubens' Tube. However, simply sealing it off with duct tape to create a good seal is a perfectly acceptable method to create a functioning device. In either case, be careful what you set the opposite end on to make sure you preserve airtight seal.

Fifth - Finally, if you're using ventilation ducting like I am, it's worth while to run a bead silicone sealant down the seam of the tube. I pressed it into the joint using my finger, and then cleaned up the excess. Now we have our tube completely sealed where we want it to be.

Step 9: Construction 3 - Fuel Holes.

Now it's time to build the fuel delivery system. The basic idea is to evenly distribute the fuel within the tube.

First - Determine the location(s) where fuel will enter the ducting. I recommend using what will be the "back" of the tube - this is simply a right angle from the line of holes on the top. Additionally, depending upon whether or not you choose to have fuel delivered at more than one location, a bit of advance planning should go into where these locations are.
In my case, I chose two locations - at distances half way out from the center hole and the first and last hole. It's also worth noting, again in retrospect, that one point for fuel delivery would probably work equally as well, so long as this location is centered.

Second - As in the prior step, tap your marked location with a nail, and drill a hole large enough for your hose splicer to fit. In my first attempt, I didn't use hose splicers, but rather 1/4 copper tubing. Using hose splicers will make hooking everything up much easier, and I highly recommend it.

Third - Using a utility knife, nail, or other sharp object, lightly score the ducting around the hole, or holes. This should allow for the epoxy to create a more secure bond.

Fourth - Install the hose splicer(s) in the hole(s). Mix up some epoxy putty, and apply liberally. You'll want to create both an airtight and secure bond.

Step 10: Construction 4 - Fuel Supply

This step's all about hooking up the propane to the tube.

First - Teflon tape on the areas where you'll be attaching the tubing is a good idea to make sure you won't have propane leaking. This includes the propane nozzle, the tee connector (if you're using multiple delivery points for fuel), and end of the hose splicer(s) that will connect to the tubing.

Second - Hooking up the fuel is a pretty straight forward process. Simply cut and attach the hose. To do this, you'll want a cup of very hot water close by to soak the ends of the tubing in, this will warm and soften it up so you can easily get it over the connections.
If you're using one entry point for fuel, you'll only have one connection to make right now, if you're using a tee connector for two entry points, connect both ends to connector, and to the Rubens' Tube.

Step 11: Construction 5 - Mouting

Considering the Rubens' Tube is round, it's a good idea to mount it on something so it won't roll around.

First - Attach the brackets to the base using screws. (I used a piece of scrap wood, I think it was part of a shelving unit once).

Second - This step may or may not be required depending upon how your tube fits in the brackets. During construction I found that the brackets were slightly larger than the ducting. To aid this, I used some scrap hosing and zip ties to put around the ducting, thus adding to the circumference.

Third - Finally, mount your Rubens' Tube inside the brackets. Tighten up the brackets and you should be left with a fairly stable setup.

At last! - Your Rubens' Tube is ready for action! Please read the safety notes before hooking up the propane and continuing.

Step 12: Safety First!

As with anything dealing with fire and flammable gas, there's some important, although fairly obvious things to consider. And although I am fairly confident that this is a sound design, just as with a propane grill, heater or lantern, I must urge on the side of caution, and cannot take any responsibility for any accidents or mishaps that may occur. In order to minimize the possibilities of an accident, do not operate the device without keeping in mind all of the following.

First - As alluded to above, propane is flammable! When using your Rubens' Tube, make sure you're in a well ventilated area or outside. In addition to the potential fire hazard, there is also a very real danger of carbon monoxide exposure from propane being less than completely burned. Carbon monoxide is DEADLY.

Second - Take note of the way you orient the propane tank. If the tank is not right side up, the propane may flow in unpredictable ways. Even a regular propane tank, used for a lantern or torch, turned on it's side, is likely to start spurting liquid after it's been turned on for some time - this can be extremely dangerous. Also allow for a safe distance between the Rubens' Tube and the fuel source.

Third - Even after the propane is shut off, the tube and hosing will still contain fuel. After turning off the fuel, you'll see the flames slowly start to lower. However, even after they're no longer visible, it's possible that they're still burning within the pipe itself. After shutting off the gas, remove the propane tank from the hose and allow plenty of time for the remaining fuel to burn off - there's more fuel than one would expect inside the tube itself!

Fourth - While the entire Ruben's Tube isn't likely to reach very high temperatures while being operated for reasonable lengths of time, the top part of the tube will become hot, even after short runs. Be very careful to allow adequate cooling time before handling the device.

Fifth - In case of emergency, be sure a fire extinguisher is close by at all times. Finally, if you're a minor, never operate your Rubens' Tube without the supervision of a responsible adult.

Finally - Use common sense! As dire as the above warnings may sound, assuming you use common sense and play it safe, your Rubens' Tube will serve as an amazing scientific toy!
Let's move on to how to use the device.

Step 13: Using Your Rubens' Tube

Assuming you've already gone over the safety notes, the first step in using the Rubens' tube is to "prime" the tube.

First - Attach the fuel source. Again, make sure there's a safe length of hose between the propane tank and the tube itself.

Second - We're going to want to create some positive pressure inside the Rubens' Tube. Because the flow of propane is fairly slow, and there's a lot of volume within the tube, we need to seal it up temporarily. To do this, use a strip of masking tape to cover all the holes on top of the ducting. Then turn on the gas.
Wait about two minutes, by this time enough propane should be in the tube to create a decent pressure differential. Depending upon the size of your tube, and the pressure of your fuel source, it may necessary to wait a greater, or lesser time.

Third - Remove the tape covering the holes. Then, using a long match or fireplace lighter, try lighting the gas by one of the holes. Assuming there's enough pressure, each hole should ignite down the tube. However, it may be necessary to light the tube in several places.
If the flame is very small, it may be indicative of lack of pressure within the tube, so you'll want to wait a bit longer the next time around before removing the tape.

The tube's ready for prime time when each flame is roughly an inch in height.

Fourth - Introduce an audio source near the diaphragm. You should be able to excite the flames by lightly tapping on the diaphragm, or even snapping next to it. However, for the most fun, and scientific pursuit, you'll want to use a speaker. See step 5 for notes on the sound source. Refer to the safety notes for turning it off.

You should be able to literally see the wavelengths of various audio sources. Head back and take another look at steps 2, and 3 to get a better understanding of exactly what's happening. As entertaining as dancing fire can be, there's a lot of science behind it too.

Notes
1 - I'd imagine one could complete this project in no more than an afternoon assuming all the supplies were on hand. I obviously worked with a bit of trial and error, so it took me some extra time.

2 - My average run times range from about 5-10 minutes. I'd imagine one could run the tube for longer than that, however, I'd err on the side of safety and keep the times limited.

3 - Keep an eye on the diaphragm. It's completely possible that the diaphragm may exhibit signs of wear over time. Although this has not yet been an issue for me, I could foresee possible diaphragm maintenance being needed one day.

And finally - Be safe, learn, and have fun.

Step 14: Further Information.

Beyond Wikipedia, I was able to find some interesting articles relating to the Rubens' Tube, and its creator.

ScienceWeek - An article mentioning Heinrich Rubens' work with Max Planck on early quantum physics and their correspondence with Albert Einstein.

FYSIKbasen The English translation of a Rubens' Tube design on a Danish website for physics demonstrations. There's a number of other interesting demonstrations here as well.

ISU Physics Department Another Rubens' Tube design. This one uses a signal generator for its audio source.

Also-
Thank you for all the feedback thus far. I've updated the Instructable in several places to address the questions that have come via the comments and messages.