Rubens Tube

My friend Ellery and I built a Rubens tube last Saturday, 5/7/2011.  I used this instructable as my primary source of research information for this project.  A rubens tube is essentially just a metal pipe with a bunch of holes drilled along one side in a straight line.  You pump a flammable gas into the tube (propane in this case) and the gas comes out of the small holes.  When you ignite the gas, it creates a bunch of small flames along the length of the tube.  If you place a speaker at one opening of the pipe and pump sound waves into the tube, the speaker causes varying amounts of pressure along the inside of the tube.  This pressure causes the flames above to react accordingly.  A rubens tube allows you to visualize sound waves using fire.  How awesome is that?

We purchased a 10 foot length of EMT electrical conduit.  A helpful individual at Home Depot cut it into a four foot section and a six foot section for us.  We chose the four foot section since it would require less propane pressure and less drilling.

Next we laid out a strip of masking tape along the pipe as straight as we could get it.  We then laid out measuring tape on top of that and taped it in place so it wouldn’t move.  Based on a suggestion from the previously mentioned instructable, we made a mark every 1/2″ along the tape.  We left 4″ on either side of the pipe in an effort to keep the ends from getting too hot and melting the speaker or other components.

Next, we used a metal punch to punch divots into the pipe at every mark we made.  The divots allow your drill bit to sink into the metal a bit so it can grab on and do its job.

Next, we drilled out every single hole with a 1/16″ drill bit.  We broke one bit during this process.  This was probably because it is difficult to keep the bit perfectly straight, especially after you drill 30 or so holes.

After the holes were drilled out, we checked them for obstructions.  If there is any left over metal debris inside of a hole, the flame will not be as high as the others.  If we found any, we just drilled the hole out again until it was gone.  Then we filed down the top of each hole to make it smooth so we wouldn’t cut ourselves on the metal.

Next we had to drill the holes where the propane flows into the tube.  We measured the pipe into thirds going from the first and last output holes we drilled.  We drilled one input hole between each of the thirds which makes for two input holes in total.  We wanted the propane input to be as even as possible to make all of the flames an even height.  We drilled them about 90 degrees from the output holes.  When drilling, we started with a very small bit and worked our way up to the 1/2″ bit.

The above image shows both of the input holes after drilling them out with the 1/2″ drill bit.

The 1/2″ fittings we purchased would not quite fit into the holes we drilled, so we filed the holes out a bit until we could get the fittings in place.

The above image shows one of the fittings sitting in the hole nice and snug.  The other fitting was a bit loose but that didn’t matter in the end.

This image shows both of the input fittings epoxied in place.  The epoxy keeps them secured in place and also helps to prevent any propane from leaking out around the fittings.

We found that the 2″ PVC fitting fit snugly into the 2″ EMT conduit on one side where the conduit was bent inwards a bit.  We had to pound this in place, but tight is good since we don’t want any propane leaking.  The small plug in the middle was wrapped in teflon tape and then screwed into place very tightly to prevent propane leaks.

This shows the end plug assembly epoxied to the EMT pipe.  We did not epoxy the small plug in place so we could remove it later for venting.

On the other side of the pipe, you need some kind of membrane between the speaker and the pipe opening.  The membrane needs to keep the propane inside the tube, but also allow the soundwaves to pass through the tube from the outside.  We cut the neck of of a latex balloon and stretched it over the end of the tube nice and tight.  You want it to look like the head of a drum.

This shows our speaker assembly.  I found these speakers at a liquidator sale.  They were almost exactly 3″ in diameter and came with an amplifier.  We purchased a rubber coupling/adapter that fit snug around the speaker.  We then clamped the hose clamp around the speaker to make sure it would not fall out.  We then slid the smaller side of the adapter over the stretched out balloon and clamped it in place with the hose clamp.  This seems to keep the balloon nice and tight and holds everything in place.

We originally used a propane torch as our fuel source, but this turned out to not output enough pressure to keep the flames ignited.  We eventually switched to a regular BBQ grill propane tank using a low pressure regulator.  I don’t have any photos of that setup yet, but we basically just found some fittings at the hardware store so we could hookup the regulator output to our vinyl tubing.

The vinyl tubing then goes into a splitter, and finally hooks up to the inputs of the rubens tube.  The above picture shows the hose assembly before it was hooked up to the tube.

We turned on the fuel source and waited a minute or so for the tube to fill with propane.  We used a long lighter to ignite the tube.  Be careful during this stage.  We discovered that the flames can start out much higher than you would expect if the pressure builds up too high…

We hooked up our speaker to an amplifier and used an Android frequency generator application to generate different tones.

We also tried some different music to see how the tube would react.  I find that music is fun to watch but the pure tones are much more interesting to see.

Finally, we hooked up an electric guitar and amplifier up to the Rubens tube to see what would happen. The result was pretty fun.