What you Need
Rubens' tube was invented by Henrich Rubens in 1904, it consists of a large diameter (normally metal tube) with a series of holes drilled in the top. A loudspeaker is attached to one end, and a flammable gas supply to the other, and the gas coming out of the holes is lit.
I saw this demonstrated by Bryson Gore (of Christmas Lecture fame) and loved it, so I had to build one. It involved drilling 360 1mm holes in an aluminium pipe, and arranging to attach a speaker to one end and a propane supply to the other. So I now am the proud owner of a Rubens' Tube.
As you can see waves of flame seem to be created and their wavelength changes with the frequency (pitch) of the sound being fed in. The basis for this effect is resonances in the gas in the tube.
When the loudspeaker vibrates it sends a series of waves of air down the tube at the speed of sound, these then reflect off the far end of the tube, so there are two sets of waves moving through the tube in opposite directions.
For most wavelengths this does not produce a very interesting effect, but when the length of the tube is a multiple of half the wavelength the two waves add together to form what is known as a standing wave. In some places the two waves add together forming extra large changes in pressure (antinodes) and in other areas they cancel each other out so the pressure is constant (nodes).
(It is actually slightly more complex in the Rubens' tube as the holes act as damping, broadening the resonances and allowing it to form patterns even when the wavelength is not quite right.)
How does this standing wave affect the flames?
That is in fact a very good question, the average pressure at all points of the tube is the same, so the answer is not that some parts of the wave are at a higher pressure than others. This means the answer must lie with how the flames react to the changes in pressure.
Just to make things more complex Rubens actually discovered that his tube could behave in two completely opposite ways. When the sound is relatively quiet the flames get shorter at the antinodes but when the sound is very loud they get longer.
These two completely different behaviors seem to have different causes.
When the sound is relatively quiet the changes in pressure due to the sound are less than the gas pressure. For relatively small pressure differences the flow of gas through a hole is proportional to the square root of the pressure difference (pressure difference is energy per unit volume of gas, and the change in kinetic energy of the gas is proportional to its velocity squared, so the velocity and so the flow rate is proportional to the square root of the pressure difference).
This relationship means that the increase in flow when the pressure increases is less than the decrease in flow when the pressure decreases so on average the flow, and therefore the flame length, is less where the pressure is varying a lot, at the antinodes.
This means that the flames are shorter at the antinodes (loud parts of the tube) and the same length at the nodes.
When the sound is very loud
I have not found any discussion of this case, Rubens seemed to think that it was so obvious that he didn't bother writing it down. It isn't obvious to me, but this is what I can work out:
When the speaker is set to loud the changes in pressure due to the sound are much larger than the gas pressure, and the behavior is very different. Now the gas is squirted out of the holes and forms a jet traveling away from the tube.
The pressure then becomes lower in the tube than outside so gasses from outside are sucked in. However these gasses are not just the propane that was squirted out; suction doesn't have a direction, so gases are sucked in from all around the holes, most of which are air.
So gas is squirted out and air is sucked in allowing more gas to be squirted out in the next cycle. This hugely increases the flow of gas out of the tube, for a while... the mixture of gases in the tube is slowly made up of more and more air. This makes the burning cleaner and cleaner, forming a bluer flame, until eventually there isn't enough gas to sustain combustion and it can put itself out.
What to do
What may happen
Why does it happen?
Are you CORGI registered? Bored chemist, Wed, 21st Apr 2010
The way the flames were escaping from the end of my small test tube indicates not... I really need to work out some way of soldering/welding the end pieces on to make a good seal. daveshorts, Wed, 21st Apr 2010
Dump the aluminium and use copper instead. You'll have no problem soldering it.
They register people trained to do gas installations. I think it stands for something-or-other of registered gas installers, but I'm not sure about that.
I am in the process of making a rubens tube for my physics class and having some difficulty. After lighting the tube I will only recieve a small blue flame instead of a larger orange flame. Also it won;t stay lit. It will go out almost immediately after lighting. Any help or suggestions would be much appreciated. Jake, Fri, 4th Jun 2010
What would the optimum diameter for the tube be ? As this could cause a problem if copper were to be used and what distance did you put between your holes ? Thanks RobCheese101, Thu, 10th Jun 2010
Aluminum is a pain make your job easier and just use JB weld. It'll hold at about 500 degrees, it's permanent, and a 100 times easier than working aluminum. Mayhym, Mon, 14th Jun 2010
To be honest I haven't done a lot of development work after getting it to work. I have a feeling making the tube slightly larger than 50mm for the 1.8m I used would be a good idea.
Does anyone have a practical use for this device? Apart from being used as a physics demo. I saw it on Garage Science (Did not look like a kitchen ) and it looks pretty cool but...... whats it for? There must be some practical uses? Aaron_Thomas, Wed, 4th May 2011
Yeah, play your instrument of choice through it for a ridiculous live show jgoldbeck, Thu, 5th May 2011
I haven't tried building one of these but may have an idea to help. Has anyone tried to bevel the holes similar to an orifice in any consumer gas appliance? If I remember correctly it greatly effects the organization of the flow of gas molecules passing through. There are calculators available online to determine size and desired effects. Maybe finely tuning the tube will accentuate the effect. Jlfonz, Sun, 5th Jun 2011
More thoughts about this. What is at the opposite end of the speaker? If it is some sort of rounded end cap it will severely distort any reflected sound waves in such a small area. Would putting a sound deadening material in the opposite end eliminate any standing waves. Something like loose fiberglass or foam. How about a completly flat surface on the opposite end that can either move with the sounds waves or be adjusted for distance from the source? How about an identical speaker on the other end pointed in the same direction as the source speaker. I think this might create a graphic equaizer effect of the flames. It seems that this is the overall desired effect. I also believe that some of you may be dealing with pressure issues. If your fuel source does not give enough volume and pressure you would need smaller orifices (orifici?) to maintain a flame. If they are to big for the source they will not burn well as the oxygen mix would be way to high. If using propane tanks for gas grills---try using two of them. This would also help to alleviate the issue stated in the original post--of oxygen being sucked into the orifices (orifici?) at the troughs. BTW--even if I am wrong--this is fun. Jlfonz, Sun, 5th Jun 2011
Hi guys. As my first post I'd like to tell you about my Rubens Tube.
Very cool - in fact from the look of those flames, very hot as well. Look forward to the videos - but remember to hold safety first! imatfaal, Mon, 4th Jul 2011