Science Experiments

Rubens' Tube - waves of fire

Wed, 21st Apr 2010

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.

Big Rubens' tube, the speaker is just off the shot to the right and the gas comes in to the left.

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).

The speaker moves the air

The loudspeaker vibrates creating compressions in the air which move down the tube (sound waves)

The wave reflects

When they hit the end they reflect back, and travel back again. So you have two waves moving in opposite directions inside the tube.

The standing wave

The lines are a graph of the pressure changes in the tube

If the wavelength is right the two waves will add together to form a stationary pattern called a standing wave. This contains areas where the pressure is varying by a large amount (antinodes) and others where it 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.

At the start of this the flames near (but not quite at) the ends are being shortened by the sound, but as the sound gets louder the flames get longer

These two completely different behaviors seem to have different causes.

Relatively quiet

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.


Part of the tube: the flame on the left has a constant pressure, the pressure for the flame on the right is changing


Quiet low pressure

When the pressure increases the flow of gas increases a bit and the flame gets slightly longer.

When the pressure decreases the flow reduces a lot and the flame gets a lot smaller.

This means that the flames are shorter at the antinodes (loud parts of the tube) and the same length at the nodes.


Rubens' Tube running quietly

In this case the areas with the largest changes in pressure produce shorter flames and those where it is quiet will produce longer ones.

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.

Part of the tube: the flame on the left has a constant pressure, the pressure for the flame on the right is changing

Air rushing out

Low pressure high volume

The pressure difference is very large so gas is squirted out forming a fast moving jet

The pressure change is so large that air is actually sucked in. Because suction happens in all directions, most of what gets sucked in is air. So to start with lots of gas is squirted out and air is sucked back in.

Gas Depletion

Rubens tube with loud sound

After a while however the gas around the antinodes gets throughly mixed with air, and the flames become very well oxygenated and blue. If it is left long enough they can even stop burning.

The areas with the loudest sound have the longest flames

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.

Over time air is sucked in making the gas mixture weaker and weaker. Leaving the antinodes with very little gas to squirt out.

High pressure with high volume

What to do

What may happen

Why does it happen?

Dave Ansell

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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.

It's just as well I did not know about this in the sixties. If I had, I'm sure I would have made one to put in the Disco.

On second thoughts, you might not want to do too good a job sealing the ends. If you happen to create a combustible mixture inside the tube, it would be good to have a sort of safety valve.

BTW, what's CORGI. What does Her Majesty have to do with this? Geezer, Thu, 22nd Apr 2010

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. rosy, Thu, 22nd Apr 2010

It could certainly be a combustible mixture in the tube, but the holes act like a Davey lamp
The metal conducts the heat out of a flame fast enough that it cannot propagate through the hole.

I would rather avoid copper, as it is expensive, heavy and easily damaged. I think soldering aluminium isn't as bad as all that, I just need to get a powerful gas torch daveshorts, Thu, 22nd Apr 2010

If you've never tried soldering Aluminium before, you're in for some fun. (It's a pain in the neck!) Here's a link that might be helpful.

Ah yes. I understand the Davey Lamp principle, but then again, Davey didn't have a honking big loudspeaker in his lamp either . A safety valve would be good insurance.

My brother had a Hillman Imp. One damp morning it would not start. My brother is not among the most technically aware, so he kept on cranking the starter in the hope that, by some miracle, the fuel would ignite.

Evidently there was no problem with the carburettor because it did exactly what it was supposed to do and mixed the fuel and air in ideal proportions for combustion. Unfortunately this mixture was being pumped out of the engine and into the exhaust system in a sufficient quantity to completely fill the silencer.

Eventually, my brother got the result he desired. Ignition! There was an extremely loud bang and the silencer was split wide open from end to end. The silencer was also immediately de-coked and all the carbon deposits spewed forth as flaming embers in the explosion.

Fortunately no one was injured, but it certainly woke my brother up. I think he had to have a change of trousers.

Geezer, Thu, 22nd Apr 2010

Thanks! I see they even tell you "How to Spot a Cowboy". I'll try that out around here.

Well, I'm glad they are finally using trained corgis as gas fitters in the UK. I'm sure they'll do a much better job than some of the twits that used to work for our local Gas Board 
(OK! OK! - it was a joke - well, kind of anyway.)

EDIT: Come to think of it, our Gas Board really was a joke! Geezer, Thu, 22nd Apr 2010

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

Sounds like you are not waiting for long enough, or just don't have enough gas flow, so the flames are very oxygenated. I don't know what kind of gas you are using, methane would probably burn cleaner than propane anyway. daveshorts, Mon, 21st Jun 2010

I used 1mm holes at a 5mm spacing. I am not sure of optimum diameter. I think for a 1800mm tube like mine more the 50mm diameter is great for higher order modes, but the fundamental doesn't work quite so well. I also had a test tube which was only 20cm long which would only do fundamental and first harmonic, I think because the cross section was larger than the second harmonic, so complicated 3D standing waves started happening. daveshorts, Mon, 21st Jun 2010

Great post!  I've been working with Ruben's tubes for a few years now and have come to the same conclusion you have about the loud volume behavior.

I was wondering if you could expound a little more on the effects of different diameter tubes.  We're working to make a fully optimized tube.  :-)  More details after we've got it up and running.

Also, I second the JB-weld. jgoldbeck, Wed, 4th May 2011

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.

My feeling is that if the tube is too narrow, the low frequency resonances will be weakened (I definitely see this) but as the tube gets wide, the high frequencies will start to have a wavelength similar to the radius of the tube, and I imagine that you will get all sorts of exciting modes of vibration across as well as along the tube. I think this will tend to smear the resonances, which is what I see with mine. It works beautifully for short wavelengths but you don't really get the first couple of modes. daveshorts, Wed, 4th May 2011

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 made one of these using 50mm PVC pipe and it worked well.
I then attempted one using 50mm galvanised pipe so it would last longer and it doesn't work.
Any thoughts on what may be wrong?
The holes in each are 2mm diameter and spaced 10mm apart.
each tube is about 180cm long.
levo1969 levo1969, Fri, 20th 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.

1 1.9m 75mm gorrigated iron down pipe (Bunnings, $10)
1 1.9m 220x15mm pine
2 0.3m 220x15mm pine
1 jaycar 100mm speaker
  silicon rubber
1 200x200x7mm plastic plate (hole in middle for blowtorch thread)
1 blowtorch
1 set of $5 computer speakers (for the amp)
1 iphone with oscillator app

65 2mm holes in the top, 20mm spacing.
Cut two big holes in the small bits of pine to house the pipe, attach on top of either end of the base. seal pipe flush to the end of the wood with silicon rubber. On one end, mount the speaker (silicon seal), on the other, the blowtorch/plate.

I've been able to demonstrate a standing wave at various frequencies, and use it as a great visualiser for music (youtube video on the way)

This is just something i knocked up as a pilot - I'm designing mach 2 now. Any suggestions? One thing I'll do is reduce the jet size, and spacing. Jets currently have difficulty lighting their adjacent flame.

By the way I'm not a pyhsics teacher, just an independent learner. I wonder if any schools would like to borrow the tube? I'd be happy to lend or demo. rationalmaximiser2, Mon, 4th Jul 2011

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

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