Rubens Tube Project

Scroll down below video for more information and lesson plan.

Introduction

A Rubens Tube is a device first demonstrated by Heinrich Rubens in 1904. It consists of a tube containing many small holes and a fitted with a speaker on one end. When the tube is filled with a flammable gas, the escaping gas can be lit to create a row of diffusion flames. Normally these flames will all stand about the same height, since the pressure in the tube is more or less constant and the holes are all the same size. However, when a sound is played through the speakers, the flames change in height such that they display a standing wave with a wavelength equivalent to that of the sound being played.

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Rubens Tube Lesson Plan

Physical Explanation

The Rubens Tube operates by the principle that the sound waves we hear are actually pressure waves traveling through a gas. These pressure waves create slight variations in the local pressure at various lengths down the tube, even though the overall pressure remains the same. From Bernoulli's Principle, we know that the velocity of the gas escaping the holes in the tube is a function of the pressure forcing the gas out, all other things being equal. In areas of higher pressure, the gas escapes the holes faster and therefore the flames are taller, and in areas of lower pressure the flames are shorter. The areas of higher and lower pressure are caused by the sound waves in this case, so the pattern created by the different flame heights across the tube are a representation of the sound being played.

Construction

Our Rubens Tube was constructed mostly from parts we purchased at Home Depot. For the main tube section we used a 60" section of 4" diameter galvanized ventilation pipe. The pipe comes unwrapped, so we had to roll it ourselves. We decided to make the flame outlet holes 1/16" diameter at 1/2" intervals. We drilled the holes before rolling the ventilation pipe.

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4" Ventilation Pipe used for Rubens Tube.
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Marks at 1/2" intervals for holes to be drilled for flame outlets.
To secure the ventilation pipe while working on it, we attached an endcap. Ultimately this endcap would be used to mount the tweeter speaker.
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Van attaching the endcap after rolling the tube.
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Van holding the rolled tube.
After drilling the holes and rolling up the tube, there were still burrs and other imperfections in the holes. To ensure that the holes were more or less uniform, we cleaned up the holes with a conical grinder drill bit.
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Grinder bit we used to clean up the flame outlets.
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Cleaning up the flame outlets.
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Closeup of holes for flame outlets.
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Rolled tube with holes in mounting brackets.
We decided to install two speakers in the tube, one at each end, since at the time we weren't sure of the best size speaker to use. We salvaged the speakers out of a discarded computer speaker system we found in a dumpster, and used one of the tweeters and the subwoofer.
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We had to remove the subwoofer from the computer speaker assembly.
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Electronics of discarded computer speaker system.
The subwoofer took the most work to attach to the tube. We had to use an expander to bring the 4" pipe out to 6" to fit the subwoofer, and make custom brackets out of duct hanger strap. The angled brackets we initially purchased were too large and didn't work.
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Subwoofer from computer speaker system.
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These brackets didn't end up working.
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Duct hanger strap used to make custom brackets.
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Closeup of custom brackets used to attach subwoofer to pipe expander.
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Gasket made from square rubber sheet.
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Subwoofer and expander assembly.
We used propane from an LPG tank (the kind used in backyard barbeque grills) for the flammable gas. We purchased an adjustable pressure regulator for the tank, which turned out to be a very wise choice since careful control of the gas flow is required for best results of the Rubens Tube. Also, we set up the electronics so that we could attach both the subwoofer and the tweeter, or just one of them at a time. As it turned out, for the most part, the tweeter is all that is required. The subwoofer only registers relatively low frequencies with long wavelengths, and tends to be so powerful that it blows the flames out. It did turn out to be quite dramatic for music with a lot of bass, but wasn't really worth the trouble, and if I built another one of these I probably wouldn't include it.
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Hose barb for propane inlet.
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Connecting the Rubens Tube to the speaker electronics.
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Subwoofer end of Rubens Tube.
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Completed Rubens Tube.