RUBENS TUBE: HISTORY, MATERIALS, OPERATION - SCIENCE - 2025

The Rubens tube is a device that consists of a tube with a speaker at one end and a row of perforations through which flammable gas comes out, which ignites to produce small flames.

This design makes it possible to visualize the sound waves within the tube produced by the speaker. And it shows without a doubt that sound is a pressure wave, since the flow of flammable gas that comes out through the holes is proportional to the local pressure of the same.

Figure 1. Rubens tube. Source: wikimedia commons

Figure 1 shows a lit Rubens tube, in which it can be seen that the height of the flames follows a wave pattern, which is precisely that of sound.

The Rubens tube is named after its creator - Heinrich Leopold Rubens (1865 - 1922) - who was a prominent German physicist and engineer. His main field of research was the study of electromagnetic radiation in the infrared and his investigations were decisive in the development of the theories of black body radiation that later led to quantum physics.

In 1904 Heinrich Rubens built his famous tube, originally four meters long with a row of 200 perforations spaced 2 centimeters apart at the top.

materials

-First, a line is traced along the tube with a pencil or marker. It is also necessary to make transverse marks 1 cm apart and 10 centimeters apart from the ends of the tube, which serve to drill the holes.

-With a bench drill the holes are made, using a 1.5 mm diameter drill, in the marks previously drawn.

-At one end of the tube, the thread-to-hose adapter is placed and the outlet of the tube is covered on the other, using a piece of the glove's latex as a membrane. This is fixed tightly with masking tape and secured with a clamp that is placed over the tape so that the membrane is not cut.

-Then the gas cylinder is connected and a speaker is placed at the end of the membrane, which in turn has been connected to an audio amplifier. The signal can be generated with the mobile, since there are apps to generate audio signals at a given frequency.

-Finally, the audio output is connected to the amplifier using the appropriate cables. The scheme is in figure 2. Once the flame is lit, you can visualize the sounds in the flames of the Rubens tube.

Precautions

-Take the pertinent precautions since you are going to work with gas and fire, so the experiment must be carried out in well ventilated spaces and remove all flammable objects and substances from the surroundings.

-Avoid drafts.

-Be careful not to leave the device on for too long, to avoid damaging the speaker.

-The wave pattern is best observed by decreasing the intensity of the lights.

Figure 2. Rubens tube scheme. Source: self made.

Functioning

When the gas exiting the perforations is ignited and the sound source is applied close to the membrane, it can be seen how the flames of varying height draw the shape of the standing wave inside the tube.

The membrane is what allows the sound pressure pulses produced by the loudspeaker or horn placed next to the membrane to be transmitted to the interior of the tube, which are reflected when they reach the other end.

The superposition of the transmitted wave and the reflected wave creates pressure differences, producing a standing wave whose pattern is reproduced by the height of the flames, such as higher areas (peaks) and areas where the flame is barely distinguishable (valleys or nodes).

Appropriate frequencies

The loudspeaker can be connected to a variable frequency wave generator that must be set to the appropriate frequencies to visualize the different standing wave modes that form inside the tube.

The appropriate frequencies depend on the length of the tube and the relationship they must meet is the following:

Where L is the length and n = 1, 2, 3, 4…

The speaker can also be connected to the output of a music player to visualize the sound of music through the flames.

The Kundt tube: predecessor of the Rubens tube

The Rubens tube is a variant of the Kundt tube, created by another German physicist named August Kundt, who in 1866 built a glass tube, placed it horizontally, and filled it with pieces of cork or spore dust from the Lycopodium fern.

Then he closed one end with a plunger and put a flexible membrane on the other, to which he attached a sound source. In doing so, he observed how the spore dust clumped together at the nodes of the standing wave that forms inside the tube, and the wavelength can then be measured.

By changing the position of the plunger, the patterns corresponding to the different modes of vibration can be displayed for a given sound source.

Kundt's tube is very useful for measuring the speed of sound in different gases and at different temperatures, and it is clear that Heinrich Rubens was inspired by this work to create his famous tube.

References

1. Acoustics Web. Rubens tube. Recovered from acusticaweb.com
2. The tao of physics. Flames in an acoustic tube. Recovered from: vicente1064.blogspot.com
3. Maciel, T. The Flaming Oscilloscope: The Physics of Rubens' Flame Tube. Recovered from: physicsbuzz.physicscentral.com.
4. Sound and Waves Demonstration with a Rubens' Tube. Retrieved from: people.physics.tamu.edu.
5. Wikipedia. Heinrich Rubens. Recovered from: wikipedia.com
6. Wikipedia. Rubens tube. Recovered from: wikipedia.com.