Sealed Tube Speaker System Shows off Standing Waves sans Fire, Mess


Kundt’s tube is one of several classic acoustics instruments used originally to measure, and now mostly to demonstrate, phenomena related to standing waves. Unlike Ruben’s tube (which uses open flame and requires a source of combustible fuel) or a Chladni plate (which uses loose fine powder) to create similar effects, Kundt’s tube is both completely safe and completely clean. It can’t start any fires and the granular material is totally contained within the tube. This makes Kundt’s tube rather more suitable for permanent installations, especially in science museums or other kid-friendly environments.

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MF14BA_Badge-01The example shown here was built by San Luis Obispo resident and nuclear engineer Mike Sullivan. It consists of a clear tube with a speaker at each end containing hundreds of fluorescent plastic beads. The speakers produce two different pitches — one (to left) fixed at A220, and one (to right) that can be varied over a range of about a half-step up or down from A220 by turning the single knob.

Interference between the two similar tones produces beats that determine the location of nodes and anti-nodes in the standing wave pattern, and hence the pattern assumed by the acoustically-levitated beads. The control panel also features an LCD readout of the two frequencies in real time, and conceals an Arduino Uno configured to function as a dual frequency generator, plus an off-the-shelf stereo amplifier to boost output power. “While the sound is quite loud inside the tube,” Sullivan writes, “the chamber is sealed and the noise is quite moderate when operating.”

3 thoughts on “Sealed Tube Speaker System Shows off Standing Waves sans Fire, Mess

  1. Jared Ficklin says:

    Awesome! I built several of these and use them all the time to teach kids about sound when fire is not allowed. Even pre-fire it helps give them a sense of colliding molecules organized into waves.

  2. Daniel Pereira says:

    There is something I don’t understand. I been trying to create my own kundt’s tube with one speaker and the end of the pipe close but until now I haven’t achieve good results, can somebody give me some tips?

    I read in the article they used two speakers but with one it is not possible?

  3. Samuel Stanton says:

    I would really like to build this for a science fair project. Is there a way that you could make a guide with the code and wiring guide for the arduino.

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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