Overview
By providing a heat source at the right location within an open tube, a self-amplifying standing wave can be produced, creating a unique pitch, which is proportional to the tube’s length. The sound, surprisingly loud and somewhat ghastly in tone, lingers as long as the tube is heated.
The goal of this project was to make a micro-controller driven Rijke pipe-organ as a Christmas gift for my nephews. The tubes would be made from thermal-shock-resistant quartz glass so the glow of the heating element–electrified nichrome wire–could be seen center stage.
Prototyping
The prototype began with scrap plastic and a lab stand turned on its side. The goal was just to get a tube to sing and make sure the idea was sound. The note produced by each tube is a function of its length, so I ordered 5 tubes of varying length to span a range of tones.
| Note | Frequency (Hz) | Wavelength (cm) | Tube Length (in) |
|---|---|---|---|
| D4 | 293.66 | 117.48 | 23.1 |
| E4 | 329.63 | 104.66 | 20.6 |
| G4 | 392 | 88.01 | 17.3 |
Fabrication
After success with the prototype, next up was building the real deal. Since this project required red-hot heating elements, I wanted the final version to be entirely fire-proof in case something went wrong. Plate aluminum, ceramic stand-offs for the nichrome, and high-temp insulation on the wiring.
With a rough CAD model, machining went fairly quickly via a combination of manual and CNC-driven work. As with most projects, McMaster-Carr was essential for sourcing the many strange parts necessary to complete this odd-ball build.
The set of 5 Rijke tubes was controlled by a bank of relays driven by an Arduino. To keep the project more family-friendly, I capped the voltage at 24V to minimize risks of shocks or sparking. The electronics sat off the main platform to further insulate them and were connected to the organ via a custom 6-pin connector. The code was fairly brief, and, once the organ was powered on, would cycle through all combinations of notes (the interference of two pipes playing at once creates a nice vibrato) and then proceed to randomly generate notes after that. While I planned to have the organ play an actual song, the Christmas deadline snuck-up on me before I had a chance to get that far.
The Final Product
A little final tweaking was needed to get the pipes singing their best (hence all the adjustable points: clamps, set screws, etc.). The nichrome has to be in just the right spot–the working window is only a few centimeters–so I suppose that’s how this instrument gets tuned.
