The thermoacoustic organ is unique among instruments in several aspects. It is one of the few that is fire-powered. The other flame instruments in existence, such as the Orgue à Feu invented by Michel Moglia, have been described as more a spectacle than a concert. These instruments, like the one at right built by Antoinette Jacobson, emulate pipe organs in their sound production but their playing is wild and unpredictable. In contrast, the thermoacoustic organ can play recognizable melodies and even be integrated into ensembles with conventional instruments.
The Pipes
A series of custom-made pipes provides the geometry necessary to sound production. As described before, each pipe in theory has three components: the heat exchangers, the stack, and the resonating tube. As manufactured, each pipe had the stack plus six metal parts (below).
A steel nipple seals off the hot end by screwing onto a 1" pipe, which in turn screws into the hot exchanger. The heat exchangers sandwich the stack and its metal casing. Finally, the open end of the resonating tube screws is threaded into the cold exchanger. The stack is made of Celcor, a material with numerous 0.5 mm channels run its length. Celcor is an extruded ceramic, originally used to filter molten metal in foundries. The stack casing is made of stainless steel for its low thermal conductivity. All of the heat from the burner should be going into the air in the tube, so any that flows down the pipe itself is wasted. The outer resonating tubes do not experience so extreme a temperature gradient so they are made of cheaper black steel. The heat exchangers are made of copper for enhanced conduction. Each has 44 holes through which the air in the tube travels. Each tube has its own hot exchanger, but a single part acts as the cold exchanger. In fact, this is the only part that the organ pipes share, so it acts to hold the tube bank together.
The Hot
As the name of the instrument suggests, the hot temperature is maintained with fire. A line of propane flames, similar to a gas stove, heats the hot exchangers. A stock regulator attached to a propane tank reduces the pressure from the high level inside the tank to a manageable one. A metering valve controls the flow rate. The final part of a grill provides a high-speed jet of gas to pull along air and premix the fuel. This jet is created by Swagelok cap with a small hole drilled in the center.
The original heating system for the organ burned denatured alcohol. A number of lantern wicks running the length of the organ sat in a reservoir. This system had several disadvantages, however. Propane burns hotter and a tank lasts much longer than the alcohol reservoir, allowing longer performances. Finally, the ability to turn off the fuel with the control valve makes for a much safer instrument.
The Cold
Removing heat from the cold end of the stack is just as important as adding heat to the hot. The thermoacoustic organ accomplishes this task with liquid nitrogen. This -170ºC fluid flows through twin channels that run the length of the cold exchanger. The nitrogen is stored in a 50-liter pressure tank (right), and the vapor pressure of the boiling nitrogen forces the liquid out.
Insulation
The heat from the flames should ideally be going entirely into heating the air at the hot end of the stack, and liquid nitrogen should be absorbing heat solely from the cold air in the tube. To keep the heat where it is wanted, an insulating blanket was installed around the heat exchangers. Made out of amorphous silica and capable of withstanding temperatures of up to 1100°C, this resin-free quilt is not destroyed even by direct propane flame.
Activation
The organ employs a tractor exhaust cap on the open end of every tube. When closed, they completely seal the pipes and no sound is created. When opened, the sound wave forms and the instrument emits a tone within half a second. While not quick enough to play Chopin's Etudes, it is sufficient to play many pieces, and far superior to the multi-minute delay of simply waiting for the exchangers to heat up and cool down.
Each cap is spring-loaded so that it is closed in the absence of an external force. A wire connects the organ keys to the cap (right). When the key is unpressed, the spring keeps the cap closed and the sound off. When the key is depressed, it acts as a lever and tensions the wire. A plastic hose snap (the circle in the diagram) guides the wire so that the cap is pulled in the necessary direction to open.
Musical Range
The range of notes for the Fire and Ice Organ is compromise between musical versatility and ease of manufacture. Increasing the number of tubes allows more diverse music to be played on the organ. However, the dual heat exchangers of the organ pipes require the drilling of 88 small holes for every single tube, so the less tubes required, the happier the college machine shop is. I ultimately decided on a 14 note arrangement: all the "white keys" form the C below middle C to the F above it, plus the included C-sharps and B-flat. This combination allows much musical variety with a relatively low number of pipes. (If necessary, allow this file to show active content that could access your computer. It is a mouse-over portrayal of the scales and chords.)
| Keys | Chords |  |
|---|---|---|
| C Major | G Maj & Min | |
| F Major | A Maj & Min | |
| D Minor | Bb Maj & Min |
Future Work
The organ is now virtually finished. The combination of propane flames and liquid nitrogen produce a temperature gradient sufficient for every pipe to sound without creating any additional noise that would drown out the tones generated. The keys activate the pipes reliably and with minimal delay. However, the tuning is less than perfect. All length calculations were made when the pipes were being operated with a MAPP gas torch and ice water. The liquid nitrogen cooling system has decreased the air temperature enough to slow the speed of sound, lowering the pitch of all the notes. The shorter tubes are cooled more than the long, so the high notes are flatter than the low. I have been able to pick out select notes from the original 14 that make an approximate scale, but it is not as in tune as I would like. Getting all the tubes the right length would greatly increase the musicality of the instrument.
Other upgrades include the possibility of additional bass notes activated by foot levers, much like the pedal keyboard on a pipe organ. More visible flames would also be a nice addition. The switch from alcohol to propane as a fuel made the instrument more reliable and safer, but also made the flames less visible. Special lever activated "flares" could easily be added to the propane system for an almost unlimited theatrical potential.
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