ABSTRACT
The traditional pipe organ is a familiar sight in sacred locations such as churches and performance halls. This project seeks to reach beyond those environments and suggest that the street has its own sound and soul. Motorrrgan is a mobile pipe organ that uses the compressed exhaust of a motorcycle’s internal combustion engine to play a series of fabricated wooden pipes.
1. INTRODUCTION
This project was inspired by research into pneumatic muscle systems and how to dynamically control them using pressurized exhaust from pneumatic air systems. Building custom air cylinders sparked an interest in the compression cycle of the internal combustion engine as it is nothing more than an air compressor outputting compressed exhaust gas instead of air. Part of my fascination with this project is that I am literally “tuning” an engine. Imagine a world where exhaust pipes played interesting rhythms.
2. THE PIPE ORGAN
A traditional pipe organ in its simplest form is a group of whistles or pipes that are grouped together through a manifold (“windchest”) and can be played individually or as a group using stops and a keyboard. Pipes are of wood or metal construction and produce a tone when pressurized air is directed into them via a valve system. Since a single pipe produces a single pitch multiple pipes are required to play a full scale. Pressurized air is generated either from manual bellows or more typically an electric blower that functions much like a super strong fan. Air is stored in the windchest at a constant pressure until supplied to the pipes to produce a tone. A modest sized pipe organ pipe plays best at around 0.1 psi or about 2-3/4 inches of water on a manometer. This figure can vary depending on the age of the organ and the number and size of pipes. For multiple ranks of pipes to play a large volume of air is required.
3. INTERNAL COMBUSTION ENGINE
A typical engine on a modern motorcycle is 2 cylinder 4-cycle running on gasoline. The pressure at the cylinder head on a fully functioning motorcycle engine is around 150psi. As the exhaust gases work their way through the exhaust system and muffler the pressure is reduced due to the expansion caused by the muffler. Unlike a blower typically used on a pipe organ the engine produces compressed air at the exhaust stroke of the engine cylinder which works more like an air compressor than a fan. The result is a higher air pressure, on a manometer the exhaust from a 650 cubic inch engine consistently measured off the chart at half throttle.
4. PERFORMANCE
The percussive pulsing of the pipes matches the pulse of the running engine. Digital actuators controlled by a programmable microcontroller control wooden valves for each pipe.
The instrument is intended to be played either while parked or while driving. This prototype has a small number of pipes making up a pentatonic scale, playing a programmed piece which can be manipulated by the engine throttle.
5. FABRICATION
Using a laser cutter the fabrication of individual pipes is quite straightforward. Wind sheet thickness, pipe length and other dimensions can be calculated or referenced from pipe charts. There are many ways to build a wooded pipe. The most traditional and best quality is to use tight grained hardwood and is skill reserved for a craftsperson. For this project fine wordworking resources were not available or desired. Using a laser cutter the fabrication of individual pipes is quite straightforward. Pieces of the pipe are cut out of MDF (medium density fiberboard) and laminated to each other to form the complex internal structure of the pipe.
6. CALCULATIONS
Calculations are derived from Raphi Giangiulio [6]
6.1 Calculation windsheet thickness
WST =I2f2ρH3/2P
I is Ising’s number
f is the frequency
ρ is the density of air
H is the mouth height
P is the air pressure
It’s important to be careful with the units in this equation. Using 1.2 kg/m3 for the density of air and putting in a factor to correct for the units, the following equation will give you the WST in inches.
WST =I2f2H3* 1.554 x 10-6P
WST is the windsheet thickness in inches
I is Ising’s number
f is the frequency in Hz
H is the mouth height in inches
P is the air pressure in inches of water
6.2 Table from Raphi Giangiulio [6]
7. REFERENCES and ACKNOWLEDGEMENTS
[1] G. Essl, G. Wang., and M. Rohs. Development and Challenges turning Mobile Phones into Generic Music Performance Platforms. In Proceedings of the Mobile Music Workshop, Vienna, 2008.
[2] Louis G. Monette, The Art of Organ Voicing, New Issues Poetry Press, 1992
[3] N.H. Fletcher, Air Flow and Sound Generation in Musical Wind Instruments, Annual Review of Fluid Mechanics, vol. 11, 1979
