The following by Bill Kibler was discovered on the net on the subject of pipe organs. Somehow this could never be quite the same if forcibly converted to metric under EC Directive EC/181/80
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A typical "open" organ pipe, or any other column of air, such as a trumpet or flute, for instance, that is approximately two feet long will produce a fundamental roughly corresponding to middle C. Doubling the length will halve the frequency, lowering the pitch an octave, halving the length will double the frequency, raising the pitch an octave.
A "stopped" pipe, which has a plug or stopper in the end, has the sound wave reflected from the closed end, so that the effective length is twice the actual length. Therefore, a stopped pipe of 8' pitch is only 4' long. In addition to providing a unique sound, stopped pipes are often used in the pedal registers of smaller organs to save space and cost. Stopped pipes are peculiar in that as a result of the reflection all the even harmonics (2x, 4x, 6x, 8x, etc.) are damped out, which results in a typical "hollow" tone. For acoustically similar reasons, a clarinet has little even harmonics, therefor a clarinet patch is very effective as a simulation of a stopped pipe sound.
Pipes are also divided into two groups or families based on the method of sound production. "Flue" pipes are essentially similar to recorders or whistle flutes, in that the air is directed against an edge, where it drives an oscillation at the resonant frequency of the pipe. "Reed" pipes use a metal tongue driven by the air to produce the oscillation, and the resultant pitch and sound is influenced by both the tongue and the length of the pipe.
The following example will demonstrate the harmonic series. You will hear the whole series from 1 to 8 followed by these pitches sounding together, followed by just the odd harmonics (1-7) individually and then together.
Play this example.
The seventh harmonic is intrinsically out of tune with the scale. The note (B-flat) that I show is really quite flat compared to the real seventh harmonic. To learn more about harmonics, just ask any horn or string player.
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Pipes in an organ are organized into groups of similar sounds, or "voicings". These groups are called "ranks". Organs can have as few as 1-4 ranks, as in chamber or residence organs, to as many as hundreds of ranks, with thousands of pipes, as in a large theater or church organ. Organists, in order to define the range of pitches available in the various ranks, use labels based on the length of the longest pipe in a rank.
Typically, a pipe organ keyboard, or "manual" will extend two octaves down from middle C, and three octaves up, so that the lowest pipe in a rank tuned to "unison pitch", or middle C, will be eight feet long. A rank that sounds middle C when middle C is depressed on the manual is called a rank of eight foot pitch, or simply an eight foot rank. A rank that sounds an octave higher is a 4' rank, two octaves higher 2', three octaves higher 1', an octave lower 16', two octaves lower 32' and so on.
Pipe organs date back to Classical Greek and Roman times but in those days all the pipes associated with a particular pitch sounded together. During the late Renaissance, a mechanism to "stop off" unwanted pipes in a particular part was invented, and for that reason a rank or group of ranks sounding together is called a "stop", (what we would call a "patch" in synthesizer terms). Stops, then, may consist of individual ranks, or combinations of ranks.
The important thing to remember is that all stops other than 8' are transposing, that is, the pitch produced is higher or lower than the note depressed on the keyboard.
Stops which reinforce the fundamental and octaves are called "Foundations" and are: unisons (8'), superoctaves (4', 2', 1') and suboctaves (16', 32', and sometimes even 64'). (Yes, the lowest pipes are really 64 feet long, although they are often folded or "mitered" to save space. The high notes of a 1' stop are very short, 1/4 inch or so.)
Stops which reinforce the odd harmonics are called "Mutations" and are: Quints (fifth [5-1/3'], twelfth [2-2/3'], and nineteenth [1-1/3'], reinforcing the fifth of the scale); and Tierces (tenth [3-1/5'], and seventeenth [1-3/5'], reinforcing the third of the scale).
With MIDI, Octaves are produced by transposing up or down 12 or 24 semitones. Quints are produced by transposing the part upward 7, 19, or 31 semitones. Tierces are produced by transposing the part up 16 or 28 semitones.
Stops which are primarily groups of unisons and octaves are called "choruses". Stops which are groups of upper harmonic reinforcements and which are commonly used in combination with foundations are called "mixtures". Stops which combine groups of foundations, mutations and/or mixtures are called, not surprisingly, "combinations".
The following example will illustrate.
Play this example.
Remember, these notes or groups of notes would all result from playing middle C on the keyboard.
Foundations:
• Measure 1 - 8 foot(')
• Measure 2 - 4'
• Measure 3 - 2'
• Measure 4 - 8', (middle C) again for reference
• Measure 5 - 16'
• Measure 6 - 32'
Chorus:
• Measure 8 - short phrase on middle C - 8'+4'+2'
• Measure 9-1/2 - pedal added 16'+32'
Measure 11 - 8', (middle C) again for reference
Mutations:
• Measure 12 - 2-2/3'
• Measure 13 - 1-3/5'
• Measure 14 - 1-1/3'
Typical mixture:
• Measure 15 - short phrase - 8'+4'+2-2/3'+2'+1-1/3'
• Measure 16-1/2 - pedal added - 32'+16'+5-1/3'
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Our next example demonstrates layering to simulate combinations. A brief excerpt from Partita No.3 by J. S. Bach, repeated with a cadence added:
Play this example.
Excerpt - violin
Organ parts (lowered an octave for clarity)
• 1st. repetition - 8' (recorder)
• 2nd. repetition - add 4' (piccolo)
• 3rd. repetition - add 2' (ocarina)
• 4th. repetition - add 16' (flute)
• 5th. repetition - add 2-2/3' (pan flute)
Cadence - right hand (great) 8'+4'+2-2/3'+2'
left hand (swell) 8'+4'+2'
pedal 16'+32'
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ENDS
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