A Crash Course in Concepts and Terminology Concerning Organs

The following is seriously oversimplified, and is not intended to be more than a vague orientation to organ concepts and terms. It may be rough going in places for some of you, but the explanations will improve as time allows - this is rather quick and dirty. First a few helpful links to other sites.

For an excellent beginner's introduction to pipe organs (most of the concepts are common to both pipe and electronic instruments), check out the American Guild of Organists' Young Person's Guide to the Pipe Organ. (Also suitable for adults.)
PipeDreams, a PBS radio program featuring pipe organs (on Sunday afternoons here in SC) has a page with a variety of links concerning how pipe organs work, how pipes are constructed, etc. Also, the two most recent programs are available as streaming audio from the main site.
There are literally thousands of named organ stops, although many are synonymous. For an excellent online resource documenting all known pipe organ stops, including sound clips and construction details for many, try the Encyclopedia of Organ Stops.

A stop is the control (either a draw-knob or tab) on the console which turns a specific tone (or voice) on or off. More loosely, the term stop is sometimes also used to refer to the specific voice (such as flute or oboe) that the corresponding tab or knob activates.
(As I understand it, the term "stop" actually derives from the fact that the control, whether a knob or a tab, "stops" that voice from sounding when keys are depressed. This definition seems a bit like negative logic to me, but I suppose my ECE background is showing through.)
The tendency is for theater style organs to have tabs and church style organs to have drawknobs, but this is by no means an immutable rule.
Organ stops carry a number indicating a length in feet. On a stop board you may see Diapason 8' or Diapason 4', or Contra-Bourdon 32' or Fife 1'. This notation is derived directly from the physical length of the pipes in the corresponding rank for that stop.
The 8' stops sound the pitches that are intended by the written notes on the page. For example, "middle C", (the C just below the lowest line on the treble clef, which is the same as the C just above the top line on the bass clef), produces a sound with a fundamental frequency of about 262 Hertz (abbreviated Hz, formerly called "cycles per second"). This corresponds to the C smack in the center of the manuals. If you go up an octave (to the next C), the frequency doubles to about 524 Hz. If you go an octave down to the C below middle C, the frequency is cut in half to about 131 Hz. The lowest note (also C) on the organ keyboard is two octaves down, thus half of half (or one-fourth) of the frequency of middle C - roughly 65 Hz.
This is all well and good, but why is this strange "foot" notation used? As it turns out, the 65 Hz frequency low C on the keyboard (standard 5 octave organ keyboard - a piano keyboard goes a good bit lower) corresponds to the resonant frequency of a tube approximately 8 feet long, thus in a pipe organ (as opposed to an electronic organ) the low C on the keyboard with an 8' stop will be generated by blowing air into a pipe eight feet long.
If you look at the photo above in the description of "rank", you can see this fairly clearly. The largest pipe on the far right immediately behind the console appears to be about 8 feet long, although the exact length is a bit difficult to judge without an accurate scale reference. (It certainly is longer than 4 feet and shorter than 16 feet, which are the other two options.) More difficult to judge due to the perspective is the length of the longest pipes in the Trompette en Chamade rank, but the longest appears to be about 8 feet also. (Actually, in reed pipes, of which this is an example, the fundamental tone is not generated by the pipe itelf, but by a reed at the base of the pipe. The pipe you see is actually a resonator designed to modify the harmonic structure of the tone, and may or may not have a length commensurate with the fundamental frequency being produced.) As you go up the keyboard, the pipes for the higher notes produced by that stop get progressively shorter. (The pipe producing the high C on the keyboard with an 8' stop selected would be only 6 inches long - can you figure out why? See next paragraph for a hint.)
A 16' stop will play notes an octave below the written note values. (Doubling the pipe length halves the resonant frequency, and vice versa.) Thus the low C on the keyboard with a 16' stop selected has a fundamental frequency of half of 65 Hz (the low C frequency with an 8' stop) or about 32 Hz, and in a pipe organ, the pipe generating that note would be 16 feet long. Some organs have one or more 32' stops. The low C on a 32' stop has a fundamental frequency of half that of the 16' stop, or about 16 Hz. This is at the very lower limit of human hearing. Relatively few pipe organs have a 32' stop due not only to the huge size of the pipes (making them very expensive) but also to the size of the blower needed to supply air to these gargantuan pipes. To give you an idea of the scale, click here for a picture of the organ in Royal Albert Hall, which has the pipes for a 32' stop prominently displayed. Note the relative size of the organist at the console near the bottom center of the picture.
To carry this idea to the absurd extreme, there are actually a few organs that have a 64' stop. The lowest octave produces frequencies BELOW the range of human hearing - you do not hear them, you FEEL them! (Well, actually, you don't hear the fundamental frequency, although you can hear the higher harmonics present in the tone.)
There is actually a less expensive method of simulating the really large pipes with two smaller pipes sounded simultaneously. Basically, for each note, a pipe sounds at the second harmonic (twice the desired frequency - half the pipe length) and another sounds at the third harmonic (three times the frequency - one third pipe length). Human audio perception "hears" the fundamental frequency (the first harmonic, or the pitch that would be produced by a full-length single pipe) when the second and third harmonic are played together. Such tones are called "synthetic stops". This is usually not cost effective in electronic organs since the cost of a single electronic oscillator depends only slightly on the desired frequency - two higher frequency oscillators would almost certainly cost more than a single oscillator at the lower frequency.
Since the pedalboard is only about 2 2/3 octaves, there may be a question concerning what the foot notation means. In this case, the low C on the pedal board corresponds to the low C on the manuals. Thus an 8' pedal stop has the same pitches as the lower 2 2/3 octaves of the manuals with an 8' stop selected.
There are four basic tonal families, not counting special effects. On the organ being renovated in this project, the stop keys for these families are color coded.
    • Green: Diapasons (or Principles). These are pure organ sounds not intended to be imitative of another instrument.
    • White: Tibia (Flutes). These are imitative of transverse flutes, recorders, etc.
    • Yellow: Strings. These are imitative of stringed instruments such as violins and violas.
    • Red: Reeds. The reeds are fundamentally different from the previous three families. In organs with real pipes, they have a part that moves, called the reed. This is much like the reed in a clarinet or a saxophone. (These are, of course, simulated electronically in the Park organ and have no moving parts other than electrons.) The other three tonal families, Diapasons, Flutes, and Strings (collectively called flues), have no moving parts. The reeds (perhaps surprisingly) are largely imitative of brass instruments, often having names like trumpet or tuba, as well as reeded instruments, such as bassoon or clarinet. The reeds also include a number of other stops such as Vox Humana (human voice)

    Sound samples of the four tonal families as implemented by the Artisan Organ Co.

    These clips are based on the arpeggio format used in the online Encyclopedia of Organ Stops. These recordings are of the four unified ranks.

    Diapason arpeggio from C0 (33 Hz) to G7 (6272 Hz) This will be recorded again later following fine tuning and amplitude adjustment. Those of you with a vaguely astute ear will cringe at a few of them.
    String arpeggio from C0 (33 Hz) to C7 (4186 Hz) These also need fine tuning of both pitch and amplitude. Also, the top octave has a loose ground connection which will require removing the board to repair. The next version will extend up to G7.
    Reed arpeggio from C0 (33 Hz) to C7 (4186 Hz) The highest stop on the unified Tuba rank is 4', thus there are no notes above C7 in this rank.
    Tibia arpeggio from C0 (33 Hz) to G7 (6272 Hz)

    This page last updated at 1:52 PM on Mon, Apr 23, 2007
    CHORD site maintained by Dr. William Park. Please address comments or suggestions to parkw@ces.clemson.edu