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.

---

Stop

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.

Ranks.

In the context of pipe organs, a "rank" is the group of pipes that sound the notes, from lowest to highest, for one stop (or voice), for example a trumpet. For each stop on the instrument (with the exceptions of mixtures and mutations - see below) there will be one rank of pipes. Within a rank, there is one pipe for each key on the keyboard. (Couplers may necessitate more pipes than there are keys on the keyboard, however. See explanation of couplers below.)





The Trompette en Chamade rank installed at Temple Beth Zion in Buffalo, New York. (Various other more conventionally mounted ranks are visible in the background, along with the Swell box, which has vertical louvers to control loudness of the pipes inside the box.) Although not all of the pipes in this rank are visible, I can clearly count 53, with hints of a few more hidden behind those clearly visible. There probably is a total of 61 pipes in this rank, corresponding to the 5 octaves plus one note (61 keys) on the standard organ keyboard. The longest pipe would sound when the lowest note on the keyboard (low C) is pressed (with the Trompette en Chamade stop on, of course). The shortest pipe would sound when the top note on the keyboard (high C) is pressed.

Photo used with the kind permission of Chuck LaChiusa (http://ah.bfn.org/)








In many electronic instuments, including the one being renovated at Clemson, the term "rank" has a slightly different meaning. Again, a rank is a set of devices generating all of the notes from the highest to lowest, except the devices are electronic circuits (called oscillators or tone generators) instead of pipes. Each oscillator in the Park organ actually generates two waveforms (tones) at the same frequency, a sine wave (very mellow with almost no frequencies other than the fundamental - sounds like a flute) and a square wave (very brassy with lots of higher harmonics mixed in - more like a trumpet or oboe). These two tones are then routed to devices called "tone changers" which are specialized filters that combine the two tones in various ways and change the harmonic content as needed to simulate the sounds of standard pipe organ voices.

In the Park organ, each "rank" is routed to a set of eight tone changers, thus each rank of oscillators actually produces eight voices, not just one, and has eight corresponding stops on the stopboard. You will note that the stops come in multiples of eight on the stop board (not counting the unified ranks - see below). For example, the Solo manual has three ranks, one 16', one 8', and one 4'. There are eight 16' stops, eight 8' stops, and eight 4' stops. This can easily be seen on the photo of the Solo stopboard. The pedal ranks only produce six stops each, with the exception of the 32' rank which only produces two stops.

Although there are some acoustic disadvantages to this scheme of multiple stops from a single rank, it does allow a large number of stops to be implemented relatively inexpensively. Perhaps I will write further comments on this topic later.

• The "foot" ( ' ) notation.

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.

• Tonal Families:

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)

• Other stops.
  • In addition to the simple voices, there are also mixtures, which are exactly what they sound like - mixtures of similar tones, but at two or more pitches. When a mixture is played, two or more pipes (or electronic tone generators) sound when a key is depressed. On the Park organ, the mixtures are color-coded by the family from which they are derived. Typically, the stop for a mixture will bear a Roman numeral indicating how many pitches are mixed together. For example, Fourniture IV would sound four simultaneous pitches when a key is depressed.
  • Mutation stops activate tones that are not at octave intervals from the base (8') octave. Mutation stops will have foot values that are NOT a power of 2, such as 3' or 5 ¹/₃'.
  • On this organ, percussion stops are mostly black (there are a few that are dark green). This includes both real percussion instruments such as cow bells or snare drums as well as electronically synthesized percussion sounds.

• Special Effects: In addition to stops that turn the various voices on or off, there are stops to control vibrato (a slight repetitive variation in frequency), tremolo (a slight repetitive variation in loudness), and sustain (the note dies away gradually after the key is released instead of turning off immediately). There are also stops for celeste, which shifts the frequency of an entire rank slightly sharp (higher frequency) or slightly flat (lower frequency). In normal usage, a celeste stop is selected along with an unshifted stop, and the slight difference in frequency causes what is often called a beat frequency, which is heard as an undulation (for lack of a better word) in the sound. It is sort of like tremolo, but usually a bit slower as well as being qualitatively distinct. On the Park organ, these stops are light blue.
  
  
• Couplers (the rectangular white switches along the bottom in some of the pictures) allow the organist to play the stops associated with one manual on a different manual. They also allow the organist to shift the entire pitch of the manual up an octave or down an octave. For example, Great to Pedal 4' allows all of the stops on the Great manual to be played one octave higher on the pedals. For example, the standard organ keyboard is five octaves. However, with the couplers, the pitch can be shifted both up an octave and down an octave, so each manual rank has a total of 7 octaves worth of oscillators - five in the middle, plus one on each end for the octave shifts up and down.
  
  
• Unified Ranks: Most of the stops on the top rows (excepting the black percussion stops on the Great manual) are from what are known as Unified ranks. The unified ranks can be played on ALL manuals and pedals (assuming that manual has the appropriate wiring) with various shifts of pitch. The unified ranks are the sources of the mutation stops on this organ. Each of the four stop families has a unified rank. Examples include 16' which is an octave lower than the norm, 2' is two octaves higher, 2 ²/₃' is an octave plus a fifth higher, etc. (If you do not know the terminology of octaves, fifths, etc., this may seem like Swahili or Urdu to you - sorry.)