A modern organ keyboard generally has a compass of five octaves, from two octaves below middle C to three octaves above, making a total of 61 notes. (Some older organs have shorter compasses, extending to only two-and-a-half octaves or so above middle C, since older organ music did not call for the highest notes.) Organ pipes may be made out of either wood or metal, the latter being a lead/tin alloy similar to pewter. At unison pitch (the same pitch as a piano), for an open stop the lowest pipe, two octaves below middle C, will have a length of approximately 8 feet from the mouth to the top, and is thus called an 8 ft. stop at the console. Open pipes are normal organ pipes open at the top. In the diagram below, the points of the greatest vibration of the air are at the antinodes (A), which are at the top and at the mouth. The point of least vibration is at the node (N), which is approximately in the middle of the pipe. Very little of the vibration takes place from the body of the pipe, and most of the vibration emanates from the top and the mouth, setting up standing waves in the building, capable of being heard. λ is the wavelength, and l is the speaking length of the pipe. As can be seen in the diagram, n2, n3, etc., represent the harmonics. An open pipe producing the second harmonic will be 8/2 = 4 ft. and will play an octave higher than unison pitch. The third harmonic will be 8/3 = 22/3 ft. and will play an octave and a (pure) fifth higher than unison pitch. The fourth harmonic will be 8/4 = 2 ft. and will play two octaves above unison pitch. The fifth harmonic will be 8/5 = 13/5 ft. and will play two octaves and a (pure) third above unison pitch. The sixth harmonic will be 8/6 = 11/3 ft. and will play two octaves and a (pure) fifth above unison pitch. As well as unison stops the organ has stops at all these different pitches in order to reinforce certain harmonics and create more volume and different timbres. These stops will be marked at the console with their appropriate pitch such as 22/3 ft.
Stopt (or stopped) pipes are closed at the top, either by a wooden stopper or tampon, or by a metal canister, or by having a top soldered on. As will be seen from the second diagram, the stopt pipe has its node (N) at the top, and a single antinode (A) at the mouth. A stopt pipe of 8 ft. speaking length will produce a pitch one octave below unison pitch and the stop will therefore be marked 16 ft. at the console, meaning 16 ft. pitch. It will be noticed that in the case of stopped pipes only the odd harmonics are produced, i.e., 16/3 = 51/3 ft., 16/5 = 31/5 ft., etc.
Generally speaking the wider the pipe the flutier the tone, and the narrower the pipe the brighter the tone. Organ stops are generally divided into three categories -- flutes (wide scale), diapasons or principals (medium scale) and strings (narrow scale). The stops representing the harmonics are also produced in pipes of different scales, e.g., a flute scaled Nazard 22/3 ft., a principal scaled Quint 22/3 ft. (sometimes called Twelfth 22/3 ft., because it plays a twelfth higher than unison pitch), or a string scaled Salicet 4ft. Stops whose pitch is an odd-numbered harmonic are known as mutation stops. String mutation stops are relatively rare, as are stops sounding the seventh harmonic, 11/7 ft. Flat Twenty-first (principal scale) or Septième (flute scale).
The diapason and principal stops and their harmonics on a particular manual or in the pedal are known as the diapason or principal chorus. They will often include ranks of high pitch such as Nineteenth 11/3 ft., Twenty-second 1 ft., Twenty-sixth 2/3 ft., Twenty-ninth 1/2 ft. and so on. For convenience sake these higher pitched stops are often grouped together in what are called mixture stops, and a Mixture with 4 sets of pipes or ranks to a note would be called Mixture IV or Mixture 4 rks. at the console, and would typically start at low (8 ft.) C as something like 19-22-26-29. To reinforce the treble, and also because the upper pitched pipes become impossibly small to make and may even be above the human threshold of hearing, it is normal to "break back" or move the pitch of the ranks down by, say, a fifth at intervals of, say, and octave. Thus a typical Mixture IV might be:
c2 8 - 12-15-19
c3 1 - 8 - 12-15
(Note here that the Quint or Fifth, 51/3 ft., is normally avoided because it is part of the 16 ft. and not the 8 ft. harmonic series, so the top octave here is 1(unison), 8 (octave), 12th. and 15th., rather than 5-8-12-15)
|Diagram of the vibration of air in an open organ pipe|
|Diagram of the vibration of air in a stopt organ pipe||