Organ/Types of Pipes
There are four basic tonal groups of organ pipes that each individual stop can be sorted into, depending on their sound quality and construction.
Flue pipes cause air to vibrate by driving air against a sharp lip at the base of the pipe, similar to whistles or recorders. The pitch of a flue pipe is determined by its length. Flue pipes make up most of the pipes in an organ, and can be categorized into three tone qualities.
- Principals (or diapasons)
The principals are the most common stops on an organ, and the backbone of many stop combinations. All instruments will have this type of pipe. The principals don't imitate any orchestral sounds but are the pure, basic "organ" sound often associated with hymns.
On a console, they can be found under the name "Principal", "Diapason", "Prestant", or the German and French equivalents, "Prinzipal" and "Montre". On the manuals, they occur most commonly at 8' pitch, usually with 4' and 2' octave variants available (which are typically called "Octave" and "Super Octave" instead of "Principal"). On the pedals, you can usually find them at 16' and 8'.
Flutes are another common type of organ pipe. They are wider than principals and produce a tone that has more fundamental and less upper harmonics. They are usually softer and breathier than the Principals and sound somewhat similar to orchestral flutes. Flute pipes can be either open (like the principals), or stopped at one end, which makes the sound more muffled and reduces the amount of even-numbered harmonics.
Common open flute stops include Flute, Harmonic Flute, Piccolo, Waldflute and Sifflote, while common stopped flutes include Gedackt, Rohrflute, Stopped Diapason, Bourdon and Subbass. They can occur at any pitch, usually 16', 8', 4', and 2' in the manuals, and 16', 8', and sometimes 32' and 4' in the pedals. Mutation stops like 2 2/3' and 1 3/5' are generally made to have a flute tone, although some may have a principal tone instead.
String pipes are narrower than principals and produce a thin, bright tone. They are less common than principal and flute pipes, and occur only at 8' pitch on many organs; larger organs may have strings at 4' and 16'. Strings also often have celestes, which are a second rank of string pipes that are slightly detuned to produce an undulating effect when both regular and celeste ranks are drawn. Common string stop names include Gamba, Violoncello, and Violone.
Some stops contain tone qualities of two different classifications of flue pipes. Examples include the Geigen Principal and Salicional (combination of String and Principal), and the Gemshorn and Spitzflute (combination of String and Flute).
Reed pipes produce sound by means of a vibrating brass reed that beats against a shallot, similar to a mouthpiece of a clarinet or saxophone. Unlike flue pipes, the pitch is determined by the reed and not the pipe, which instead serves as a resonator. The shape and length of the resonator affects the sound; for example, an inverted-conical resonator tends to produce a Trumpet or Oboe-like sound, a cylindrical resonator produces a Clarinet sound, and a shortened resonator (known as a Regal) produces a buzzy sound with little fundamental.
There are two main types of reed stops. Chorus reeds (such as the Trumpet, Clarion, Oboe, Bassoon and Trombone) blend in with flue pipes and serve to add power and brilliance to the full organ, and are usually the loudest stops. They are not necessarily imitative of their orchestral namesakes. Some organs have extremely loud stops such as the Tuba, which is characterized by a "smooth" tone; and the Trompette en Chamade, a horizontally mounted Trumpet often visible on the exterior of the organ case.
Solo or orchestral reeds come in a wide variety of tone colors, often imitating orchestral instruments. They are used in quieter solo passages, and generally do not blend well with other stops. Some examples include the Clarinet, French Horn, Orchestral Oboe (distinct from the non-imitative Oboe), and Vox Humana (a regal that somewhat imitates the human voice).
A different type of reed construction is the free reed, which beats through a shallot instead of against it. This produces a unique sound that is "mellower" than that of traditional reed pipes. Free reeds are rare in pipe organs (most often found on 19th-century German romantic organs), but are the primary method of sound generation in the pump organ or harmonium, as well as accordions and harmonicas.
Some organs also feature percussion instruments. The most common percussion stops on church and concert hall organs are chimes and the Zimbelstern (a series of bells producing a continuous tinkling sound when activated). Theatre organs (a different type of pipe organ originally intended to accompany silent movies) tend to contain a larger variety, including xylophones, bass drums, cymbals or even pianos. While not technically pipes, they nonetheless merit inclusion here. They usually should not be used as part of a large chorus because they don't blend in well with other organ pipes.