Organ, keyboard musical instrument in which compressed air vibrates within tuned pipes to produce sound. An organ consists of flue pipes and/or reed pipes, an air supply, and the keys and other controls.
Flue pipes are made of metal or wood, and they work much like whistles. Air enters at the foot of the pipe, moves as a sheet against a narrow slit or flue, and begins to vibrate as it passes across a sharp lip set in the pipe above the flue. This initial vibration causes all the air in the pipe to vibrate, producing a musical tone. The pitch of the tone depends on the length of the pipe; the shape and material of the pipe influence the color or quality of the tone.
Some flue pipes are closed at the top; "stopped" pipes produce pitches an octave lower than open pipes of the same length. In reed pipes, the reed and the metal trough against which it beats (called a shallot) are encased in a pipe into which air is released from the air supply. The incoming air causes the curved end of the reed to beat against the shallot and set the surrounding air into vibration. The musical pitch produced is low for long reeds, high for short reeds. The shallot is connected to a pipelike resonator, the shape of which affects the color of the sound. A set of pipes all having the same tone quality is called a rank. The most characteristic organ sound is produced by metal flue pipes called diapasons or principals; pipes of this kind form the central core of classic organ sound
Because the lowest note on most organs (two octaves below middle C) is produced by an open diapason pipe about 8 ft long, ranks of pipes at normal pitch are spoken of as 8-ft ranks. Ranks sounding an octave lower than normal are called 16-ft ranks, and those sounding an octave higher, 4-ft ranks. Mutations are ranks of pipes sounding at pitches other than octaves above normal pitch, such as an octave and a fifth above normal (for example, two G's above a C). Mixtures are ranks of pipes of different pitches operated as a single unit (by a single stop). Mixtures often contain ranks sounding several octaves above 8-ft pitch as well as mutation ranks. The high pitches of mutations and mixtures blend together to produce the incisive, bright quality that is associated with organ sound. On large organs the ranks are grouped together into several divisions, each controlled by a separate keyboard, or manual, and having one or more wind chests, airtight boxes that act as air reservoirs. The main division is called the Great Organ; the other most common divisions are the Choir Organ, the Swell Organ, and the Pedal Organ. The pipes of the Swell Organ are enclosed in a "swell box," a chamber having a slat-covered opening similar to a venetian blind. The slats can be opened and closed by a pedal lever, allowing gradual changes in volume.
The air for the pipes is supplied from a wind chest, on which the pipes are mounted. Air, which is produced by bellows or by an electrical blower, enters the wind chest at a constant pressure. When a key is depressed, small valves open to allow air from the wind chest to enter the pipes and cause them to sound. A "stop" mechanism allows any rank of pipes to be prevented from sounding. The ranks are controlled by knobs or switches (called stops) set near the keyboard. By extension, the ranks of pipes they control are sometimes called stops. Until the 19th century the connections linking the keys and pipe valves -- including mechanisms to couple keyboards so that ranks of pipes may be multiply controlled -- were achieved mechanically by a system of levers and cranks connected by strips of wood called trackers and stickers.
Builders in the 19th century began to devise electrical and pneumatic actions to make the key-to-valve and stop connections. Because many organists believe these actions to be less responsive and sensitive than direct mechanical linkages, in the 20th century organs were again being built with the traditional tracker action. The keyboards, wind chest, and pipes of small organs are contained in one unit. In large organs the keyboards and other controls are built in a separate unit called the console. Many organ consoles have a number of controls, called pistons, which allow the organist to bring into play at one stroke a combination of several ranks or stops. Each organ is unique in that it must suit the acoustics and architecture of the room that houses it. The room itself has an intimate acoustic relation to the organ, profoundly influencing the sound of the organ by the amount of reverberation it allows.
The earliest organ, the hydraulis, was developed by the Greek inventor Ctesibius (flourished 3rd century BC). It utilized a large chamber partly filled with water. The wide mouth of a funnel-like extension from the wind chest was set in the top of the water; as air pressure in the wind chest fell, water rose in the funnel and compressed the air, thus keeping the air pressure constant. The hydraulis was used for public entertainments in ancient Rome and Byzantium. Bellows-type organs were also known to the ancient world. This was the organ that reappeared in Europe in the 8th and 9th centuries, imported from Byzantium and from Arabs who had discovered ancient Greek treatises. Although some ancient organs had a stop mechanism, this device was forgotten, and on early medieval organs all ranks sounded at once, creating a formidable effect.
By the 15th century the stop mechanism had been reinvented, pedal keyboards came into common use, and reed stops (not found on ancient organs) were developed. Smaller organs had also become common: the portative organ, carried by a strap around the player's neck, which had only one rank of pipes and was supplied with wind by a small bellows pumped by the player; the positive organ, self-contained and portable; and the regal, a small instrument with one rank of nasal-sounding reed pipes.
Between 1500 and 1800, various national styles of organ building developed, each distinguished by characteristic-sounding ranks. German organs of the 17th and 18th centuries were particularly outstanding, and it was for such organs that the music of Johann Sebastian Bach was written. Reed Organs - Keyboard instruments in which the wind supply is directed toward free metal reeds like those of a harmonica or accordion are called reed organs. They include the melodeon, developed in the United States about 1825, and the harmonium, developed in Germany about 1810.
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