II.

Pipes

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.