The pipe organ can make a wide variety of sounds. How are these different sounds made?

The Pipes

The majority of organ pipes are made from an alloy of lead and tin (a high tin content improves the tone) but lead, zinc, copper and wood are also used.

Organ pipes come in two types

1. Flue or Labial -  these use the same principle as a whistle or recorder. The kind of sound they make is determined by: the wind pressure, the height and width of the mouth, the diameter of the pipe relative to its length, its shape, whether or not it has a stopper in the top, and a number of other tone-altering devices
2. Reed - here a beating brass reed at the bottom (boot) of the pipe makes the sound which is then amplified or modified by the pipe extending from it. The type of reed tongue, the pipe shape, its length, tone-altering attachments, and the wind pressure all have a bearing on the sound.

Tones

The organ's pipes can be divided into five main tone types:

1. Principal or diapason - the organ's basic and most distinctive sound
2. Flute - several types. Not as bright but sweeter than principals
3. String - thin edgy tone similar to, but not really an imitation of, the orchestra's strings
4. Trumpet - loud reed pipes used to provide the fortissimo
5. Orchestral - usually reed pipes, more or less imitating orchestral counterparts

Pipe Length

The length determines the pitch. The longer the pipe, the lower the pitch; the shorter, the higher. An open pipe two feet long produces a note at the same pitch as middle C on the piano. A pipe one foot long with a stopper in the top gives the same note, but in a different tone. In large organs, pipe length ranges from half an inch (1cm) to 32 feet (10 metres).  A 32ft pipe sounds at 16 cycles per second, just below the threshold of human hearing and is felt rather than heard.

Chorus

A Chorus is stops of pipes of the same tone but of differing pitches, used to vary the dynamic and harmonic range in much the same way as the orchestra's string section (double bass, cello, viola and violin) does.

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