Flue pipe

Air under pressure (called wind) is driven through a flue and against a sharp lip called a labium, causing the column of air in the pipe to resonate at a frequency determined by the pipe length (see wind instrument).

The term only applies to flue pipes, not to reeds, and is practised by a specialist voicer,[1] who may also be the tuner.

Flat pieces of metal or wood called ears may be attached to the sides of the mouth for tuning purposes, and a horizontal dowel called a roller or beard may be affixed at the pipe to ensure prompt pipe speech.

When this low pressure area reaches a critical stage, it pulls the airstream past the edge of the mouth, filling the vacuum.

The column of air in the resonator thus vibrates at a frequency determined by the pipe's dimensions.

Flue pipes generally belong to one of three tonal families: flutes, diapasons (or principals), and strings.

The Flûte harmonique (French for "harmonic flute"), whose use the great 19th-century French organ builder Aristide Cavaillé-Coll advocated, is a metal flute pipe of double length with a hole punched in the center, which causes the pipe to speak at its first partial with a very round, intense sound.

The "Ripieno" includes many Diapason stops, all separate, in contrast to the German and French style "Fourniture" and "Mixtur".

Often, an organ will feature two similarly-voiced stops, one tuned slightly sharp or flat of the other.

When these stops are played together, a unique undulating effect results due to alternating constructive and destructive interference (beat frequency).

Examples include the Voix céleste (French for celestial voice), typically tuned slightly sharp, and the Unda maris (Latin for sea waves), typically tuned slightly flat.

Rare outside Italy is an undulating diapason, as in the Italian "Voce Umana" (not to be confused with the Vox Humana, which is a soft reed stop with a short resonator).

Examples include the Saxophone, the Muted horn, the Clarinet flute, and the Echo oboe.

While recent scientific studies have shown that the nature of the metal used in making the pipe has little or no effect on the final sound[citation needed], organ builders agree that a tin/lead alloy, for example, creates a very different tone than does zinc or copper metals or spotted or frosted alloys.