Flue pipe

A flue pipe (also referred to as a labial pipe) is an organ pipe that produces sound through the vibration of air molecules, in the same manner as a recorder or a whistle. Air under pressure (called wind) is driven down 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.) Thus, there are no moving parts in a flue pipe. This is in contrast to reed pipes, whose sound is driven by beating reeds, as in a clarinet. Flue pipes are common components of pipe organs.

Flue pipes include all stops of the "Principal", "Flute", and "String" classes, and some stops from the "Hybrid" class.

Flue pipes may be metallic or wooden. Metal pipes are usually circular in cross section; wooden pipes are usually square or rectangular, though triangular and round wooden pipes do exist.

A flue pipe has two major parts, a foot and a resonator. The foot is the bottom portion of the pipe, usually conical. At its base is the toe hole, through which wind enters it. The length of the foot does not affect the pipe's pitch, so organ builders vary the foot lengths of their flue pipes depending on other factors, including the desired shape of the pipes in the façade, the height of the rackboard in which the pipes are seated, and the weight of the completed pipe.

The resonator supports the oscillations of air generated at the mouth of the pipe, a horizontal opening at the juncture of the resonator with the foot. The voicing, the length of the resonator, and the resonator's volume all determine the fundamental pitch of the flue pipe. The conical taper of the pipe determines the overblown pitch. If the pipe is metal, a tuning sleeve or tuning collar may be fixed at the top of the resonator and raised or lowered to vary its length, thereby adjusting the pitch produced.

Between the foot and the resonator, the side of the pipe containing the mouth is flat. A plate of metal or wood called a languid, fixed horizontally here, blocks the airway, except for a small slot called the windway alongside the mouth. This allows air to flow as a sheet of wind directed towards the pipe's mouth. 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.

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