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 (referred to as wind) is driven down a Flue and against a sharp lip called a Labium, which causes the column of air in the pipe to resonate at a frequency determined by the pipe length. See Von Karman vortex street . Thus, there are no moving parts in a flue pipe. This is in contrast to reed pipes, which are driven by a beating reed, similar to the clarinet. Flue pipes are common components of pipe organs.
Flue pipes may be made of either metal or wood. Metal pipes are normally circular in cross section, while wooden pipes are most often rectangular or square, though triangular and cylindrical wooden pipes have been made.
A flue pipe comprises two main portions: the foot and the resonator. The foot is the bottom portion of the pipe. At its base is the toe hole, through which wind enters the pipe. The length of the pipe foot does not affect the pipe's pitch. Thus, organ builders vary the foot lengths of their flue pipes depending on several 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 vibration of air generated by the mouth of the pipe, which is a horizontal opening cut at the joint between the resonator and the foot. The voicing, the length of the resonator, and the resonator's cubic volume all determine the fundamental pitch of the flue pipe. The conical taper of the pipe will determine the overblown pitch. If the pipe is metal, a tuning sleeve or tuning collar may be attached to the top of the resonator, which can be raised or lowered to change its length, thereby changing the pitch produced.
At the joint 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 is affixed horizontally inside the pipe at this location, completely dividing the resonator and the foot, except for a small slot (called the windway) parallel to the mouth. This creates a chamber inside the pipe foot, allowing air to escape only 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 a beard) may be affixed at the pipe mouth to aid in prompt pipe speech.
When wind is driven into the foot of the pipe, a sheet of wind is focused by the windway across the mouth to strike just above the edge of the upper lip. This creates a Bernoulli effect, or "siphon effect," causing a low pressure area to be created just below the mouth. When this low pressure area reaches a critical stage, it pulls the air stream past the edge of the mouth, filling the vacuum. This pressurizes the opposite of the previous state, an alternating pressurization and rarefaction of the air column contained within the pipe's resonator. This is described by the Von Kármán vortex street phenomenon. The column of air in the resonator thus vibrates at the tuned frequency determined by the pipe's dimensions.
The end of the pipe opposite the mouth may be either open or closed (also known as "Gedecked" or stopped). An open pipe produces a tone in which both the even-numbered and the odd-numbered partials are present, while a stopped pipe, produces a tone with only the odd-numbered partials. In stopped pipes, the wind travels both up and down the body of the pipe, doubling the length of the column of sound; thus, a Closed tube sounds an octave lower than an open pipe of the same length. The tone of a stopped pipe tends to be gentler and sweeter than that of an open pipe, though this is largely at the discretion of the voicer.
A stop of diapason type may or may not actually be labelled "Diapason." The "Diapason" label is most commonly used in English-style organs, whereas the same type of stop is known as a "Prinzipal" or "Principal" on German-style organs and for French language organs they would typically be called "Montre" (literally on "Display" - i.e. the pipes at the front of the organ case) or "Prestant" ("standing in front" Latin 'praestare'). Furthermore, diapasons at pitches higher than 8′ pitch (pronounced 8 foot, referring to the length of the resonator part of the longest pipe of the stop) are often labelled with other names. For example, on English-style organs, the stops called Principal and Fifteenth sound one octave and two octave pitches respectively above the 8′ Diapason; on German-style organs, the name Octav is used to indicate the stop an octave above the 8′ Prinzipal, and similarly for French instruments, the names Octave and Octavin for 4′ and 2′ pitches respectively are commonly used.
The diameter of a flue pipe directly affects its tone. When comparing pipes of otherwise identical shape and size, a wide pipe will tend to produce a flute tone, a medium pipe a diapason tone, and a narrow pipe a string tone. These relationships are referred to as the scale of the pipe: i.e., wide-scaled, normal-scaled, or narrow-scaled. As a pipe's scale increases, more fundamental will be present, and fewer partials will be present in the tone. Thus, the tone becomes richer and fuller as the pipe's diameter widens from string scale to principal scale to flute scale.
The material out of which the pipe is constructed also has much to do with the pipe's final sound. 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, 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.