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 Flue Pipes

Flue pipes may be made of metal or wood, with or without plugs or "stoppers" in their tops. They all sound in the same way, which can be demonstrated by blowing across the top of a piece of rigid tubing, of, say, half an inch in diameter, until a sound is heard. A current of air strikes a sharp edge, producing a sound, which is characterised into a musical note by the column of air within the tube, which will vibrate at a set frequency, dependent mainly upon the length of that air column.

This is exactly what happens in flue pipes, where air passes through the foot of the pipe, is forced through a slot by the languid, and then strikes the upper "lip" of the pipe with great velocity. The length of the pipe, its shape, the materials of which it is made and its other characteristics, such as whether it has a stopper or not, determine which note it will produce and what tone quality it will have. Most metal flue pipes have vertical "ears" on either side of the mouth, which help to stabilise the note. In some pipes, the upper lip is covered with a strip of leather, to achieve a particular tone quality. The front edge of the languid is usually "nicked" with small notches to enable the pipe to speak clearly and promptly, without an initial burst of white noise or "chiff". On thinner pipes, producing string tones, the mouth is frequently masked by a rounded wooden dowel, called a "bridge" or "harmonic brake" (both terms are used), again to assist these thin-scaled pipes to speak clearly.

 

 

 

Diagrams by Geoff Wyatt

An open flue pipe measuring approximately eight feet from the upper lip to its top will sound the note "C" at the bottom of the organ keyboard, two octaves below Middle C. There is no particular magic about this, just a law of physics, that a column of air eight feet long will vibrate approximately 64 times per second, generating a note of that frequency, which our forebears in their wisdom designated "C".

This would all be academic but for the fact that the stopkeys on the organ show not only the name of the stop, but also its pitch (e.g., 8). This means that if the Diaphonic Diapason 8 stopkey for the great manual is depressed, sixty-one of the pipes of the Diaphonic Diapason rank can be sounded from the great manual, and the pipe sounding the bottom note (C), 64 vibrations per second, will be long feet long from mouth to top (the "speaking length). All stops marked "8" sound at the same pitch as the piano.

The same law of physics decrees that a pipe of twice that speaking length (i.e., sixteen feet from mouth to top) will sound exactly on octave lower. A pipe of four feet speaking length will sound an octave higher than one of eight feet speaking length. Thus, as any stop marked "8" will sound at the same pitch as the piano, any stop marked "16" will sound an octave lower. and any marked "4" will sound an octave higher. One marked "2" will sound yet another octave higher. Stops marked with fractions, e.g., 2_, are rather special, and are used to colour combinations of stops. They are known as "mutations" (from the Latin mutare, meaning to change), and are not intended to be used alone.

With your piece of tubing you can conduct a further experiment. While blowing across one end and sounding a note, place a finger over the other end, to seal it. As if by magic, the pitch of the note will fall one octave. At the same time, the tone will have become less bright.

This illustrates another of the laws of physics - that a "stopped" flue pipe sounds an octave lower than an open pipe of the same speaking length. Stopped pipes are also found in theatre organs, and have a characteristically duller tone.

Flue pipes in the theatre organ may be constructed of wood or metal, and may be open or stopped, depending upon the quality of tone required. In the Style 260 organ, the tones available from flue pipes range from the keen Viol d'Orchestre (thin, open metal pipes), to the rich, broad-sounding Tibia Clausa (wide, stopped wooden pipes). Apart from the large Diaphone bass pipes, only the Flute and Tibia pipes are made of wood, with open and stopped pipes respectively (to save space, the bass pipes of the Flute rank are also stopped). Compton rarely used wood pipes, except for basses, and on some Christie organs the Tibia was made of metal. Because it is very difficult to make very small pipes from wood, the extreme treble parts of wooden ranks are usually formed of metal pipes.

From what has been said above, it can be deduced that the Tibia Clausa pipe sounding the same pitch as the eight foot Diaphonic Diapason pipe will have a speaking length (because it is stopped) of four feet. The stopkey, however, is still labelled "8", as this reflects the pitch of the notes, rather than the physical length of the pipes. Thus, all stops labelled "8" will sound at the same pitch as the piano regardless of whether the pipes are open or stopped.

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