The classical pipe organ

There is a great variety of organ types.  The description of the mechanics of the organ that is given here refers to the type that is most common in the province of Groningen:  the organ with mechanical slider chests.  This type of organ was in general use in European organ building from the 16th through the 19th century.  After a period in which pneumatic and electromagnetic key action were common, the mechanical organ has again since 1960 been the most frequently built.  For this explanation we have chosen an organ that could have been built in our province in the 17th or 18th century.  

What is an organ, really?  Only what one sees on the outside:  a beautiful wooden case with a number of shiny pipes?  This would be a very incomplete description, as will become apparent in the following paragraphs.  The exterior of many organs shows that they are composed of various sections.  Depending on the size of the organ, the sections might be called Bovenwerk (1 in fig. 1), Hoofdwerk (2 in fig. 1), and Borstwerk (3 in fig. 1).  The Rugwerk (4 in fig. 1) is placed in a separate case, behind the organist’s back (5 in fig. 1).  The large pipe towers, often found on the sides of the organ, belong to the Pedal (not shown in fig. 1).  
An organ is really a big wind instrument.  The sound that it makes is a result of blowing air (usually called wind) through one or more pipes.  The principle is that of a collection of recorders and clarinets, one for each tone.  Instead of blowing them himself, an organist presses down one or more keys, making a connection between the pipes and the air under pressure in the bellows.  The pitch depends mostly on the length of the pipe, and the tone color (the character of the sound) on the form and the material of the pipe.  A row of pipes sounding a scale in the same tone color is called a “register”.  The most important parts of a pipe organ, also called a church organ, are these:

- the pipes;
- the wind system;
- the playing mechanism. 

When a recorder player wants to alter the pitch of his instrument, he closes or opens a number of finger holes.  Thus the recorder becomes as it were longer or shorter.  But an organ pipe can produce only a single pitch.  This means that for each pitch a separate pipe is required with a different length:  the shorter the pipe, the higher the pitch.  For a keyboard (manual) with 54 keys each register requires at least 54 pipes.  Pipes are made in a wide variety of shapes, each with its own tone color.  Pipes may be made of metal or of wood.

Organ pipes fall into two categories according to the way they produce their tones:  the flue pipes and the reed pipes.

The flue pipe is the most common and most important pipe form.  The pipes in the front of an organ usually belong in this category.  The “flue” of the name is the opening (8 in fig. 2) between the lower lip (7 in fig. 2) and the languid (3 in fig. 2).  The opening on the front of the pipe is called the cutup (5 in fig. 2).  Air (wind) that is blown through the toe hole (9 in fig. 2) into the pipe, is forced through the flue (8 in fig. 2), producing a thin sheet of air that flows against the underside of the upper lip (6 in fig. 2).  The swirls of air this makes become a standing wave in the body of the pipe (1 in fig. 2), resulting in a vibration of air that is perceived as a tone.  In a long body, a low tone is produced;  in a short body the vibrations are faster and the tone is higher.  

The reed pipe differs completely in its construction and its functioning from the flue pipe.  The most important part of a reed pipe is of course the reed (5 in fig. 3), a thin strip of springy brass.  As with the flue pipes, the wind enters at the bottom through the toe hole (7 in fig. 3).  The foot of the pipe is called the boot (2 in fig. 3).  The wind makes the reed vibrate, and these vibrations close and open a hole in the shallot (4 in fig. 3).  The pulsing stream of air going through the shallot makes the air in the resonator (1 in fig. 3) vibrate, and this is what is heard as a tone.  The length of the vibrating part of the reed can be changed using the tuning wire (6 in fig. 3), and this changes the pitch of the sound.  This tuning wire is used when the reed pipes need to be tuned.  

Tone color
We explained above how the sound is produced in an organ pipe.  But the tone color of one pipe may be very different from that of another, just as the sounds of various instruments are very different.  A note played on a piano sounds very different from the same note a played on the bagpipes or sung by a voice.  That’s because a perceived tone is always composed of a fundamental and a whole series of audible overtones, sometimes as many as 20.  Some instruments and organ pipes sound clear, silvery and radiant:  they produce many high overtones.  Others sound hollow and woolly:  they produce only a few high overtones.  A row of pipes with the same tone color for each tone of the keyboard is called a register or stop.  
A register that produces the same pitch as a piano is called an 8’ register.  The longest and lowest-sounding pipe of that register is 8’ long.  Pipes of a 4’ register sound an octave higher on the same key.  A 4’ register of which the pipes are closed on top with  stopper or cap sound the same pitch as an 8’ register.  For very low tones, i.e. the longest pipes, this can save a lot of space and material.  The stopped pipe produces fewer overtones than an open pipe of the same diameter, and so it’s less clear in its tone color.  

The word “scale” is used by organ builders to refer to the relationship between the diameter and the length of a pipe.  Flue pipes can have a narrow, a normal, or a wide scale.  Examples of pipes with various scales are shown in figure 4.  The first pipe comes from the register called Prestant or Diapason.  This is a register with a normal scale, and it has a clear, full tone with many overtones.  The prestants are the most important registers of an organ.  Pipes with a wide scale are found in fluty registers like the Open Flute or the Nachthoorn (2 in fig. 4).  These pipes produce fewer overtones and seem therefore weaker and less assertive in tone.  Registers with a narrow scale are called strings;  they produce many overtones making their sound reminiscent of bowed instruments.  Examples are the Viola da Gamba (3 in fig. 4) and the Salicional.  Figure 4 shows a number of stopped pipes.  The Holpijp or Bourdon (6 in fig. 4) has stopped pipes of normal or wide scale.  A narrow scale is found in the stopped pipes of the Quintadena (7 in fig. 4).  Other pipes are conical, like the Gemshorn (4 in fig. 4), funnel-shaped like the Trechterfluit (5 in fig. 4), or half-stopped like the Rohrflöte (8 in fig. 4).  The Rohrflöte is called half-stopped because there’s an open tube soldered into the cap.  

The wind system
Whatever the shape or size or construction of the pipes, in order to make a sound they need wind.  Just as a flutist blows into his flute, similarly the organ pipes will have to be blown.  
This used to be done with large bellows, with one or more people to tread them.  That was heavy work, but since the advent of the electric blower, this is no longer absolutely necessary.  Here we’ll describe the various parts of the wind system.  In figure 6 the path of the wind is schematically shown.  The blower (1 in fig. 6) blows the air into the bellows (2 in fig. 6).  The bellows, a wooden container with a weighted movable top, provides a regular amount of wind to the organ pipes under a more or less constant pressure.  On the lid of the bellows there’s a mechanism that can stop the air from entering the bellows from the blower.  If a lot of air is needed to make a large number of pipes sound, then the top of the bellows falls a bit and the opening into the bellows is opened wider by a valve.  And if there’s less wind needed, the opening into the bellows will be almost closed.  Thus the wind arrives at the windchest under constant pressure.  

Key action
As can be seen in figure 1, the pipes of an organ stand on a wooden chest, called the windchest (6 in fig. 1).  In figure 7 there’s a cross-section of a windchest.  An average windchest is about two yards long, one yard wide, and 10” thick.  On the top there are as many holes as there are pipes.  The pipes stand in these holes.  When there is too little space on the windchest or if some pipes don’t stand directly on the windchest, e.g. the front pipes of the prestant, then there may also be holes in the sides of the chest.  Conductors are used to get the wind to these pipes.  Under the windchest there are metal trackers (3 in fig. 7), one for each key of the keyboard.  The wind trunk is attached to the side, to fill the windchest with wind.  The stop action is also on the side.  The path traveled by the wind is as follows:  the wind passes from the bellows through the wind trunk and into the lower part of the windchest, called the windbox (1 in fig. 7).  There the air is stopped by valves, called pallets (2 in fig. 7).  Attached to each pallet is a tracker which is connected at its other end, via an ingenious mechanism, with a key.  If the organist plays a key, the pallet is opened and the air can pass further.  When the key is released, a spring closes the pallet.  The air has now arrived in a narrow rectangular “channel” (4 in fig. 7).  

Each channel is closed at the top by a strip of wood called a sponsel, which is bored with holes so that the wind can pass to the pipes.  Between the top of the chest, the toeboard with the holes that the pipes stand in, and the sponsels, there are movable strips of wood called sliders (5 in fig. 7).  A slider, which lies perpendicular to the channels, is bored with holes placed so that when the slider is moved into a certain position, the holes in the slider are aligned with the holes in the sponsel.  By moving the slider, the wind can be permitted to pass through to the pipes, or blocked.  The movement of the slider is controlled by a mechanism that is connected to a stopknob next to the keyboards.  To keep the pipes standing vertically, they are supported about 15 cm. above the toeboard by the pipe rack.  The space between toeboard and slider has to be air-tight, of course, so that no air will leak away to pipes that aren’t intended.  Felt washers around the holes and sometimes more complicated expansible means are sometimes used.  Now the wind can reach the desired pipes and bring them to speech at the correct pitch.  

It will be clear that this story is far from complete.  There is for example much more to be said about the key action and the stop action, and also the mixtures and compound stops.  We hope that the interested layman will have learned something about the way a pipe organ or church organ with mechanical slider chests functions.