Pipe Organs 101

So, just what is a “pipe organ”?

A pipe organ is a musical instrument that produces sound by air vibrations created in an organ pipe, which is controlled by a musician from a keyboard. The pipe organ has been around for quite some time, much longer than the piano.

How is sound made by the pipes?

Sound is air that is vibrating. It's like when you blow across the top of a soda bottle- it makes a “note”, a sound that has a specific frequency of vibration called pitch. Similarly, one organ pipe produces one tone at one pitch; a given pipe is not mechanically manipulated in some way to produce multiple notes like a flute or guitar string. Since there is just one pipe for each note, a keyboard with 61 notes (5 octaves) would have 61 pipes, one for each note. The one set of pipes for each note on the keyboard makes just one kind of sound. The organ will have several sets of pipes; each of these sets of pipes is called a “rank”, which can make different sounds. This is similar to an orchestra which has many different types of instruments, each having its own characteristic sound. If a keyboard of 61 notes controls a group of 10 ranks of pipes, there will be 610 pipes playable from that keyboard. There are many more pipes in an organ than the ones you typically see.

So why can't you see all the pipes?

The pipes are arranged on a windchest as shown in the drawing below, and typically only the main Principal stop is visible. The windchest and pipes are enclosed on the sides and back by a case so that the sound projects out the front more clearly. It's like when you're yelling to someone- you hold your hands up by your mouth to help project the sound so they can hear you better. The organ case acts the same way, projecting the sound into the room.

How does an organ make different sounds?

Cross-Section of an Open Flue Pipe (front and side views) and Reed Pipe

There are several basic methods an organ builder has at his disposal to make different sounds come from the pipes. These are:

The tone of a musical note can be very complex, but basically consists of the main or fundamental pitch, and harmonics of that fundamental pitch. The number and strength of the harmonics are what defines a tone- it's like the human voice: people have unique sounds to their voice that differ slightly because the harmonic content of each person's voice differs. If a friend walks up behind you and says “hi there”, you can often guess who it is just by the sound. You can probably identify several hundred people when hearing just a few words from them or your name because of the unique harmonic content of each voice.

What exactly are harmonics?

When you play a middle C note on a piano, the middle C string inside the piano vibrates to make the middle C note sound. However, while the string is producing this middle C note, the string can also simultaneously vibrate in such a way as to make the note an octave above middle C. An organ pipe has this same characteristic as a vibrating string, except it is a vibrating column of air. It not only makes the sound of the middle C note, but an entire series: the C one octave above, the G one and one-half octaves above (12 notes up), the C two octaves above, the E 17 notes up, and so on. Of course these harmonics are not at the same loudness and the volume relative to the main note is what creates the characteristic tone.

There's underlying mathematical logic behind these harmonics. If a note is producing a certain pitch, the harmonics are simply that pitch multiplied by whole numbers, so the harmonics are a 2x, 3x, 4x, 5x, 6, and so on. Twice the pitch is the octave, 3x is the G that is 12 notes up, 4x is two octaves, 5x is the E 17 notes up, and so on. These principles of sound are true of the human voice and all musical instruments, not just organ pipes.

How does the keyboard control the pipes?

The pipes are made to sound when air under a low pressure is blown into the bottom or foot of the pipe. The foot of each pipe sets on a hole on the top of a box-like structure. The box has a separate channel for each note. There is a chamber of low pressure air supplied by a small blower, and a set of valves which control the flow of air into the individual note channels. The proper pipe from each rank sits above the channel for that note, so for example the low C pipe sits above the low C channel which is controlled by the low C on the keyboard. Organ builders call the air supply wind and the chamber the wind chest. In our example, there would be 61 valves leading to 61 separate channels, one for each note. Each valve is opened and closed by playing the corresponding key on the keyboard. The valve is actuated by either a direct mechanical linkage from the key or an electromagnet controlled by the key.
Side view of windchest containing 10 ranks of pipes. The organist plays a key which is linked by a tracker which pulls down a valve in an air chamber to the channel supplying air to any pipes whose slider is in a position where the holes line up, allowing air to reach the pipe and make sound. Note the “Mixture IV” stop connects air to 4 pipes in the toe board so that the 4 pipes play as one.

Click here to view photo of an actual organ illustrating the pipes on the windchest like this drawing.

It can be noted that the current music keyboard originated in its present form in the 1300's, thus making it the oldest “standard man/machine interface” in the world!

What is a “stop”?

In the very early days of organ building, all the ranks of pipes were played together and there was no way to separate which types of ranks were used. It's sort of like a big choir where everyone sings together and there weren't any solo voices. Over time, there was a mechanism invented called a slider that was used to stop or isolate a ranks of pipes. The slider was a thin piece of material with holes in it that would run cross-wise to the channels and be underneath the pipes. At the organ console, a lever would move this slider between two positions- one where the holes would line up and allow all the pipes in a rank to receive air from the channels and another where the holes would not line up and thus block the air flow. Because the historical precedence was to have all the ranks on, the slider was viewed as stopping the sounding of pipes. (Besides, having a “start” just doesn't sound right!)

So what are the main sounds an organ can make?

The main sound of an organ is called a “principal” tone. These pipes are straight and are usually the ones that are visible when you look at the organ. Often they are made of highly polished tin, which produces a bright sound and looks nice. The sound is probably most like a modern orchestral flute, which also produces a bright sound, has a long, straight tube, and has a small “attack” sound when the sound is first produced. The initiation of the sound, called chiff can be made somewhat stronger or weaker depending on how the organist plays the keys, and provides articulation to the notes.

The main principal sound is at a range that is called eight-foot (8') pitch. This is because the lowest C on the keyboard has a pipe with a resonating length of 8', and is the same pitch as you have on a piano. If we have a set of 61 pipes, with the low C being 8' long, we can add a second set where the pipes are one-half the length of the 8' stop, so the low C is only 4' long. This second set we will call a 4' Principal or 4' Octave. Each note has its main pitch one octave above the 8' pitch note.

What other ways to change the sound are there?

As we described before, adding sets of pipes at successively higher harmonics can change the sound. As we alluded to earlier that the shape of a pipe can also change its sound. In the organ there are two main categories of pipes: flue pipes and reed pipes. Flue pipes have no moving parts and generate their sound by vibrating air in a column like a flute or recorder. Reed pipes have an actual mechanical reed, like a clarinet, at the base and then have a resonator above.

Flue pipes can be further classified into two sub-categories: open pipes and stopped or closed pipes. Open pipes are open at each end and have a full harmonic series with the main pitch and harmonics at a frequency of 2x, 3x, 4x, 5x, etc. Stopped pipes have a cap on the top and thus the pitch is one octave lower than the pipe would be if it was open. These pipes have only the odd harmonics: 1x, 3x, 5x, 7x, etc. Pipes having only odd harmonics have a uniquely different sound than open pipes.

As a general rule of thumb, the narrower the pipe, the more harmonics it will have, and the wider the pipe, the fewer harmonics it will have. Straight pipes also have more harmonics than tapered pipes. So an organ builder has plenty of things to change to make different sounds: tube shape (straight, tapered), whether is it stopped or not, and pipe material composition.

Pipes are typically made from a tin-lead metal, with the tin content being 30%, 50%, or 70%, and sometimes 90% for the visible pipes. Tin is more expensive than lead (yes, plain old economics is a factor!) and also produces a brighter sound. Pipe metal made at the 50/50 tin/lead mixture is sometimes referred to as “spotted metal” because of an unusual metallurgical property where the metal surface has round patterns approximately 3/8" in diameter. Pipes also can be made of wood. Wooden pipes are typically square or slightly rectangular in cross-section, and are most often stopped pipes.

We had said earlier there are harmonics at successive octaves, but also in-between the octaves. For example, the third harmonic from the low C is a G, up 12 notes from the C The fifth harmonic is close to the E, 17 notes up from the low C. There are stops where the primary pitch of the pipe is made to be at these non-octave harmonics, so when you play a C, the pipes plays a G or E! These stops are called mutations and their designated length a fraction instead of the usual 8', 4', 2' etc. A common mutation would be a 2 2/3' (the G, or 12th) and a 1 3/5' (the E, or 17th) stop. This may sound strange on paper, but when you hear it, it blends perfectly and sounds like another sound “color”.

Another stop type is called a mixture. These stops have several pipes that sound together and are not separable. (They may also be called “compound” stops.) These pipes are typically principal-types and sound above the 2' Octave at the G, the next C, the next G, then the next C. These pipes reinforce the main harmonics and have a sound that is unique to the organ.

How would an organ builder use 10 ranks?

In our example, we would see a “stop list” that might look like this:
8' Principal
8' Flute
4' Octave
4' Flute
2' Octave
1 1/3' Mixture IV
8' Trumpet

In this example, shown in the sketch above, we see an 8' Principal, which would be the main “foundation” sound. These pipes would stand at the front of the windchest and be the ones you would most likely see. The 8' Flute would likely be a stopped set of pipes, so the low C would really only be 4' long. The 4' Octave is one-half the length of the 8' Principal, speaking one octave above. The 4' Flute would possibly be a tapered open set of pipes, not as bright as the 4' Octave. The 2' Octave would be principal-style pipes. The 1 1/3' Mixture would be a set of 4 pipes that play together. The low C would have a G 2-and-one-half above the low 8' C, then the successive C, G, and C above that G. The mixture pipes would be principal-style pipes also, and add brightness and power to the ensemble. The 8' Trumpet would be a set of reed pipes. This would make a group of 610 pipes of 10 ranks in 7 stops controlled by a keyboard of 61 keys.

Why do organs have several keyboards and pedals?

So far we've explained everything pertaining to just one keyboard. Just as a human body is made of several organs like the heart and lungs that serve different purposes, an organ is in reality a collection of several organs which serve different musical functions.

The second keyboard will control a second windchest with its own pipes. Traditionally, this second keyboard would control a positive organ which would be lighter in character and an octave higher in pitch. The main principal stop would be a 4' pitch and its first octave would be at 2' pitch.

The pedal organ is played with the feet and is typically designed at an octave lower than the main organ, so its would likely have a 16' Principal as its main stop. Pedal boards typically have 32 keys ranging from low C to G two and one-half octaves higher.

Are organs used only in churches?

The organ certainly grew up in the church, primarily in Germany, France and Holland. Before the electronic reproduction of music, the church was a primary source of music in one's life. Because the harmonic structure of the organ is so highly developed and it can sustain its tone unlike a guitar, piano, or drum, it makes the organ an ideal instrument to accompany singing.

Today, organ builders have been very busy building organs for universities and modern concert halls. The popularity of the organ has also blossomed in the Far East; many new concert halls in places such as Japan and Singapore are being equipped with fine modern pipe organs.

Aren't electronic organs just as good?

With modern digital technology, electronic organs have improved greatly. However, just because something has a computer in it, it doesn't mean it's better. Digital sound samples from real pipes are used now to create the sounds, but one must still pay attention to the balance of sound within an individual rank, between ranks in one division, and between divisions. These issues apply to any organ design, pipe or electronic. Like the modern trend in software of "feature-itis", organ builders must be careful not to get caught in the trap of making organs that focus on unique stop sounds randomly collected and miss the balanced design of the overall instrument so masterfully done by the best builders in history.

What kind of music is available for the organ besides “Phantom of the Opera” and funeral dirges?

Unfortunately organ music has often been clouded by ignorance and gained a reputation of being either melodramatic or boring. The truth is the organ has a very long history and its current repertoire spans a period of over 400 years. Certainly Johann Sebastian Bach is the most famous composer for the organ, but there are many others from the same period in Germany as well as France, and there are modern composers writing for the organ today. Many great composers have contributed to a wealth of literature including Pachelbel, Bach, Handel, Mozart, Lizst, Reger, as well as Couperin, Franck, Vierne, Widor, Dupré, Langlais, and Messiaen.

After World War II a reform movement began in organ building where a return to some of the underlying principles used by the historic masters of organ building were followed. Organs had become muddy and inarticulate in sound. Organs built by some German and Dutch builders were imported to North America in the 1950's, and these instruments were part of an awakening of what an organ could really be. A good example is the Dutch-built Flentrop organ at Harvard University installed in 1958. Although it was not a large organ, it was bright, fresh, and perky, and became popular through a number of recordings made on it.

OK, I'm getting a feel for this- what else can I read?

The next article in this series, Pipe Organs 105: What is 'Organ Music', examines the music of the organ and some features of the instrument that make its music so special. Another step would be “A Brief Look At The French Classical Organ, Its Origins and German Counterpart” by Lawrence Phelps. It explains the history of the organ in more detail and examines how the organ evolved in its development. The articles “Trends in North American Organ Building” and “A Short History of the Organ Revival” examine in detail Mr. Phelps' own efforts and experiences in the organ reform movement.


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© Steve Thomas 2003