Saturday, January 17, 2015

The Pipe Organ: Development and Design

Written by: Ginny Moe



The design and music of the pipe organ are a result of its history  as builders responded to changing societal needs but more importantly  as organists, composers, and organ builders  learned to take advantage of developments in science  and industrial techniques. This session is on the two main types of pipes and how they speak,  or make sound.

At its most basic the pipe organ is a set of pipes  similar to flutes or clarinets through which wind blows to make sound usually musical sound and which, in the modern organ, is controlled by an organist who plays a keyboard shaped like a piano keyboard.

Almost all pipes on the modern organ are of two types. Flue pipes comprise the majority of the pipes, and they are almost always made of either metal or wood with a foot through which air flows into the pipe. Most organs also use reeds (sometimes called reed pipes) in which the speaking mechanism is covered by a boot into which air flows, vibrating a tongue. As an example, I currently play a pipe organ which has 17 sets (ranks) of flue pipes and 4 ranks of reeds, plus some extras which combine several ranks of flue stops.

Now the sounding mechanism of these two types pipes is very different. In the flue pipe air enters the foot of the pipe through a toe and is directed toward the mouth of the pipe by a languid. Air goes outside the pipe at the mouth, and the the speedy air outside the pipe reduces air pressure inside the pipe, drawing the airstream into the pipe. This is the Bernoulli effect in practice; the same reason jets can fly. IT IS THE AIR ITSELF WHICH VIBRATES, setting up sound waves.

By contrast, in the reed pipe a shallot extends into the boot. An aperture in the shallot is covered by a tongue. The boot surrounds this mechanism, and air enters the boot through a toe in the bottom. The wind presses the tongue against the shallot, and the tongues bends to cover the the aperture. The tongue springs back, allowing the wind to enter the shallot, and again the Bernoulli effect is observed, and sound is produced by the vibrating tongue. Here, IT IS THE TONGUE, OR REED ITSELF, WHICH VIBRATES.

Since the wind itself makes the sound in flue pipes, the pipe, which shapes the trajectory of the wind, directly affects the sound, and the most noticeable difference is made by the width of the pipe. Very narrow pipes generally produce more overtones, like bowed string instruments, and are called string pipes. Very fat pipes generally produce fewer overtones, like flutes, and are called flute pipes. And the most important pipes in any organ are the medium width flue pipes, called diapasons or principals. Various modifications around the mouth of the pipe also change the sound. The number of strings, diapasons, and flutes varies, but as an example, my current instrument has three independent string ranks, six independent flute ranks, and seven independent diapason ranks.

The reeds make a very distinctive sound, but most of them sound more like each other than they sound like any flue pipe. Usually they are louder, and the variation in sound is mostly caused by different shaped and length tongues and apertures in the shallot. What are often called reed pipes are properly referred to as resonators, and they amplify and change the sound. They are designed in many inventive and sometimes bizarre shapes, some of which make a difference in the sound quality, or timbre.

To summarize, organ pipes are normally either flue or reed pipes. In flue pipes, the air vibrates, and in reeds, the tongue vibrates. Most organ pipes are flues, and the length and shape of the pipe make changes in timbre, or sound quality. The most important pipes are the principals, or diapasons, and if you think of the sound of a pipe organ, you are probably thinking of the sound of the diapasons. They are of medium width, and the narrow pipes are called string pipes, while the fatter pipes are called flute pipes. A few reeds are usually part of a pipe organ, providing distinctive solo stops and fiery color. In them the sounding mechanism is covered by a boot, inside of which a tongue vibrates against a shallot to produce sound, which is then shaped modified by a resonator.

Check in again next month, when the subject will be organ keys, and various ways the keys control the pipework. I am Ginny Moe, and this is a series on The Pipe Organ, and how its design developed throughout history.
Twitter: @GinnyMoeRHSCwebsite: GinnyMoe.net

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