It’s not entirely incorrect to describe the pipe organ as ‘just a big box of whistles’ but that obscures the fascinating mechanisms that turn a box of whistles into a musical instrument.

Let’s start with the whistles. A pipe organ makes sound by pushing air through pipes. Pipes may be made of wood or metal. There are two categories of pipes in an organ (although every firm pronouncement of ‘every’ or ‘none’ has exceptions). The majority of the pipes are called flue pipes. They work much as blowing across a bottle produces a sound – a vibrating sheet of air is alternately pulled into and pushed out of a resonating tube.

 

 

The length of the pipe determines pitch. Width and shape (square, tapered, cylindrical) determine tone. Fine tuning the tone is fiddly and beyond the scope of this article, but comes down to wind pressure, mouth width and height, and the gap between the moth and the languid (the horizontal bar in the image above). Flue pipes may be ‘stopped,’ which is to say ‘plugged.’ Stopped pipes sound at a pitch twice their length; a convenience for longer pipes, possibly affecting the tone.

The other type of pipe comprises the reeds. These produce sound by vibrating a reed, typically a brass ‘tongue,’ against a tube, called the shallot, which has a rounded or tapered opening through which the raw sound passes to the resonator.

The reed and shallot are contained in a boot, so-called, and are adjusted for pitch by a tuning wire which controls the length of the reed which is permitted to vibrate. You can see Matthias Havinga adjusting the tuning of a reed pipe at ~2:30 in the video which Scruffy kindly shared.

Unlike the flue pipes, reed pipe pitch is controlled (almost) entirely by the length and shape and curve of the vibrating reed. Tone is shaped primarily by the pipe. Unlike the labial pipes, reed pipes are rarely the length shown on the stops. Reed pipes are often oddly shaped, as also seen in the Havinga video.

 

 

Now the complexity: Each specific tone class (that is, consistent tone varying only in pitch) produced by an organ requires its own pipe (or pipes, see ‘mixtures’ below) per keyboard key or pedalboard pedal. In a modern organ, that’s almost always 61 keys. Older organs often have 54. Pedalboards typically have 32, older 30, sometimes fewer. Now consider that an organ may have as few as 6-8, or as many as 29 (or more), stops per manual, and pedal, e.g. the Sydney Opera House Grand Organ, the world’s largest mechanical action pipe organ. Yikes!

We’re going to restrict ourselves to mechanical action organs here.  Pipes sit in rows on windchests.  Each key is connected to a pallet, a spring loaded lever, held closed, that opens when the key or pedal is pressed. Like this, but with a column of pipes lined up behind (to the right of) the single one shown:

 

 

But… that implies all pipes associated with a given key or pedal will sound when the key or pedal is pressed. And yes, that’s how the first organs worked. Creative minds came up with the notion of sliders, which can be pulled to line up a hole under a specific set of pipes, or pushed to shift the hole away from the foot holes to stop that set from sounding. Again, there’s a good shot of Havinga pulling a stop in the video linked above. An organ stop is a mechanism to turn on or off a specific set of pipes. A set of single pipes is called a rank.

Mechanical stop action uses the same mechanisms as key action, although more robust, requiring more force and greater range of motion.

Stops are labeled with the name of the rank and the length, nominal, of the longest pipe in the rank. So, for example, 8′ Diapason, 4′ Principal, 16′ Trumpet, and so on. 8′ pipes represent ‘standard pitch’, 4′ sound an octave higher, 16′ sound an octave lower. The longest pipe will be on the low C key or pedal. The commonest pipe lengths/pitches run from 32′ to 1′, although there are vanishingly rare 64′ stops for really chest rattling lows to 1/2′ stops, or shorter (see the Sydney stop list for examples) for ear-piercing shrillness.

Now, if you look at the stops on an organ console, you’ll find two deviations from what I’ve just described. What the heck is a Quint 2 2/3? Or a Mixtur VI?

Fractional pipe lengths represent harmonics relative to the 8′ pipes, or to 16′ and/or 32′ in the pedals, known collectively as mutations. Never intended to be drawn alone, only in combination with a whole-number pipe, they add or reinforce harmonics and thus color the sound.

Mixtures, identified by Roman numerals, are multiple ranks, denoted by the Roman numeral, of high pitch, drawn as one, to add brilliance to the ensemble. To the best of my knowledge, there are no reed mutations or mixtures.

A full organ chorus is at least 16, 8, 4, 2; or 8, 4, 2; or 32, 16, 8, 4 and a mixture, generally III or IV, plus mutations. Reeds are often drawn as well, for an even more powerful chorus. You will sometimes see a mixture labeled IV-VI, which indicates that the number of pipes varies as you move up or down the keyboard.

Pipe ranks are given a variety of different names, representing smaller tonal differences and/or national origin influences. The stereotypical classical organ sound is provided by the Principals (Diapasons, Principals, Montres, etc). The ‘lighter’ but more variable in tone organ sound is from the Flutes (Bourdon, Rohrflöte, Chimney Flute, etc.). There are also Strings (Gamba being common), more astringent than flutes or principals, and orchestral imitative stops (more rare).

Reeds don’t really come in families, other than the Trumpets (Posaune, Trumpet, Trumpette, Clarion, Clarine, etc.).  Trumpets en Chamade are mounted horizontally, projecting from the organ facade, and are brilliant and loud, but hopefully never shrill. A good Trumpet en Chamade chorus should part your hair but not pierce your ears. They crown a full organ chorus of all manuals coupled or provide a triumphant melody line above a principal chorus.

To be continued, with the action!