Passing on to the next size of unit, we come to the sectional organs mentioned. Each of these organs is played from its own clavier or manual and has its own special timbre and utility. An organ of five manuals (where found) would commonly comprise a Great Organ, a Swell Organ, a Choir Organ, a Solo (or Orchestral) Organ, and an Echo (or else Bom barde) Organ. Moreover, save only in the tiniest instruments, there is also a Pedal Organ played by the feet of the executant. The normal range or compass of the manuals is 61 notes, i.e., 5 octaves, from CC in the bass to C"" in the treble (though cer tain organ builders of the United States have adopted a manual compass of 7 octaves) ; that of the pedals is 32 notes, i.e., 21 octaves from CCC in the bass (lowest C on the pianoforte) to the G below middle C. In other words, a manual stop ordinarily com prises 61 pipes and a pedal stop 32 pipes.
These are primarily divisible into two main genera, namely flue and reed. Flue-pipes bear a strong resemblance to the humble "penny whistle," both as to appearance and physical behaviour—except that a flue-pipe does not sound more than one note. In reed pipes the wind vibrates a curved brass tongue over the surface of a reed or shallot, so that the oscillations of the tongue cause the speech of the pipe. Associated with the reed is a metal resonator, which exercises much the same function as the "loud-speaker" horn of a radio-outfit.
Both genera have in common the properties of pitch and quality. With a flue-pipe the pitch is lower as the tube or body is longer; with a reed-pipe pitch depends on the vibrating length of tongue and on the position of the spring wire, which can be made to increase or diminish this length. The quality of a flue-pipe is affected mainly by the diameter (or scale) of the body relative to the length, the wider scale giving the fuller tone and vice versa; the quality of reeds is affected by the length of resonator, their shape and scale, the thickness of the tongues and other factors.
But the souls of organ pipes are all in the hands of the "voicer," and their ultimate character depends more on the process known as "voicing" than on anything else.
Although most of the flue-pipes in an organ are open at the ends to the atmosphere, there is also a considerable class having its bodies closed by a cap or stopper. Known as gedeckts or bourdons, these pipes are peculiar in that they speak a note nearly an octave lower than an open pipe of the same length. Another class has the bodies pierced about midway with a small hole, the result being that they sound an octave higher than ordinary pipes of the same length. These are called harmonic pipes.
Voicing consists for the most part in adjusting the mouth-pieces of a flue-pipe or the tongue-curvature of a reed in such a manner that the desired quality of tone or timbre is permanently obtained. The final process of voicing is known as "regulation" and means the making uniform in power of every pipe in a stop. This the voicer achieves by admitting a greater force of wind through the "foot" of the pipe, if he wishes to louden its tone; or by reducing the foothole and so reducing the wind, if he wishes to soften it.
For tuning various methods are employed, according to the character of the particular class of pipe being dealt with. Thus some flue-stops are tuned by moving up or down a metal sleeve which fits over the top of the pipe, the speaking length of which is in this way increased or diminished. Others—especially "upper work" stops—have to be coned in or expanded slightly at the ends, according to whether it is intended to flatten or sharpen the pitch. Others again are tuned by pressing in or pulling out a metal piece affixed in or near the tops of the pipes. As for gedeckts the caps or stoppers afford an easy means of altering the pitch.
In making the acquaintance of organ stops for the first time many persons fall into the error of putting far too much faith in the names. Organ stops, it is true, resemble orchestral instruments to the extent that they bear names which are supposed to distinguish them from one another; but so far from the "orchestral" analogy holding good generally, it fails for two reasons—first, because organ builders have invented a hun dred and one fancy names to denote pretty much the same kinds of stops ; secondly, because they do not all agree as to what a given name is to denote.
Proceeding then on these lines, we may group flue-stops into three broad species—flute, diapason and viol. Flutes are either open or closed (i.e., gedeckts), harmonic (i.e., of double-length) or non-harmonic. Diapason stops on the whole have a natural and normal organ tone, familiar to everybody. Viols are of smaller scale than diapasons and so are of comparatively thin and stringy tone. Several stops stand on the borderland between two species; but the majority can be brought under one of the fore going heads, quite apart from their loudness and intensity.