There are two main categories of organ pipes: flue pipes and reed pipes. Flue pipes (made either of wood or metal; their construction is basically similar in principle) account for about four-fifths of the stops of an average organ. Figure 1 shows a front view and a vertical section of the most typical sort of metal flue pipe. The pipe consists of three main parts: the foot, the mouth, and the speaking length.

The pipe stands vertically on the wind-chest, and wind enters at the foot hole. The foot is divided from the speaking length by the languid, a flat plate; the only airway connection between the foot and the speaking length is a narrow slit called the flue. The wind emerges through the flue and strikes the upper lip, producing an audible frequency, the pitch of which is determined by and amplified in resonance by the speaking length of the pipe. A pipe of this kind is, in fact, identical in principle with a recorder or a tin whistle; but, whereas they have holes along the speaking length, which the player covers and uncovers with his fingers to secure the notes of the musical scale, in an organ there is a separate pipe for each note.

The tone of a pipe is determined by many factors, including the pressure of the wind supply, the size of the foot hole, the width of the flue, the height and width of the mouth, and the scale, or the diameter of the pipe relative to its speaking length. The material of which the pipe is made also exerts an influence; it may be an alloy of lead and tin, wood, or, more rarely, pure tin or copper, and for the bass pipes zinc. The pipes may also vary in shape, a common variant being an upward taper in which the pipe is smaller in diameter at the top than at the mouth. Or, the top of the pipe may be completely closed by a stopper. Such a pipe is said to be stopped; a stopped pipe sounds an octave lower in pitch than an open pipe of the same speaking length.

Open pipes of large diameter are said to be of “large scale,” and open pipes of small diameter are said to be of “small scale.” Large-scale pipes produce a fluty or foundational quality of tone that is free from the higher harmonics (the numbered series of partials, or component tones). Small-scale pipes produce a bright quality of tone that is rich in harmonics, recalling bowed strings. Stopped pipes can be particularly foundational in tone, and they favour the odd-numbered at the expense of the even-numbered partials. Tapered pipes are somewhere between stopped and open pipes in tone quality.

Flue pipes are tuned by increasing or decreasing the speaking length. In the past, several methods of tuning were employed, but in modern times this is often done by fitting a cylindrical slide over the free end of the speaking length and sliding it up and down, lengthening or shortening the pipe as required. In stopped pipes the stopper is pushed farther down to sharpen the pitch or is pulled upward to lower it.

The pipe maker thus broadly fixes the type of tone that a pipe will produce; but this is further controlled within fairly wide limits by the wind pressure and, finally, by the voicer, who adjusts the tone of each pipe by manipulating the foot hole, flue, and upper and lower lips. The attack of the note may also be greatly influenced by cutting a series of small nicks in the edge of the languid. Heavy nicking, commonly practiced in the early 20th century, produces a smooth and sluggish attack. Light nicking or no nicking, as used up to the 18th century and in some more advanced modern organs, produces a vigorous attack, or chiff, somewhat like tonguing in a woodwind instrument. If not excessive, this chiff enhances the vitality and clarity of an organ. The voicer is the artist upon whom the ultimate success of any organ depends. The tonal designer or architect is hardly less important, however; it is he who decides upon the choice of stops, their disposition in the organ, and the scales to be followed by the pipe maker. A completely successful organ depends upon the effective cooperation of designer and voicer.

Reed pipes

Organ reeds were probably originally copied from instrumental prototypes. A reed stop may have a beating reed like that of a clarinet or a free reed (a type discussed below in connection with reed organs).

The shallot of a beating reed pipe is roughly cylindrical in shape, with its lower end closed and the upper end open. A section of the wall of the cylinder is cut away and finished off to a flat surface. The slit, or shallot opening, thus formed is covered by a thin brass tongue that is fixed to the upper end of the shallot. The tongue is curved and normally only partially covers the shallot opening. But, when wind enters the boot, the pressure of the wind momentarily forces the tongue against the shallot, completely closing the opening. Immediately, the elasticity of the brass asserts itself, and the tongue reverts to its curved shape, thus uncovering the opening. This process is repeated rapidly. The frequency of the pulsations of air that enter the shallot is determined by the effective length of the reed and, in turn, determines the pitch of the note. Thence, the pulsations pass out into the tube, or resonator, which further stabilizes the pitch and decides the quality of the note. Most reed resonators have a flared shape. As in flue pipes, a wide scale favours a fundamental tone, and a narrow scale favours a bright tone. Cylindrical resonators produce an effect similar to that of stopped flue pipes, the note being an octave lower than the equivalent flared pipe and the tone favouring the odd partials. Some reed pipes, such as the vox humana, have very short resonators of quarter or eighth length. Pipes the resonators of which have no mathematical relationship to the pitch are known as regals; regal stops were popular in the 17th century, particularly with the North German school, and their use has been revived in modern times. Their short resonators have varying and peculiar shapes, which produce a highly characteristic snarling tone; they can be difficult to keep in tune.

Reed pipes are tuned by moving the tuning wire, thus shortening or lengthening the tongue. As in flue pipes, the scale and shape of the resonator largely determine the quality of tone to be produced; but the wind pressure, the shape and size of the shallot, and the thickness and curvature of the tongue also have important influence. The tongues may also be weighted with brass or felt; this weighting produces a smoother quality of tone, especially in the bass notes.