How Reed Pipes Produce Sound

The boot is the sealed chamber that receives the wind, and the block is the plug in its top that carries the whole sounding assembly. Wind enters the boot through the toe hole in its base. Boots are usually hollow metal cylinders. For larger pedal reed pipes the boot may be made of wood, usually square in section, and a wooden boot may be fastened to the toeboard so that the strong vibration of a bass reed pipe cannot shift it. That is the Organ Historical Society’s description of prevailing practice, and individual instruments vary.

The block sits in the top of the boot. The shallot hangs from its underside, the tongue and wedge are held in it, the tuning wire passes into the boot to reach the tongue, and the resonator rises from its top. Every part that decides what the pipe sounds like is carried on or in this one piece.

The shallot is a strong brass tube attached to the underside of the block and opening upward into the resonator, and along one side it carries an opening whose surrounding face the tongue beats against. The reed tongue is a thin, springy strip of brass or phosphor bronze laid over that opening. Both are held in the block by a wooden wedge.

The shallot’s own shape, cylindrical or tapered, greatly affects the timbre, and so does the width of its opening: a wider opening in an open shallot lets more sound out and develops more of the fundamental in the tone, while a narrow opening emphasizes the upper partials. The face of the shallot around the opening matters too. If it is bare, burnished brass, the tongue strikes metal and the sound is bright; if it is covered with leather, an option some builders use, the tone is darker.

The wedge fixes the assembly in the block and the tuning wire fixes the tongue’s working length. The wedge is a piece of wood driven into the block that clamps tongue and shallot together in position. The tuning wire, typically phosphor bronze, passes into the boot and holds the tongue against the shallot at one point along its length; the part of the tongue beyond that point can vibrate and the part behind it cannot. That is why the wire is the tuning device of a reed pipe. How the free length relates to pitch, and why the wire is not an owner’s adjustment, comes later on this page.

The resonator is the tube above the block that receives the pulses from the shallot and gives them their pitch and fullness. Metal resonators typically take one of three forms: cylindrical, open or partially closed at the top; conical; or a combination of the two. Wooden resonators, used mainly on heavy bass reed pipes, are most often square in cross-section, with sides near parallel or flared outward. A resonator can be mitered, folded through an angle, to save height or to direct the sound, and a hooded reed pipe is one whose resonator turns through a right angle at the top so that the sound projects forward rather than upward. Length and form do more than fit the pipe into its space; they shape the tone, and the sections below explain how.

The note begins when wind entering the narrow passage between the tongue and the shallot sets the tongue vibrating. The tongue is a springy strip, clamped where the tuning wire holds it and free at its tip, and voicing gives it a slight curvature so that its free end stands off the shallot’s opening at rest. Once wind flows through that gap the tongue beats toward and away from the shallot, alternately covering and uncovering the opening, and each cycle admits a pulse of wind into the shallot. Angster, Rucz and Miklos of Fraunhofer IBP describe the motion becoming periodic by way of an aerodynamic-acoustic feedback loop between the vibrating tongue and the air in the shallot and resonator, which is how the tongue settles into a steady, repeating note.

Whether the tongue fully closes the opening is described differently by different sources. High-speed photography reported by Bob Mitchell in a 1974 Theatre Organ treatise showed the reed tongue never completely closing the shallot’s aperture, while other descriptions treat the tongue as closing against the shallot on each beat. What is not disputed is that the tongue’s rest position against the shallot has a narrow working window: it must be free to vibrate, yet not stand so far off that the opening is too wide for any vibration to start. Setting that position is voicing work, and it is described here only so that the window is understood.

The boot admits wind through its toe hole and lets it out through nothing but the shallot, so every pulse of wind the tongue admits must travel through the shallot and out through the resonator. That single exit is what turns a vibrating strip of brass into an organ pipe. The pulses are not released into the room; they are delivered into a column of air, the shallot and resonator together, that has resonances of its own. The REEDDESIGN project, coordinated by Fraunhofer IBP, showed by laboratory experiment that the vibrating tongue excites an acoustic resonance of the shallot itself, and that the shallot’s shape, dimensions and diameter change both that resonance and the sound the pipe finally produces. The shallot is not a passive channel; it is the first stage of the resonating system.

The pitch of a reed pipe is set by neither the tongue alone nor the resonator alone but by the coupling between them. Angster, Rucz and Miklos state that the pitch is determined by the coupling of two oscillating systems, the vibrating reed tongue and the acoustic system formed by the resonator and shallot. The tongue has a frequency of its own, fixed by its free length, thickness and curvature; the air column has resonances of its own, fixed by its length and form; and the sounding note is where the two agree. The resonator alone does not set the pitch, as a reader coming from flue pipes might expect. It pulls the tongue toward its own resonance, and how far it pulls depends on how strongly the two are coupled.

That coupling strength differs from one kind of reed pipe to another. The same researchers note that Trumpet pipes are characterized by strong interaction between the resonator and the tongue, whereas in a Vox Humana the coupling is weak. In a Trumpet the resonator has a firm hold on the note; in a Vox Humana the tongue is much more on its own, and the two kinds of pipe behave differently when tuned for exactly that reason.

Without a resonator, the tongue’s frequency simply follows its free length. Rossing, Angster and Miklos reported to the Acoustical Society of America in 1998 that when a reed tongue is sounded without its resonator, its vibration frequency changes continuously as the vibrating length is changed, but that once the resonator is added, the frequency jumps abruptly from below to above each resonance frequency of the resonator. The resonator does not merely color the tongue’s note; near each of its resonances it captures the note, and the tongue’s frequency cannot be walked smoothly through that region by changing its length. This measured behavior is the basis for the coupled-system account above, and it is also why the tongue’s free length and the resonator’s length are not independent settings on a finished pipe.

A reed resonator can be as long as a flue pipe of the same pitch, shorter, or longer, and each is a normal design rather than a mistake. The Organ Historical Society’s course material classifies reed resonators three ways: equal in length to a flue pipe of the same pitch, called full-length, with Trumpets and Oboes as examples; shorter than the body of an open flue pipe of the same pitch, called fractional-length, with Clarinets and Krummhorns as examples; and longer than an open flue pipe of the same pitch, called harmonic.

Within the shorter class, a practitioner’s registration handout separates half-length cylindrical ranks such as Cromorne, Krummhorn and Dulzian from short fractional-length ranks such as Vox Humana, Regal and Rankett. The Encyclopedia of Organ Stops describes the Krummhorn as usually made with narrow cylindrical half-length resonators, and the Vox Humana as most often having cylindrical resonators of a quarter or an eighth length, usually partially or fully capped.

A short resonator on a low-pitched reed pipe is possible because the pitch comes from the coupling described above: the tongue supplies a frequency of its own, and the resonator need not be long enough to sound that frequency by itself. A stoplist may or may not say which construction a rank has. Parkey OrganBuilders notes that specifications sometimes mark reed ranks L/2 or F/L to record half or full length, which means the distinction is disclosed only when the writer of the specification chose to record it.

Form shapes tone as much as length does. Conical resonators allow the full development of the overtones in the sound created by the tongue and are found in Trumpets, Oboes and Tubas. Cylindrical resonators are found in Krummhorns, Clarinets, Vox Humanas and several other ranks generally called regals, and capping a cylindrical resonator, partly or fully, is usual on the Vox Humana. A cylindrical resonator does not by itself decide the tone; shallot, tongue and wind pressure act with it.

Two conventional classes group reed ranks by their tonal role rather than by construction: chorus reeds, such as the Trumpet, Clairon and Bombarde, and solo or orchestral reeds, such as the Clarinet, Oboe and Cor Anglais. The BYU OrganTutor describes solo reeds as imitative and distinctive in quality and as not usually blending well with the flue ranks. The two-class division is a convention, and it leaves out regals and free-reed ranks. The same teaching resource makes a point that matters to anyone reading a description of a reed rank: reed scaling is a different quantity from flue scaling, so a reed pipe is not called wide-scaled or narrow-scaled in the flue sense.

The tuning wire sets the free vibrating length of the reed tongue, and that length sets the tongue’s frequency. The wire holds the tongue against the shallot at one point; the portion of the tongue beyond that point, toward the free tip, is what vibrates. A longer free length gives a lower note and a shorter free length a higher note. Because the tongue and the resonator act as a coupled system, the free length of the tongue and the length of the resonator are not independent of each other, and a change at one has consequences at the other, including at any regulating scroll the resonator carries.

This page describes the wire; it does not tell anyone to move it. Reed tuning is a trained technician’s task, and even the Theatre Organ Fact Finder, an enthusiast reference, closes its account of the mechanism with the advice to leave well alone when in doubt. Which way a wire is moved, and by how much, is deliberately not stated here.

Tongue thickness and tongue curvature are design variables that the voicer settles, and they decide much of what the pipe sounds like. Thickness first. Thicker reed tongues produce a louder sound than thin ones, and that sound is darker, with a stronger fundamental; thinner tongues give a brighter sound. The relationship is steep. Michael McNeil, writing in The Diapason, notes that a tongue’s stiffness varies as the cube of its thickness, so doubling the thickness makes the tongue eight times stiffer.

That is why tongue thickness is measured in thousandths of an inch, and why a published measurement set for an eighteenth-century organ records tongue thicknesses in hundredths of a millimeter while every other dimension is given in millimeters. Where more mass is wanted than thickness alone provides, a small brass or lead weight can be fixed to the tongue. These are associations reported by teaching and organbuilding sources, not a measured curve for any one pipe.

Curvature second. A reed tongue is voiced by giving it a critical degree of curvature, which fixes how the tongue stands off the shallot at rest and therefore where it sits within the narrow working window described earlier. A 1961 US patent assigned to the Aeolian-Skinner Organ Company recorded that a full day was commonly required to voice one reed tongue for a 32-foot Contre Bombarde pipe. That was a statement about the hand practice the patent sought to replace, not an independent labor standard, but it is a fair measure of how sensitive the curve is. Even the REEDDESIGN research project, set up in 2011 to give reed-pipe design better tools, stated at the outset that a better understanding of the role of the shallot and resonator, and of the voicing steps, on the attack and timbre of reed pipes was still necessary.

Everything on this page that can be changed is technician work: the position of the tongue against the shallot, the curve of the tongue, the position of the tuning wire, and the length of the resonator. None of it is described here as a procedure, and none of it should be attempted from a description. The tongue’s working window is narrow, its stiffness answers to the cube of a thickness measured in thousandths of an inch, and its pitch is coupled to a resonator that captures the note in jumps rather than smoothly, so a change to any one of these moves the others. What an owner can usefully do is leave the pipe as it stands and let a technician assess it, so that whatever is found and whatever is agreed is documented against the instrument as it actually is.