Pipes and Tone Production: How a Pipe Organ Makes Its Sounds

A flue pipe starts its sound by directing a thin sheet of air at the upper lip of its mouth, and that moving air is what sets the pipe speaking. How the air jet sustains a note once it has begun, and which physical variables shape a flue pipe’s pitch, speech and timbre, is explained on how a flue pipe produces its sound.

A reed pipe starts its sound with a thin metal tongue that vibrates when wind flows against and around it into a shallot. What happens inside the pipe once that tongue is moving, and why it gives a reed pipe a different character from any flue pipe, is explained on how a reed pipe produces its sound.

Principals, flutes and strings are all flue-voiced: each of these three tonal families is produced by flue pipes. A stoplist that names a principal, a flute and a string stop is therefore naming three characters of flue tone, not three different ways of starting a sound.

The count of four is a teaching convention rather than a fixed property of the instrument. The Organ Historical Society’s teaching pages, for example, divide flue tone into five categories, adding hybrids and compound stops to principals, flutes and strings. Either scheme is a way of sorting what the pipes do; the pipes themselves are the same either way.

Reed tone is the one tonal family produced by reed pipes, in which a vibrating tongue beats against a shallot. It is the only one of the four not produced by flue pipes, which is why a reed stop in a stoplist points you to a different mechanism from the flue stops around it.

The number on a stop control, whether 16, 8, 4 or another figure, is a pitch designation: it tells you the pitch at which that rank speaks relative to the keyboard, with the 8-foot figure marking unison pitch. The footage figure comes from the speaking length of the lowest pipe of the rank, and the page on footage explains where that reference comes from and how the other figures relate to it.

The footage figure is a pitch label, not a promise about the actual body length of every pipe in the rank. For some kinds of pipe construction the body departs from the nominal figure, so the number on the control and the pipe you could measure are two different things. Both that difference and the separate question of scale are explained on what the numbers on the stops mean, and how speaking length differs from scale.

Scale is a principal determinant of tone, and it is not recorded in an ordinary stoplist. Scale describes how wide a pipe is built in relation to its length, and two stops that share a name may be scaled quite differently. Scaling is explained alongside footage on the same page as the stop numbers above.

Pipe material is a second determinant a stop name does not record. One organbuilder’s general characterization is that hard metal such as tin gives a bright tone and soft metal such as lead a darker one, and that softwoods such as spruce or fir give darker tones than hardwoods such as oak or maple. Treat that as a practitioner’s working characterization rather than a measurement; how much the material itself contributes to tone remains contested in the acoustics literature.

Wind pressure is a third. The same organbuilder links the bright, transparent choruses of classic-style organs to low pressures and the broad, rich voices of a romantic or symphonic organ to high pressures. Pressure is set by the wind system rather than by the pipe, but it is still one of the variables a stoplist does not disclose.

Voicing is the set of adjustments made to each individual pipe, and it is what finally determines the timbre that pipe produces. Two ranks of the same name, scale and material can be voiced toward different results, so the name on the control cannot fix the tone by itself.

Stop, rank, voice and division each count a different thing, which is why one instrument can carry several different totals without contradiction. Because one stop control may govern several ranks, a published stop count, meaning the number of controls, does not say how many ranks or pipes an organ contains, and unification, which lets one rank supply several controls, makes the reverse true as well.

The definitions, what voice counts, and why the totals diverge are on what stop, rank, voice and division each count.

A coupler changes which pipes a key reaches: it makes the pipes of one division sound on a keyboard other than their own, or at another octave. Couplers are non-speaking stops that add no pipework, so a long coupler list expands the combinations available to the player without expanding the instrument’s tonal resources. Unification is a different thing, a matter of how a rank is wired to its stop controls, so that one rank can be played at more than one pitch or on more than one keyboard, with each control that provides such a facility called a unified stop. Borrowing is the third word a reader meets alongside these two. How the three relate, which kinds of coupler exist and what each does to a key are explained on what a coupler changes about which pipes a key plays.

Pipe families, footage, scale, voicing, ranks, stops, divisions and couplers all belong to working instruments, and the summary below sets out how Craftsman Pipe Organs works on them.