How Flue Pipes Produce Sound

The air jet is formed where wind leaving the pipe foot squeezes past the languid. The languid is a horizontal plate inside the pipe that almost completely closes off the top of the foot; the narrow opening left between it and the lower lip is the flue, or windway. Wind arriving in the foot has nowhere to go but through that gap, and it leaves as a thin, fast sheet of air aimed across the mouth.

In a wooden pipe the languid’s job is done by a block, a piece of wood as wide as the inside of the pipe body, and the flue is formed between the top of the pipe’s face and that block. The parts differ from a metal pipe; the geometry the jet meets does not. Everything that follows depends on this sheet of air: its speed, its thickness and the distance it travels before it meets anything.

The upper lip is the relatively sharp edge across the top of the mouth, and the jet is aimed at it. The jet does not arrive as a steady stream. A thin air jet is unstable by nature, and any small sideways disturbance it picked up as it left the windway grows as it travels. At the lip that grown disturbance decides which side of the edge the air goes: alternately into the pipe body and out through the mouth. The lip itself does not vibrate. It is the fixed edge that turns a wavering jet into an alternating flow.

The pipe body is an air column with natural resonances of its own, and it is the column, not the jet, that fixes the pitch. The alternating flow at the lip pushes on that column, the column answers most strongly at its own resonant frequencies, and the note heard is the column ringing. A 2024 multi-sensor study of a sounding flue pipe describes it as three coupled systems: the air jet blowing from the flue and striking the upper lip, the air column excited by that jet, and the pipe wall set in motion by the acoustic waves. The authors present this as a way of modeling the pipe, not as a claim that each part matters equally in every pipe; the first two systems carry the mechanism this page explains.

The loop closes because the resonance in the pipe reaches back to the windway. In Fletcher’s account, acoustic motion at the mouth displaces the jet where it leaves the flue; that displacement travels down the jet and grows; at the lip the amplified deflection switches the flow between the inside and the outside of the pipe; and that switching drives the air column, which sustains the very resonance that started the displacement. Each part of the loop feeds the next, and the pipe keeps sounding for as long as wind keeps the jet flowing.

For the loop to lock in, the time the jet takes to cross from windway to lip must stand in a particular relation to the period of the note: roughly half a wavelength of the disturbance traveling along the jet. That timing condition is why the height of the mouth, which sets how far the jet travels, and the speed of the jet, which sets how long the crossing takes, have to be balanced against each other rather than chosen independently.

Once the oscillation is steady, the pipe’s partials lock into exact whole-number multiples of the fundamental even though the passive resonances of the air column are not perfectly harmonic. That locking is a product of the nonlinear coupling between jet and column, and it belongs to the sustained tone; the first instant of speech behaves differently, as described below.

The feedback account above is shared, but the sources describe what happens at the upper lip in two different ways, and this page does not choose between them. Fletcher’s model treats the deflected jet itself as the drive, with acoustic feedback through the mouth and the mode locking described above. Rucz describes the excitation as an edge tone: the jet is destabilized when it meets the lip, and vortices detach from the upper lip in a quasi-periodic way, with that edge tone providing the excitation for the air column. Adachi, Angster and Miklos modeled vortex shedding at the upper lip as a nonlinear radiation and energy-loss mechanism at the mouth rather than as the drive alone.

These are different emphases within physics that agrees about the loop, not a dispute about whether the loop exists. As of September 2026 this page treats the question as open rather than settled, and nothing in the mechanism described above depends on choosing between them.

Cut-up, the mouth height measured from the lower lip to the upper lip, is the most-cited flue variable because it acts directly on how far the jet travels before it meets the lip. Steenbrugge and De Baets record that a low cut-up is associated with a thin, bright spectrum and a weak fundamental, while a high cut-up brings a broad sound that they describe as not necessarily very interesting. The Organ Historical Society’s teaching page adds that the cut-up is not settled until the pipe is voiced, and that it is made higher to produce more sound. Cut-up is therefore a voicing dimension rather than a scale dimension, and it is the variable that most directly trades brightness against breadth and power.

The width of the flue and the edge of the languid set the thickness and character of the jet before it ever reaches the lip. Flue width appears in Britannica’s list of determinants in its own right, and it appears again in the rule of thumb for overblowing described below, where it stands alongside cut-up and jet speed as one of the quantities that must stay in proportion. The two boundaries of the flue, the languid edge and the lower lip, are also where nicking is applied, which is why the flue is treated again under nicking and the attack. Charles Fisk, writing in 1975, described open flues combined with nicking as giving the tone a grainy, shimmering, sometimes silvery quality; that is one builder’s characterization, offered in words rather than measurements.

The toe opening and the wind pressure together set how much air reaches the jet and how fast it leaves the windway, and neither can be changed without touching what cut-up does. Michael McNeil, writing in The Diapason in 2025, states that larger toe diameters allow more wind and yield more power, and also make the timbre much brighter if the cut-up is not raised. Steenbrugge and De Baets state the same trade-off from the aerodynamic side: a bright, full sound can be obtained through a relatively low cut-up, but strongly increasing loudness by opening the toe also requires increasing the cut-up, which makes the sound less bright. Toe and cut-up are a coupled pair, not two independent controls.

The foot cavity is not passive either. A 2024 numerical study reports that the closed foot of a flue pipe acts as a Helmholtz resonator and influences the oscillation in the pipe, with the amplitude falling when the foot’s own resonance approaches the sounding frequency. That result comes from one modeled geometry rather than a survey of real pipes, but it shows that the wind path below the languid is part of the acoustic system, not merely a supply.

Scale, the relationship between a pipe’s diameter and its length, belongs on this list as one determinant among the others, and this page leaves it there. Britannica’s enumeration places it beside wind pressure, foot hole, flue width and mouth dimensions rather than above them. How scale is chosen and progressed through a rank, and how the footage printed on a stop control relates to the pipes behind it, are explained on Pipe Length, Scale and Stop Footage Explained.

The sources disagree about which dimension governs cut-up, and this page reports both accounts rather than choosing one. The Organ Historical Society page and general reference works express cut-up as a fraction of mouth width. The figures commonly quoted with that convention, as reported by Raphi Giangiulio, are about one quarter of the mouth width for principals and about one third for flutes, with the caveat that the fraction varies through a rank rather than holding constant. Giangiulio himself, citing Fritz Noack’s The Art of Organ Voicing, argues the opposite: that mouth height is more a function of a pipe’s length, or frequency, than of its width, and that cut-up is independent of the width of the mouth.

The disagreement is unsettled as of September 2026, and the position attributed to Noack reaches a reader through Giangiulio’s summary rather than from the book itself. The practical point is that a quoted ratio is a convention some voicers describe, not a standard that fixes how any given pipe was made.

Nicking is a series of small cuts made across the flue on both of its boundaries, the languid edge and the lower lip. The cuts let the wind reach the upper lip not as an unbroken sheet but with some irregularity, and the Organ Historical Society page describes the result as a more direct production of the pipe’s primary tone with a reduced or eliminated chiff. Britannica summarizes the effect by era: heavy nicking, commonly practiced in the early twentieth century, produces a smooth and sluggish attack, while light nicking or no nicking, as used up to the eighteenth century, produces a vigorous attack, or chiff. Nicking’s most-cited effect is on the attack; some builders also hear it in the sustained tone.

Voicers disagree about nicking because they disagree about what a pipe’s speech should sound like, and each position is scoped to its holder and to the pipes in question. Lawrence Phelps, writing in 1954 from inside the organ reform movement, held that nicking should be as light as practical and absent whenever possible, and blamed overnicking, done to give the voicer and finisher more latitude, for draining the quality out of many organs. He characterized the movement’s program as nickless, more or less open-toe, low-mouth and low-wind-pressure voicing in pursuit of brilliance of tone and vitality of speech.

Charles Fisk, in lectures and essays of 1975 and 1976, agreed that more and more nicking diminishes the chiff until it becomes inaudible, yet his own classical voicing used a few nicks in the windway together with nicking or antiquing of the languid edge, and he described open flues combined with nicking as a positive tonal resource rather than only a way to suppress chiff. Francesco Ruffatti, in 2010, described nicking as a normal condition for string-toned pipes.

None of these positions is a general rule, and none of them is a finding of fault. The amount of nicking a pipe carries records a voicing philosophy applied to that pipe, and reading it as a defect mistakes a choice for a symptom.

The attack transient is the brief, unsettled sound before a pipe’s steady tone establishes itself, and it is what a listener notices first. Angster, Rucz and Miklos state that the sound quality of an organ pipe is mainly influenced by its attack transients, and that the onset involves an edge tone before the resonator’s behavior comes to dominate. Chiff is the audible form of that transient. Charles Fisk defined it as the little incise or transient a pipe gives out naturally when it begins its tone, wrote its sound as ‘KAA’, ‘CHAA’ or ‘SHAA’, and treated it as the consonant that precedes the vowel of the tone. His definition is bound up with his advocacy of articulate speech, and the word is used more loosely elsewhere.

During the transient the partials have not yet locked into their exact harmonic relationship; the mode locking described under the feedback loop belongs to the steady tone that follows. That is why nicking, cut-up and the toe, which shape the jet in the first instants of speech, can change how a pipe begins without changing the pitch it settles on.

A flue pipe overblows when it sounds a higher mode of its air column instead of the fundamental it was built to give. This is a regime the pipe enters, not a failure of the mechanism. The voicing literature quotes a rule of thumb: a single voicing number formed from the frequency, the cut-up height, the jet velocity and the flue width predicts overblowing once it exceeds an approximate threshold. That framing places overblowing at the point where cut-up, jet speed and flue width fall out of proportion with one another, rather than treating it as an effect of pressure alone.

Fletcher makes the same point from the pressure side: for any note there is only a limited range of blowing pressure over which the phase relations in the jet will sustain that note, and the bounds depend on the individual pipe’s geometry rather than on any universal figure. A pipe that overblows is a pipe whose jet-to-mouth relationship has crossed that band. Why a particular pipe on a particular instrument has crossed it is a question for inspection, not for a page.

Every variable above is described for what it does, not for how it is set. The Organ Historical Society’s teaching page states that voicing, through adjustments including cut-up height, nicking, ears and beards, actually determines the timbre any pipe produces. That is why a stop name or a footage figure cannot fix a tone, and it is also why a written account of the variables is no substitute for the voicer’s judgment at the pipe. Nothing here asks a reader to touch, move or adjust any part of a pipe.

When the flue speech of an instrument raises questions, the useful starting point is a documented inspection of the pipework as it stands: its wind, its mouths and its speech as found, recorded so that any later decision rests on what was observed rather than on a quoted ratio or a remembered rule.