The physics of voicing organ flue pipes
23 June 2018
Revised: 23 June 2018
Copyright © C E Pykett 2018
Abstract. Despite its apparent simplicity, details of how the organ flue pipe works have fascinated musicians, organ builders and scientists for centuries and it still continues to attract attention today. One reason for this is that the actual mechanisms are extremely complex, to the extent that they still lie somewhat beyond the reach of physics. This also applies to the techniques used in voicing the pipes, and it is this aspect on which this article focuses. However the rationale of the article is not so much to describe what a voicer does, which has been covered by others, but to shed light on the physical principles involved. This does not seem to have been attempted before, at least not without resorting to advanced mathematics. However maths makes no appearance here because the article is intended for the general reader, and in any case there still exists an unfortunate gulf between theory and experiment relating to organ pipes. Nine of the most important adjustments available to the flue pipe voicer are discussed in detail, including the somewhat mysterious technique of 'nicking' often applied to the languid and lower lip. The attack transient of a flue pipe is also covered. In each case the discussion draws out the physics involved as well as the consequential aural effects such as changes to timbre and power. Comprehensive cross-references to other articles elsewhere on the site are provided, and these can be accessed immediately using the links provided.
(click on the headings below to access the desired section)
Flue pipe voicing adjustments
Upper lip sharpness
Upper lip offset
Foot hole size
Attack transients in flue pipes
Notes and references
When an organ pipe has been made it has to be 'voiced' before its sound is judged to be correct. This vital process is carried out using a compendium of tools and techniques going back many centuries to the earliest days of organ building. Much of what a voicer does is not well known nor understood outside the craft even today, partly because there still remains an element of secrecy about it. Beyond the patent literature, which today is sparse, organ builders are not generally known for revealing technical details of their work in the public domain, though perhaps one cannot blame them for this when such a time-consuming activity would generate little revenue at best and might diminish it at worst. Thus the diminutive literature which does exist has been largely generated by scientists working in a university environment rather than by organ builders themselves, though here the amount is also small in view of the difficulty of obtaining funding for a niche area of research with little public significance. Much of it is also highly mathematical, making it difficult to assimilate for most readers, and in any case a good deal of the theory does not agree well with experimental results. Therefore this article reviews the main techniques used in voicing flue pipes for the benefit of the general reader, but it also goes further by explaining the physical background to each one without resorting to mathematics. As an example, and probably uniquely, the physics underlying the mysterious 'nicking' procedure often applied to them is explained here.
A voicer must work with a pipe which has already been made, and therefore its tone quality has been partly defined beforehand once and for all. In particular, the cross-sectional area of a pipe relative to its length (termed pipe scale) influences its power (loudness) because this depends on mouth size, which in turn is obviously constrained by the width of the pipe. Furthermore, pipe scale also strongly affects its radiated frequency spectrum or harmonic content, and thus its subjective tone colour or timbre. Nevertheless, these and other acoustic attributes can still be modified at the voicing stage, and dynamic speech characteristics such as the attack transient can be extensively controlled or suppressed altogether. These and many other factors have been discussed already in other articles on this website, but it was thought to be useful to bring those which relate to voicing together in one place. However to keep the article to a reasonable length, repetition has been largely avoided so it will sometimes be necessary to refer elsewhere to expand particular issues. To facilitate this an extensive list of references is provided at the end of this article. They refer mainly to other articles elsewhere on the site which can be accessed immediately just by clicking on the links provided.
Flue pipe voicing adjustments
Simplified structure of a typical flue pipe
Organ pipes are of two types - flues and reeds. And before going further, do not confuse the similar-sounding names 'flue pipe' (a generic type of organ pipe) and 'flute pipe' (a flue pipe having a specific flute-like sound). Although a flute pipe is always a flue pipe, a flue pipe is not always a flute pipe! This can confuse the unwary if they do not keep their wits about them while reading organ literature.
Flue pipes, in the form of actual speaking pipes or merely as unvoiced dummies included for visual effect, have adorned the external casework of virtually every organ ever made, so it is unnecessary to dwell on their appearance and construction in detail. However a cross-sectional diagram of the salient features of a flue pipe is included above at Figure 1. Yet despite its apparent simplicity, the details of how it works have fascinated musicians, organ builders and scientists for centuries and it still continues to attract attention today. One reason for this is that the actual mechanisms are extremely complex, and they still lie somewhat beyond the reach of physics. This is particularly true of the oscillating air jet at the pipe mouth: how it develops fleetingly when the pipe valve first opens to admit wind, and how it then evolves from that into a more stable form as the pipe settles down to steady speech.
For those who desire further insights, the jet-drive mechanism underlying flue pipe speech is described generically in reference
, and this is expanded for the particular cases of flutes, principals (diapasons) and strings in
,  and  respectively. Briefly, the timbre or tone colour of any organ pipe is dominated by its harmonic content received at the listener's ears when it is speaking in its sustained or steady-state phase, and in turn this is influenced strongly by the harmonic spectrum generated in the first place at the mouth of a flue pipe. But this sound is never heard in isolation, nor can it be, because it is always modified by the pipe body or resonator which applies a frequency-dependent filtering action governed by the important pipe scale parameter mentioned already. (These two processes - the oscillation at the mouth and the effect of the resonator - are not independent because they are intimately coupled by a complicated generator-resonator feedback loop which renders an exact mathematical analysis of flue pipe speech impossibly difficult). A second stage of filtering then occurs as the pipe launches its sound into the auditorium because pipe scale also determines the particular and differing efficiency with which each harmonic is radiated. Yet a third filtering stage occurs due to the acoustic ambience of the auditorium itself. However the voicer can only modify the harmonic spectrum initially generated at the mouth because s/he cannot vary the scale of a pipe nor the acoustic environment into which it speaks.
The number of harmonics and their strengths generated at the pipe mouth depend on several factors, including the cut-up (mouth height relative to its width), sharpness of the upper lip, upper lip offset relative to the flue or to the air jet, and languid height relative to the lower lip. If you have difficulty visualising what these factors actually relate to in terms of pipe construction, referring back to Figure 1 might help, and they are also discussed individually in more detail below. All of them are readily adjustable by the
voicer, at least in metal pipes. With wooden ones it is more difficult to adjust the languid height and lip offset for practical reasons. Other adjustments include the flue slit width, nicking of the languid and lower lip, the foot hole diameter and adding ears or a 'beard' at the mouth. Most voicing adjustments interact with each other to some extent so it is not possible to focus too strongly on one at the expense of the others. Nor should one be too dogmatic, because we are discussing here not merely a craft but an art in which good results stem from skill, experience and an unusually sensitive ear rather than a mandatory knowledge of physics. However it is necessary to introduce a degree of simplification so that the subject can be treated at a relatively elementary level. There exists a body of descriptive and qualitative literature in the public domain dealing with organ pipe voicing in terms of the operations carried out, and a representative selection appears in the references sections of the articles mentioned at
 through . However no attempt is made to summarise the literature here because only the physics underlying the various adjustments is of interest, and this aspect is not well discussed elsewhere. This article attempts to remedy
The cut-up of a pipe mouth is its height expressed as a proportion of its width. As a pipe gets longer, its width also increases according to the particular scaling law applied to the complete rank of pipes (see reference
 for a discussion of scaling). Its mouth therefore gets wider to suit the fatter pipe, and so mouth height must also increase to keep the ratio of the two - called the cut-up - the same. High mouths attenuate the higher harmonics whereas low ones encourage their formation, thus the former are used for flutes and the latter for keen string tones. The mouth heights of principals (diapasons) fall somewhere between these extremes, and typically these are cut up to around 1:4, meaning that height is one quarter of width. The reason why cut-up affects the number of harmonics is that the thickness of the oscillating air jet or wind sheet (measured through the mouth into the pipe) increases as it propagates upwards from the flue. In other words, the jet becomes wider and less well defined in this dimension as it moves towards the upper lip, mainly due to drag against the stationary air of the atmosphere at the edges of the jet. As the jet flips back and forth across the upper lip, fewer harmonics will be generated by a vaguely defined thick jet than by a tightly constrained thin one. This follows because the repetitive pulses of air injected from the jet into the pipe body above the upper lip will be narrower in the latter case. They will also have faster rise and fall times, and these give rise to more harmonics in the spectrum because the harmonics in a wave train of sharp, narrow pulses do not fall off so rapidly in amplitude as their frequency increases. In summary, sharp narrow pulses contain more harmonics, and they are generated by mouths with low cut-ups. Conversely, the more vaguely defined pulses generated by higher cut-ups result in fewer harmonics being generated. These effects, that is pulse shapes and their corresponding harmonic spectra, were treated in more detail in reference
 (see the section entitled 'Harmonic
Upper lip sharpness
Sharpening the upper lip by bevelling its front surface results in more harmonics being generated than if it had been left blunt. The reason is much the same as above because it means the air jet is able to pass more rapidly from inside to outside the pipe, and vice versa, when the lip is sharp. Thus the pulses in this case become narrower and better defined, again leading to more harmonics being generated.
On the other hand, some upper lips are deliberately made very blunt by increasing their width using a thin leather covering. This broadens the air impulses delivered to the pipe body, resulting in fewer harmonics being generated. Leathered lips are used for extremely dull flutes such as the tibias of the theatre organ, or for loud diapasons of heavy and ponderous tone.
Upper lip offset
Assume we have a pipe whose upper lip lies vertically above the flue slit. Further assume that the position of the languid relative to the flue results in an air jet travelling vertically upwards rather than at an angle. In these circumstances the oscillating jet will flip back and forth across the upper lip with a 'mark-space ratio' close to 1:1, meaning that it spends much the same time inside the pipe as outside during each cycle of oscillation. Other examples of such waveforms are the symmetric square and triangular waves used in electronics, and these have nulls in their frequency spectra at the positions of the even-numbered harmonics. In these cases the even harmonics are therefore absent. For hollow-sounding flutes this is fine, indeed it is a sought-after voicing situation as explained at note
. But it is decidedly unattractive for principals and strings because their even-numbered harmonics must not be attenuated relative to the odds if they are to sound as one would expect and require. To achieve this, principals and string pipes require the injected pressure waveform to consist of an asymmetric train of short pulses rather than an approximate square or triangular wave, both of which contain a preponderance of odd harmonics. In practice, for organ pipes the even harmonics in these latter types of wave will not be entirely absent as they are in the electronic case just mentioned. However they will still be too weak for principals and strings and they will therefore degrade the tone quality in these cases as just explained.
To remedy this state of affairs, the upper lip can simply be offset from the flue by pushing it slightly into the pipe or pulling it out, provided of course that the pipe is of metal rather than wood. Then the oscillating air jet spends more time inside the pipe than outside (or vice versa), changing the
exciting waveform generated at the mouth from square-ish into the asymmetric train of narrower pulses which we have seen is desirable for principal and string pipes.
An expanded discussion of the frequency spectra corresponding to the various types of waveform discussed above can be found in reference
 (see the section entitled 'Harmonic
The languid of a metal pipe can be raised or lowered slightly relative to the lower lip by levering it upwards or tapping it downwards using suitable tools. This has the effect of altering the angle at which the air jet travels towards the upper lip. It therefore changes the mark-space ratio of the pulse train delivered to the interior of the pipe, and in this respect the results are similar to those described above by altering the offset of the upper lip. Languid height adjustment cannot be done so easily, at least in the same way, to a wooden pipe for practical reasons.
Adjusting languid height or upper lip offset also has another effect in that it makes the pipe speak 'quicker' or 'slower'. These adjectives refer to the time taken for the pipe to reach its stable speaking regime. Flutes tend to be 'quick' whereas strings are 'slow' to come onto speech. In the latter case the air jet is initially thrown so far outside the mouth at the level of the upper lip that it takes more time (i.e. more cycles of the fundamental frequency) to fully engage with the travelling air impulses inside the pipe after the pipe valve is opened. Many organists will be familiar with this effect in which some ill-adjusted pipes in a string-toned stop struggle to reach stable speech at all, and during this process they can emit the most peculiar sounds. Longer pipes are more prone to the problem than short ones because their mouths are higher, thus a given adjustment of the languid produces a correspondingly greater deflection of the air jet at the upper lip than in a smaller
pipe having a lower cut-up. Below about tenor C on an 8 foot keen-toned string stop the adjustment becomes so critical that it is virtually impossible to get the pipes to speak stably or at all. This behaviour can be remedied by fitting ears and a beard to the pipe mouth, and these are discussed below.
Figure 2. Ears and a wooden beard fitted to a Gamba string-toned pipe
Many flue pipes are fitted with vertical ears soldered onto each side of the mouth, as shown in Figure 2. They increase the range of mouth heights and languid positions which will still result in stable speech. This is of special benefit in view of the small mouths which have to be used for strings, and in view of the reduction in the criticality of the adjustments which ears provide. In particular, it is possible to use even lower cut-ups when ears are used than would otherwise be the case, and this further augments the acoustic power at the high frequency end of the harmonic spectrum which is necessary for realising keen-toned strings. It is reasonably obvious that ears will act to constrain the lateral propagation of the oscillating air jet, which would otherwise dissipate horizontally into the atmosphere beyond the pipe wall without them. In physical terms the effect is to form a 'plug' of vibrating air at the mouth, and this results in the advantages mentioned because the relatively massive plug does not have time within each oscillation cycle to completely escape the confines of the ears. Thus the plug prevents the air jet from 'bending' too far away from the pipe as it oscillates, resulting in more stable speech especially during the attack transient when the pipe valve is first opened. The fundamental frequency (pitch) of the pipe is reduced somewhat as a result of the increased air mass at the mouth (i.e. it goes slightly flat), though this is not an issue because the pipe can of course be re-tuned subsequently.
Fitting a 'beard' or bar between the ears of a string pipe at its mouth further controls and stabilises the position of an air jet which has been deliberately offset from the upper lip by deflecting it outside the pipe. Various constructions have been used but a wooden beard is seen in Figure 2 above. As described earlier, the offset jet is necessary to generate a narrow-pulse type of waveform containing the many harmonics which are necessary for strings, and to a lesser extent for principals. Although stabilisation can be achieved by adjusting the languid position as described above, the adjustments become unfeasibly critical for the longer pipes. However a beard physically prevents the jet straying too far away from the upper lip when the pipe valve opens, thereby helping the pipe to come onto speech when wind is first admitted to it. While the pipe is speaking it also helps the jet to 'bounce' back into the mouth as it oscillates across the upper lip. Thus a beard augments the effect of ears in that both help to produce and maintain a stable plug of air at the pipe mouth, which in turn restricts dissipation of the air in the oscillating jet into the atmosphere.
The flue is the narrow rectangle which gives an initial shape to the air jet as it passes between the lower lip and the languid. Increasing the width (the smaller dimension of the rectangle) increases the area of the flue and hence allows a greater air flow rate, which increases the acoustic power generated by the pipe. However the increased width of the jet at the flue is amplified as it blows upwards, thus it becomes considerably wider and more diffuse at the upper lip. This can result in a reduction of the number of harmonics generated, which might be undesirable from a tonal point of view. On the other hand, a flue which is too narrow can result in the pipe having a weak, scratchy and windy tone. In organs from the baroque era and earlier, varying the flue width was frequently employed to control the loudness of a pipe rather than using the foot hole for this purpose as described below. This process, regardless of how it is achieved, is called regulation.
Foot hole size
The effects of foot hole size cannot be divorced from those related to flue width because both control air flow rate across the mouth and thence into the pipe body and the external atmosphere. However differences arise in terms of the consequential effects on tone quality, because the foot hole is acoustically isolated from the oscillating air jet above the flue by virtue of the considerable mass of air contained within the volume of the pipe foot. In contrast, adjustments to the flue affect jet dimensions directly as described above. Thus the foot hole controls the overall power of the pipe, and today its diameter is usually varied mainly for this purpose rather than for tweaking tone quality, at least when an organ is working on medium to high wind pressures in the chest below the pipe.
Nevertheless, there is a definite effect on tone quality related to foot hole size, with larger holes resulting in more harmonics being generated than with smaller ones. This occurs because a larger hole increases the air pressure inside the pipe foot just below the languid (which is always less than the pressure in the chest unless the foot hole is unusually large compared with flue area), and in turn this increases the speed at which the air jet leaves the flue. However, unlike variations in flue slit width, altering foot hole size does not affect the dimensions of the air jet as it leaves the flue - obviously, the initial cross-sectional size and shape of the jet is always that of the flue itself. But changing foot hole size nevertheless results in a variation of jet dimensions when it arrives at the upper lip. Here's why. Enlarging the foot hole results in a narrower jet at the upper lip (measured through the mouth and into the pipe) because the faster-moving, higher-energy jet has less time to expand as it travels upwards towards the upper lip, and we saw previously that narrower jets generate more harmonics than wider ones. There is consequently an interaction between foot hole size and cut-up in that both adjustments affect the harmonic content of the waveform generated at the pipe mouth, and thus the tone colour or timbre of the pipe.
Figure 3. Flue pipe showing a nicked languid
As its name implies, nicking is the process of cutting a series of small notches in the languid and sometimes in the lower lip as well (Figure 3). Their number, spacing and depth vary widely but for keen high-pressure strings the nicking is about as dense as it gets. While an Open Diapason might have 5 or 6 nicks per
centimetre, a Salicional (mild string) will typically have about 12 and a Viol d'Orchestre (keen string) about 15.
Of all flue pipe voicing techniques, nicking is probably the most mysterious. There is no doubt what it achieves regarding the tone and behaviour of the pipes so treated, but scarcely the vaguest qualitative description of how it works appears anywhere, not even in the most rigorous and specialist literature, and one searches in vain for a more complete treatment. Therefore some space is devoted to it here with the intention of illuminating the matter to some extent.
Firstly let us recall how nicking modifies the sound of a pipe. It might speak less promptly when nicked but it will do so with less of a tendency to emit a pronounced attack transient. Nicking also reduces some of the rather objectionable 'buzz' or 'fizz' noises which principal or string pipes can emit, effects which are more noticeable at close quarters. They occur because of edge tones
 generated at the upper lip, often in the form of short bursts of noise or high frequency oscillations unrelated in frequency to the pitch of the pipe. These parasitic bursts appear one or more times during each cycle of the fundamental frequency as the air sheet flips back and forth across the lip, and they are immediately identifiable when observing an oscilloscope trace of the emitted sound waveform. (Similar parasitic effects can occur with electronically-generated waveforms such as those used in radio transmitters, and the effect is sometimes called
'squegging'). Another advantage of nicking reduces the tendency of a pipe to speak in two different modes (frequencies) of oscillation at once, an effect which is occasionally heard as the pipe switches randomly from one to the other. This particularly afflicts string pipes, which are sometimes heard to wander haphazardly between the two frequencies while the note remains keyed.
From the above it seems reasonable to summarise the effect of nicking as a tendency to stabilise the speech of a pipe, but going further requires a digression into laminar and turbulent air flow. Laminar flow in the air jet at the pipe mouth exists when it moves in an orderly fashion thoughout its volume, like ranks of soldiers marching in step. There is no lateral mixing, nor eddies or swirls. The jet is simply a homogeneous thin sheet of air flowing uniformly upwards from the flue. This kind of flow can and often does exist in a pipe working at low pressure with no nicking. However if one wants to increase the pressure to get a louder sound, one might encounter a point at which instability becomes a problem. For example a pipe might work perfectly well in the voicing room of an organ builder's workshop, yet require major attention to make it work in an organ for reasons which are sometimes unclear. Or its pitch and other aspects of its speech might be seriously affected by another pipe speaking nearby. Other forms of instability include mode switching as mentioned above. All these, and more, can happen when the desirable state of laminar flow is disrupted by the onset of turbulence. So what is the solution? Perhaps surprisingly, it is to encourage turbulent flow from the outset by nicking rather than striving to retain laminar flow. Once the air jet exhibits turbulence none of the former attributes characterising laminar flow exist. The flow becomes highly irregular, and the instantaneous speed of the jet at any point ceases to have a fixed and unvarying value. Rather, it assumes a mean value at a given height above the flue but with positive and negative excursions which occur randomly across the jet. The onset of turbulence is illustrated in the photograph below of a smoking cigarette (Figure 4).
Bear in mind that what you see is laminar flow followed by turbulence in the
air. The smoke particles merely render visible what the air is doing,
thus the same effects would occur from a hot-tipped object which did not emit
smoke, though you could not then see them.
Figure 4. A smoker unwittingly demonstrating the onset of turbulence in previously laminar flow
How does nicking result in turbulence? The answer might seem obvious, and it is, but it is worth explaining nonetheless. One can imagine the air issuing from a nicked flue to consist of an array of vertical narrow ribbons, each one arising either from a nick or from the space between adjacent nicks. A ribbon flowing from a nick moves at a different speed to its two neighbours which do not issue from a nick because flow rate depends on nozzle size, as with the water issuing from a garden hose. Therefore, as the many adjacent ribbons move upwards, they rub and nudge against each other because of their speed differences. This results in rapid lateral mixing of the air in the ribbons with the result that the flow swiftly becomes turbulent as it moves away from the flue.
We now only have to understand why a turbulent air jet results in stable pipe speech. The reason is so obvious that it is easy to overlook, and it is that once turbulence has set in it is there for good. Unlike the fragile state of laminar flow which is easily disturbed, nothing can disrupt a turbulent jet to stop it being turbulent while the pipe continues to speak. The cigarette of Figure 4 demonstrates this eloquently because it is inconceivable that the turbulent swirls of smoke towards the top of the picture could ever re-form themselves into the thin column of laminar flow again. The probability of this happening is comparable with that of a shattered piece of crockery magically repairing itself after crashing to the floor. Turbulence is a physically degenerate state in the same way that heat is the most degenerate form of energy into which all other forms eventually descend. Both are consequences of the second law of thermodynamics which shows that randomness and disorder (high entropy) are the ultimate and preferred state of the universe. Turbulence is therefore an extremely stable state, though one which is far more difficult to characterise and understand than laminar flow. But because of the fortunate fact that it is obviously possible to make organ pipes work satisfactorily under a turbulent flow regime, they will by definition speak with more stability.
The only issues remaining are those relating to speech differences between laminar and turbulent flow, such as the type of attack transient, and voicers have long learned how to cope with these
. Parasitic noises such as 'fizz' are less of a problem because the speed at each lateral point of the air jet oscillating across the upper lip no longer has a well defined value. The air at all points across the lip will move at much the same average speed over time, but each one will depart randomly from this value at any instant. Consequently artefacts such as parasitic edge tones are less likely to arise across the lip as a whole in this situation. As for transients, they are sometimes reduced by nicking because the initial air impulse applied to the body of the pipe is less well defined for a turbulent jet than a laminar one and so it excites fewer natural resonances of the pipe, or it only excites them with lower amplitudes. It is more like a cloud than a sharp impulse when it first reaches the upper lip. Looking again at the smoking cigarette in Figure 4, you can actually see the turbulent air cloud because the smoke
particles render it visible, and it is then easier to appreciate how improbable
it would be for the cloud to re-form itself into the thin laminar cylinder which
precedes it. However there is a lot more to transients than this and they are discussed further below.
Attack transients in flue pipes
Flue pipes can emit pronounced attack transients as they come onto speech. One reason is that the air jet is often thrown considerably outside the mouth when the pipe valve first opens, and it can take many cycles of oscillation before the jet takes up its stable operating position. This period can typically occupy fifty cycles or more at the fundamental frequency, and during this interval an attack transient is heard.
We need not consider transients in more detail here because an entire article is devoted to the subject elsewhere on this website
. It deals with the particular case of a large string toned Violone pipe but this does not affect the issues discussed, which are generic to flue pipes as a whole. The issues include the initial anharmonicity of the partials in the transient, how they are pulled progressively into phase-lock to become the exact harmonics of the steady-state tone, the importance of choosing an appropriate operating point for the pipe on the frequency-pressure curve, and the type of action employed to open the pipe valve.
Thanks are due to the University of New South Wales for permission to use the photograph of a nicked flue pipe (Figure 3 above). This first appeared in a paper by the late Neville Fletcher and the late Suszanne Thwaites in
Scientific American in 1983, and subsequently in the book 'The Physics of Musical Instruments' by Fletcher and
Rossing. Professor Fletcher himself attributed the copyright of this picture to
Scientific American, but on enquiring whether I might use it here they directed me back to him. As he is, very sadly, no longer with us, I am therefore grateful to UNSW for permitting me to use it.
Notes and References
"How the Flue Pipe Speaks", an article on this website, C E
2. "The Tonal Structure of Organ Flute
Stops", an article on this website, C E Pykett, 2003.
3. "The Tonal Structure of Organ Principal
Stops", an article on this website, C E Pykett, 2006.
4. "The Tonal Structure of Organ String
Stops", an article on this website, C E Pykett, 2012.
5. For virtually all flute pipes the voicer, by accident or design, adjusts the mark-space ratio of the oscillating wind sheet or air jet to be approximately 1:1. This means that the sheet spends the same amount of time inside the pipe as it spends outside over each cycle of oscillation. The condition occurs automatically for many wood pipes because the flue slit often lies directly below the upper lip, and the positions of neither (nor that of the languid) can readily be adjusted. Thus the air jet blows vertically upwards from the flue to the upper lip until it begins to oscillate. With metal pipes the situation is more amenable to adjustment because the voicer can vary the direction of the air jet relative to the lip either by adjusting the height of the languid, or by slightly pulling/pushing the lip out of or into the pipe. With a 1:1 mark-space ratio we have something approaching a square or triangular wave of air pressure injected into the pipe body rather than a waveform containing a string of narrow pulses, and it can be shown that both have harmonic nulls at the positions of the even-numbered harmonics. Although they are not completely absent in practice, the power of the even-numbered harmonics is attenuated below that which principals and strings would normally exhibit and this gives rise to the somewhat 'hollow' sound of flute pipes, even in those which do not have a stopper. These effects, that is pulse shapes and their corresponding spectra, were treated in more detail in reference 
above (see the section entitled 'Harmonic Generation').
6. Edge tones can arise at the upper lip of an organ pipe quite independently of the resonating tube above it. In fact experiments have been done to show that the tube can be removed yet edge tones will still remain at the mouth. They also occur when the wind howls through sundry sharp apertures in door and window frames, and they are caused by a succession of eddies arising alternately on either side of an air sheet. These flip the sheet across the edge at relatively high frequencies and thereby generate tones. It is similar to a flag waving in the breeze, whose fabric is jostled back and forth in a wave-like manner by eddies arising alternately on either side of it. Because they have little connection with the way in which an organ pipe actually speaks, edge tones are referred to as parasitic oscillations here.
7. It is no coincidence that the 'modern' voicing techniques which surfaced in the 19th century arose because of the contemporaneous appearance of blowing plant powered successively by water, town gas and oil engines, and finally mains electricity. Before that, pressures were perforce low when the only available source of power was from human muscle. In those low-pressure days organ pipes would have worked mainly in a laminar flow regime, especially as open foot voicing was common and nicking was not used routinely. But when pressures began to rise to create the power and profundity so beloved in Victorian organs, together with the plethora of imitative stops such as keen string tones, the voicer had to invent and embrace a new repertoire of techniques to cope with the turbulent air flow in the pipes he then routinely had to handle. That he succeeded so well is a tribute to the craft, especially as physics has taken over a century to catch up.
8. "A Second in the Life of a Violone", an article on this website, C E