Touch Sensitivity in Mechanical Actions
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by Colin Pykett


Published in Organists' Review: November 1996

This version last revised: 21 December 2011

Copyright C E Pykett 1996-2011



This article first appeared in Organists' Review, November 1996 and it is reproduced here because of the number of requests received for reprints.   It created a favourable response and an abridged version appeared by request in the Diamond Jubilee Year Souvenir Book of the South Dorset Organists' Association, when again it received a complimentary mention in a subsequent edition of Organists' Review.  The article also has been cited in some subsequent research papers and dissertations by other authors.




One hears more and more today of the advantages which well-designed mechanical action organs offer to the discerning player in the realm of phrasing, articulation and touch sensitivity in general.  However it is rare for these sought-after effects to be identified quantitatively or even qualitatively; and it seems that we usually have to make do with ex cathedra pronouncements  from those in the know that the effects are real and to be promoted as a matter of priority.  This is unfortunate, because it (quite reasonably) causes many to question whether the effects are significant to the listener as well as the performer, or whether they are even real.  Thus in the columns of this Journal we see letters from those representing both polarities of the argument, at the same time as articles which go so far as to imply that practice on any instrument other than one having these attributes is futile [1] .  This article will attempt to define some of the major effects which are at the centre of these discussions, to show that they are indeed real and that in many cases they are significant enough to enable measurements to be made which demonstrate their existence unequivocally.


Attempts have been made for centuries to develop a keyboard instrument having something of the qualities which make, say, a violin seem to be part of the player (when played well, that is).  The unity of the instrument and the performer is demonstrated by the enormous variety of tone colours, dynamics and subtle articulations which almost approach the human voice in expressive power.  The invention of the forte-piano was the first major breakthrough enabling a keyboard player to have direct control over the way the notes are sounded, and the expressive capability of a modern grand piano is elegant testimony to the developments that followed.


Against this background the organ, particularly the nineteenth century organ, came a very poor (one might almost say laughable) second.  Moreover, the development of the organ at that time was proceeding in exactly the opposite direction from that of an instrument with which a sensitive player could strike up a rapport.  Organs were increasing in size to a point at which they became virtually unplayable, as exemplified by the huge tracker instruments of Hill in the middle of the century.  Whatever their virtues may have been, touch sensitivity was not one of them [2] .  Yet instead of designing tracker organs so that they became more player-friendly, organ builders abandoned mechanical action in favour of those novelties of the day, pneumatic and electric actions. Then the organ became an engineering showpiece much like the electronic organs of our own era, and in the process they had developed to a point whence no touch sensitivity at all was possible.  (In passing we might note that the much-maligned Robert Hope-Jones seemed to have appreciated this, and he tried a number of innovations to endow an electric action organ with touch sensitivity.  His second touch, sforzando effects and dual power stops were at least a recognition of the shortcomings of the organ as it was then developing, and some of these inventions were incorporated later in cinema organs.  But such crudities had little impact with sensitive musicians of the day, whether players or composers).  The inevitable consequence of allowing the organ to degenerate into little more than an ivory covered switchboard [3] was that it grew to a size quite out of keeping with the necessities of a musical instrument.  High wind pressure became of no account because of the availability of arbitrary amounts of power with which to open the pallets, and in turn this led to the widespread use of imitative tonalities and forms of voicing such as leathered diapasons.  With the simultaneous abandonment of the  classical tonal structure based on complete and balanced diapason choruses, the slide into decadence had become complete and we did not attempt to extricate ourselves from it for at least fifty years.  In the meantime even small organs were endowed with detached consoles concealing widespread extension, borrowing and duplication of stops, and when these went wrong they were often replaced with awful first generation electronics in the sixties.  It can be argued that all this occurred simply because the Victorians in Britain, and their contemporaries in other countries, did not develop an understanding of how to design good mechanical actions ! 


The importance of a good mechanical action  

It is curious that theoretical and experimental studies aimed at devising responsive mechanical actions have delivered results only in recent years.  With hindsight so many of the conclusions of this work seem like simple common sense, like minimising total mass in the action to avoid the player having to provide the energy to overcome excessive inertia in the moving parts - this is saying no more than it is easier to throw a tennis ball than a medicine ball.  Or in balancing out the weights of the vertical elements of the action so that those which move upwards more or less counterbalance those which move downwards. Or to remove the need for long horizontal tracker runs and long splayed backfalls (examples of inertia minimisation).  Or to minimise the size of the aperture covered by the pallet which opens into the groove below the pipes - this reduces the initial force required to open the pallet against the wind pressure, as does lowering the wind pressure itself to the absolute minimum.  Or to minimising the number of separate components in the action to avoid lost motion and friction in the various joints and pivots.  Considerations such as these have led to more organs having extremely simple actions such as those in which the key below the wind chest just pulls open the pallet via a light rod, usually via a roller board.  What could be simpler?


When assessing a mechanical action organ for its susceptibility to touch sensitivity one should test the pluck, or top-resistance, of the keys.  This arises from the static force caused by the wind pressure against the pallet before it opens, after which the pressures on both sides of the pallet become equal almost instantaneously. Thus the force required to open the pallet is greater than that required to hold it open.  Pronounced pluck is widely considered to be an  attractive attribute since it certainly assists in clean playing, to the extent it is simulated in many organs with electric action.  However this may be a superficial conclusion.  Some maintain that too much top resistance will prevent a skilled player from executing the repertoire of muscular skills needed to vary the speed with which the keys are depressed.  Practising on such an instrument may even inhibit the development of these skills in the first place.


The effects of the couplers are also important as these generally degrade touch sensitivity when they are brought into operation.  But the recent phenomenon of electric coupler actions embedded in otherwise fine mechanical actions seems to negate everything we are striving for - once the couplers are drawn, the result is a curious hybrid instrument in which the played manual retains its touch responsiveness whereas the coupled departments have (by definition) none. Also they allow in principle if not in practice a large and possibly unmusical organ to masquerade under the banner of a touch-responsive one.


From the foregoing it is more or less inevitable that many of the Victorian tracker instruments which still remain in our churches are not going to be good from the point of view of developing a responsive touch.  In by far the majority there is too much top resistance to the touch even before the manuals are coupled, and when they are coupled the results are a travesty as far as touch is concerned.  In criticising these instruments one has to pay tribute also to the worthiness of their construction, as many of them have withstood the ravages of a century or more with little attention. But one has to face the fact that sound construction does not correlate necessarily with sound design.


Touch sensitive effects

We come now to a consideration of some factors which are under the control of the player of an organ having the right sort of action.  It would be presumptuous to claim that all such factors on the organ can be isolated, described or measured any more than the multitude of subtle touches possible on the pianoforte can be so treated.  To do so would be to imply a limit to the artistic freedoms of the executant.  However, it is considered useful to show that there exist at least some effects which are real in the sense they can be heard and also measured in a physical sense, and that they can be varied by the performer.  As all of these effects occur either at the beginning or end of a note as the key is either depressed or released, they fall into the categories of the initiation or termination transients making up the complete sound.


Transient phenomena are of considerable importance in musical acoustics.  A characteristic of the human auditory system is that it is remarkably responsive to changes in a sound as opposed to one which is unvarying.  Many of us will be familiar with those demonstrations of various instruments in which the initiation or termination transients have been removed or altered, and in fact many of us have the facilities to do this at home these days with the ubiquitous personal computer equipped with a sound card.  Thus, and colloquially, pianos come to sound like harmoniums, oboes like violins, etc., etc.  All this takes place without any alteration of the quasi-steady-state sound which occurs between the transients.  Therefore it should not be surprising that even relatively small changes in the way a pipe begins or ends its speech are noticeable to the player and the performer.


Initiation Transients

Consider an organ having a slider soundboard in which there is compressed air in the wind chest below the pallets, and air at atmospheric pressure in the grooves between the pallets and the pipes.  As a pallet opens, air begins to move into the groove through the pallet aperture.  Eventually the pipes whose stops have been drawn will settle down to stable speech, but before that happens we must recall that air is compressible.  Thus the air in the chest will expand immediately into the groove, and pressure will be maintained in the chest only if sufficient wind can get back into it from the winding system. This will take time, measured typically in thousandths of a second, since not only does air have inertia by virtue of its mass, but the transient pressure changes we are describing can only propagate at a maximum speed equal to the speed of sound (about 1 foot or 30 cm per millisecond.  Readers accustomed to metric units will hopefully forgive a natural tendency to lapse into imperial ones when speaking of organs, since such is not without precedent!).  In practice the propagation speed of rapid pressure changes will often be less than this because they are moving in the confined spaces of the wind chest or the wind trunks.  A wind chest will have a length of several feet, so if the pallet we are considering is near the treble end of the chest and the wind trunk at the bass end, we immediately have a situation in which pressures will only stabilise several milliseconds after a pallet has been opened.  Transient changes having durations of the order of a few milliseconds are easily detectable by the ear.  Yet this is an ideal situation.  Even with pressure regulators close to the chest, such as concussion bellows or Schwimmer regulators, the inertia of their moving parts will add further milliseconds to the transient process.  If these devices are not present then the air pressure will only be restored when a pressure wave has travelled from a distant reservoir to the chest.  Indeed, it is not improbable that under certain conditions the wind pressure in the chest will not recover to its initial level while the pipes continue to sound.  This will occur if the trunking presents appreciable resistance to the passage of air when particularly air-hungry pipes are sounding.  Such issues are germane to "live-winded" organs, offering yet more possibilities to the alert player.


But we need to rein in our digressions and summarise where we have got to.  One scenario could be as follows.  Assume the key is struck with some force so that it falls rapidly.  As the pallet opens air expands rapidly into the groove and thence into the pipes, building up a rapid pressure peak at the pipe feet as it does so.  But because of the time taken for the air pressure to be replenished inside the wind chest, this pressure peak will decay somewhat after a few milliseconds, only to recover again after a further few milliseconds or even tens of milliseconds.  Therefore the pipes are subjected to a double pressure peak as the key falls.  The duration of this process from the point at which the pallet begins to open to that at which a steady state obtains is likely to be at least ten milliseconds in a typical organ, and often  appreciably longer.



Figure 1.  Frequency/time plot of a diapason pipe coming onto speech


The effect is illustrated in Figure 1, which is a display of how a small scaled diapason pipe came on speech.  The vertical axis represents the amplitude of the sound and the horizontal axis is frequency, which is plotted so that the lowest frequency (corresponding to the fundamental) is at the right hand end of the axis.  This unconventional representation was used in the interests of clarity.  The third dimension, receding into the distance as it were, represents time from the instant at which the pallet began to open.  Therefore the plot is a number of spectrum analyses closely spaced in time.  The amplitude of the fundamental frequency, corresponding to the pitch of the pipe, is clearly seen to rise rapidly to a peak, then fall away and finally to attain a steady state value rather more slowly.  Therefore the double wind pressure peak at the pipe foot is reflected in a corresponding double peak in the sound it emits.  The sharp-eyed reader might see three or even four peaks.  A few harmonics are also visible, but because of the linear amplitude scale used in this diagram rather than the logarithmic one usually expressed in decibels, the harmonic amplitudes are suppressed.  A linear rather than logarithmic scale was used in the interests of obtaining an uncluttered display.  Such transient oscillatory behaviour is entirely in keeping with a system containing air, which has both mass and elasticity.  Any physical system with these two parameters is liable to oscillate before settling down to a steady state.


What are likely to be the aural effects in these circumstances ?  It is unlikely that the ear can detect the peaks of sound individually since they are separated only by a few milliseconds.  (Mine certainly cannot).  It is at this point that we need to consider what happens if the key is allowed to fall more slowly, so that the pallet opens more gradually.  Intuition suggests that in this case the first inrush of wind into the groove will be more gradual also, allowing more time for replenishment from the winding system.  The net effect will be to smooth out the pressure peaks somewhat or even to remove them altogether, thus the pipe comes onto speech more leisurely.  Measurements similar to those in Figure 1 show that this does indeed occur, although it is difficult to correlate them with the speed at which the key is depressed without a rather more elaborate experimental set-up.  But the point is that by varying the speed with which the key is depressed, quite distinct  changes take place in the way a pipe begins to speak.  To return to the unanswered question above, therefore, it is not unreasonable to postulate that quite distinct aural differences are the result.  I can only quote the evidence of my own ears at this point, but I am convinced that the control obtained over the initiation transient of the pipe with a sensitive action is highly distinctive.  A rapid attack produces a relatively percussive sound, probably related to the oscillatory peak phenomenon, compared to the more languorous onset of speech when the keys are caressed more gently.  The effect is without doubt a most valuable addition to the performer's repertoire of touches in matters such as phrasing.  One realises the truth of this when one is confronted with an electric action and plays a piece that has been worked up on a mechanical one.


We have entered well and truly the subjective realm at this point, and the discussion can no doubt be taken forward much better by others.  It has merely been my intention to show that real, significant and measurable effects do take place at the instant at which the pallet opens, and that these can be modulated by a sensitive player with a sensitive action.  Also it is important to note that no two organs are likely to exhibit identical or perhaps even similar effects to those discussed above.  However it is gratifying that the double peak hypothesis, derived from simple intuitive thinking, was borne out so immediately by subsequent experiment.


The subject of initiation transients could fill a small book, and we have only scratched the surface here.  Not mentioned are effects such as the change in pitch as the pipe comes onto speech during a transient period in which the wind pressure is changing.  Moreover, careful measurements show that in many cases it is certain harmonics rather than the fundamental which attain stable speech first in the case of some flue pipes.  All of these effects are in principle under the control of the player given the right sort of instrument.  Nor have we mentioned chiff, which is quite independent of all that has been discussed hitherto.  Chiff also takes many forms, but one of the simplest is the case in which the pipe speaks a transient twelfth before reaching its steady state, a phenomenon familiar to many from the ubiquitous "coughing bourdon".  It is indisputable that the type of chiff emitted by a pipe can be modified by varying the speed at which the pallet is allowed to open, and sceptics have only to experiment at the console of a suitable organ in order to be convinced of this truth. 


Termination Transients

The subject of termination transients, that is, the effects produced when the key is released, seems to be regarded as less important than that of initiation transients.  This may be because the subjective effects are less important, though this seems unlikely when one recalls phenomena like the clock which suddenly ceases to tick - we almost "hear" the clock in these circumstances even though we were unaware of it previously.  Therefore the end of a steady state seems to be important psychologically, just as its commencement.  But another reason may be that with the pianoforte there is less scope for controlling the sounds produced at key release than at key down, perhaps because it is an issue connected intimately with the use of the sustain pedal in any case.  However with the organ there is a set of phenomena analogous to those at the initiation of pipe speech.  As with initiation transients, I shall discuss just one such case in detail in order to demonstrate only that the phenomena are real and that they can be brought under the control of the performer.


The case chosen is that of pitch variation as the key is released.  To illustrate this, again consider the conventional slider chest in which a note is already sounding.  The wind pressures in the chest below the pallet and in the groove above it are equal and above atmospheric pressure.  The transition to atmospheric pressure occurs chiefly within the pipe itself, at the foot and in any other constricted parts such as the mouth.  If the key is now released rapidly the pallet will close correspondingly rapidly, leaving the groove still charged with compressed air.  Clearly this will dissipate through the pipe more or less rapidly, but the time taken for this to happen will be finite and measurable.  This time interval will, of course, depend on several factors such as the wind pressure, the volume of the groove, the number of stops drawn, the types of pipework they represent, etc.  But the time involved can be measured and it is typically of the order of tens of milliseconds.  What happens to the sound of the pipes during this time?


A common feature is that the pitch sounded by the pipe will vary before it ceases to speak at all.  Many pipes are sensitive to wind pressure, particularly when it drops to zero, so this is not surprising.  What may be surprising is the magnitude of the effect.  My measurements suggest that for small flue pipes (such as those of less than one foot speaking length) the pitch variation can approach a semitone before the sound ceases altogether.  This is a large variation, and probably the only reason it is not more subjectively obvious is because of the short period over which it takes place.  Indeed, experiments I have undertaken using electronics (purely for convenience) lead to the conclusion that the pitch change has to be relatively large for it to have any subjective effect at all.  Yet the aural effect is not usually that of rapidly changing pitch.  To my ears, pipes which terminate their speech in this way simply sound more acceptable in some indefinable way than those which do not.  Going further, experiments with different degrees of pitch variation lead to the tentative conclusion that the inner parts of contrapuntal music are better defined when the effect is present than when it is not.


We have entered again a subjective minefield, and I want to extricate myself from it straight away.  What may sound good to my ears may be anathema to others, or others may perceive an entirely different range of effects.  But I do want to emphasise that here we have another example of the transient phenomena of organ pipes which have some sort of aural effect, and (most important) effects which can be controlled by the player.  For if the key were to be released slowly, the wind pressure in the groove will decay correspondingly slowly. Thus whatever effects were present in the former case will now be prolonged.  This is an exciting conclusion, because it enables us to test the range of effects possible on a particular instrument much more directly than in the case of most of the initiation transients.  By holding the key indefinitely in an intermediate position it is possible to determine precisely what the pipes will do in these circumstances when their wind supply is constricted at the pallet.  The player who is alert to these effects then opens up another portfolio of possibilities which can be exploited by sensitive articulation. 


Summary and Conclusions

We have seen that a number of transient effects occur with organ pipes, and that some of these at least are under the control of the performer at a suitable instrument.  Invariably this has to have a mechanical action designed according to sound engineering principles.  Evidence has been presented that some transient effects are measurable, so that arguments about whether they exist become difficult to sustain.  Rather, the subjective effects would seem to be the most fruitful area for further discussion.


It does not seem unreasonable for students to be expected to investigate explicitly the possibilities of transient control at the instruments they play. Thus perhaps all teachers might consider devoting lessons to these aspects of the performer's art, just as they would if they were teaching the pianoforte.  It is particularly instructive to listen to the differences in the sounds produced when individual notes are keyed at rates ranging from the very fast to the very slow, both in terms of attack and release.  These exercises should be undertaken both with individual stops and groups of stops.   This article might then provide some background for the types of effect students should be trained to listen for and to exploit. 



1.  Quoted in The Annual RCO Lecture at the IAO Organ Festival Huddersfield by Margaret Phillips, enclosed with OR, November 1995. 


2. "I am afraid I would not have the strength to [play a fugue], without a very long practice.  Perhaps you may speak to Mr Hill of these observations, and hear what he says to them... "


Letter from Mendelssohn to Joseph Moore concerning the Birmingham Town Hall organ c. 1846 (quoted in "A History of the Birmingham Town Hall Organ" by Nicholas Thistlethwaite, Birmingham City Council 1984).


This is one of many similar well-known quotations from the nineteenth century concerning large tracker organs.  Another is that of Dr Camidge of York Minster from about the same time:


"Such a difficult touch as that of York Cathedral organ is doubtless sufficient to paralyse the efforts of most men, I assure you.  I, with all the energy I rally about me, am sometimes inclined to make a full stop from actual fatigue in a very short time after the commencement of a full piece".


Letter from Camidge to C S Barker, inventor of the pneumatic lever, 1833.


Many of us will have experienced something similar even with small nineteenth century organs! 


3.  The evocative epithet "ivory covered switchboard" is not mine but it expresses everything I wish to convey.  I regret that I can no longer recall its originator.