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Swell boxes and swell pedals - Part 1: Their effect on organ pipe sounds
Colin Pykett
Posted: 19 September 2023 Revised: 31 December 2023 Copyright © C E Pykett
Abstract. The expressive capability of the organ can be enhanced by enclosing some of its pipes in a swell box with moveable shutters. However there is little or no literature describing how this affects the sounds in terms of acoustics, contrasting with the sometimes divergent opinions or desires expressed by organists which are widely aired. Therefore this article focuses on the sounds emitted by swell boxes in an attempt to better understand their behaviour. It is the first of two, with the second [2] exploring the diverse mechanisms that have been used to operate the shutters since the swell box was first introduced over three centuries ago.
Contents (click on the headings below to access the desired section)
The effect of a swell box on organ pipe sounds
The idea of enclosing some or all of the pipes in an organ in an expression or 'swell' box to vary their loudness is traditionally associated with the English organ builder Abraham Jordan, who demonstrated a practical design in 1712 (311 years ago at the time of writing) in his organ at the church of St Magnus the Martyr in London. The 'swell organ' as a separate division in a pipe organ played from its own keyboard has retained its appeal ever since, and additional enclosed divisions or even entirely enclosed instruments are found frequently. A swell box with its shutters open is shown in Figure 1 [1].
Figure 1. Swell box with shutters open
This article is the first of two which explore the technology of the organ swell box. Unusually, it consists of a detailed discussion of how swell boxes affect the sounds of the pipes which they enclose. It is unusual because, although a broad spectrum of opinion has been expressed over hundreds of years about how they should influence the sounds, hardly anything exists which shows how they actually do influence the sounds. Unfortunately the two have not always approached closely in practice. Therefore the focus here is on some acoustic measurements and subsequent analysis which I undertook of the sounds emitted by real swell boxes. This helped to distill down the overabundance of disparate beliefs and desires into a few, hopefully relatively uncontentious, properties which might characterise a satisfactory swell box.
This is followed in Part 2 (a separate article [2]) by descriptions of the diverse mechanisms that have been used to operate the swell shutters, ranging from the purely mechanical to a range of servo systems relying on pneumatic or electric assistance. Again, the gap here between intention and achievement is shown to have been excessive in rather too many cases.
The effect of a swell box on organ pipe sounds
This is not the place for a detailed account of every conceivable aspect of pipe organ swell boxes since their development and structure are covered elsewhere in the literature. The material here is limited to how effective they are in an acoustic sense, and how that effectiveness can be characterised objectively. Organists have a general understanding and appreciation of whether the effect of a swell box is 'good' or not even though they might find it difficult to articulate. Thus, although individual opinions will vary, there is probably a broad consensus on the main issues.
When a swell box moves from its open to a closed position, the volume of sound is not merely attenuated equally at all frequencies as it is when you adjust the volume control on your hi-fi system. The tone quality varies as well in that high frequencies are attenuated more rapidly with pedal position than the lower ones, so we need to understand more about this frequency dependency. As well as these aspects, a box which attenuates the overall sound too much or too little will be judged unsatisfactory, and one which gives that subtle impression of suppressed power when enclosing 'full swell' will be preferred to one which merely causes the sound to advance and retreat in an anodyne manner. Furthermore, organists seem generally not to mind that the sensitivity of swell pedals is often greatest near to the point of closure, in the sense that a given amount of movement at this point will affect the volume of sound more than the same amount when the box is nearly open. Indeed, many seem to like and expect this behaviour. However they do not like it to be overdone to the point where it becomes impossible to exercise proper control.
Although the remarks just made are generalisations, one can usefully venture further by amplifying them a little. It is probably fair to say that swell boxes are most acceptable when constructed of wood rather than other types of building material such as concrete, bricks or blocks. Although the latter can demonstrate an impressive degree of volume control, their disadvantages are twofold. Firstly, the volume control effect is exactly that - mere volume control. All frequencies from the highest to the lowest are attenuated in more or less the same way, causing the sound to advance and retreat without character as it does on many digital instruments. It is more difficult to get that sought-after impression of pent-up power and thunder when using the manual doubles, reeds and mixtures with such a box. The second problem is that boxes of this type usually have a dynamic range that is simply too great, and this compounds the difficulties just outlined. If quiet stops such as a salicional, dulciana or the celestes are enclosed in this type of box they can vanish to the point of inaudibility when it is closed. In the late Victorian era with its frenetic pursuit of engineering applied to everything which moved and much which did not, much ingenuity was devoted by organ builders such as Robert Hope-Jones to making swell boxes which cut the sound down to practically zero. This was of a piece with his habit of inventing solutions to problems which did not exist, though he was not alone. A wooden box, on the other hand, allows a crucial amount of acoustic leakage to occur in various subtle and desirable ways. Because the woodwork can vibrate, it can lend colour to the sound at various resonant frequencies. The air inside the box also resonates independently but at a frequency which varies dynamically because the box is essentially a Helmholtz resonator which is tuned by the combined apertures of all the shutters acting as a vent or port. The tuned frequency obviously changes as the organist causes the shutters to move. Although this occurs with any type of enclosure, only a wooden box can transmit the internal resonance to the panel work which will then generally vibrate sympathetically but in unpredictable ways. The unpredictability lends interest to the aural effect. The vibration also means that sound will leak out even when the box is fully closed because the woodwork itself re-radiates acoustic energy into the room, thus the pipes will seldom approach inaudibility with a wooden box unless it is so designed. Moreover, because more acoustic power is generated by the pipes at the lower frequencies, it is in the lower frequency region where sympathetic vibration will be preferentially set up, contributing to the well-liked thunderous effect of an enclosed 'full swell'. The sheer size and mass of the wood panels forming the sides of the box also make them more efficient resonators at the lower frequencies. Consequently swell boxes are unlike loudspeaker cabinets in terms of the effect they have on the respective emitted sounds, nor should they be. Concrete, bricks and blocks are excellent materials for enclosing loudspeakers because they insert negligible colouration into the programme material. Wood, although often used for cheapness and other practical reasons, is less satisfactory. The reverse applies to wooden swell boxes, where the same colouration adds to the unpredictable aural interest which fascinates the ear and which we have come to expect of an enclosed division in a pipe organ.
Taken together, these reasons explain why a wood swell box is more likely to possess the desirable characteristics outlined above. Indeed, these preferences have arisen precisely because of the hundreds of years of unconscious habituation during which we have lived largely with wooden swell boxes. Put another way, a good swell box today is a product of an evolutionary process that has been driven subtly by our subjective inclinations over many generations. When such a box is closed it acts as a low pass filter, an extreme form of treble cut EQ (equalisation) in audiophile parlance, or tone control in everyone else's. The low pass filtering effect of a wooden box allows sound to leak out progressively more effectively the lower the frequency. Indeed, at the lowest frequencies a swell box is called upon to handle - about 30 Hz, corresponding to the bottom note of a 16 foot stop - there may be little or no attenuation of sound level at all when the box is fully closed. Note that this only applies to the fundamental frequency of the pipe, not its harmonics. This is important, because it means that the subjective impression of loudness reduction when a swell box closes is achieved largely by the tone control action applied to all harmonics (including the fundamentals) of the higher-pitched pipes, and to the upper harmonics of the lower-pitched ones (with less effect on the fundamentals). This contrasts with simple volume control as generally understood which affects all frequencies equally.
Summarising, the main outcomes of the discussion so far are:
1. The overall attenuation of the sound when a swell box is closed should not be too much or too little.
2. There should be some frequency-dependent treble cut effect related to swell pedal position, rather than a simple volume control.
3. It is not necessarily a disadvantage for the sensitivity of the swell pedal to be greatest near to the 'box closed' position.
Thus a swell box has a frequency response which it imposes on the sounds produced from the enclosed pipework, the response varying with swell pedal position. The response of a given swell box can be measured, though it is a difficult and tedious process. This is partly because it has to be measured several times, in each case using a slightly different shutter setting which itself has to be measured in some way (e.g. as swell pedal position). The easiest way to generate the necessary wide-bandwidth sound source inside the box is to play a loud chord at each shutter position and record the result, using as much of the key compass as possible by employing the suboctave and superoctave couplers if available. The object is to capture the extremes of the compass as well as the mid-range in the recording and thus maximise the frequency coverage. It is more important to include the lowest notes than the highest because harmonic frequencies will generally contribute enough acoustic power in the upper frequency region even if pipes at the corresponding pitches are not actually speaking. A chord in the key of C is usually best since almost all keyboards start at C and many will also end there. Exactly the same stops and the same notes must be used each time for a particular swell box, and the microphone must not be moved between takes. Each recording is then subjected to a spectrum analysis and compared with the spectra representing other shutter positions. Repeating this for other swell boxes expands the data set which helps trends and generic features to be identified.
An example of the outcome of such experiments is shown in Figure 2, which shows representative frequency responses in the form of spectral envelopes at three shutter positions - fully closed, fully open and an intermediate position. The curves show averaged results, and they were also heavily smoothed using trendline fits to remove the 'ragged' nature of the raw spectra. Since individual swell boxes vary one from another, the averaged data here reflect generic features rather than those of a particular box. The 'fully open' curve is simply a horizontal line to which the other curves were referenced, but this does not mean, of course, that an open swell box has no effect on the sounds. It is impossible to determine what this effect would be for any given box without physically removing it, but nor does it matter because in practice we never hear the pipework in this totally unenclosed state. It is only possible and necessary to assess the effect of a swell box in relative terms by referencing it to the fully open position when processing the data from other shutter settings. Hence the 'fully open' curve is plotted as a horizontal line here.
Figure 2. Three generic swell box frequency response curves relative to 'fully open'
At the other extreme, the 'fully closed' curve shows some of the features previously discussed qualitatively. Specifically, the attenuation at the lowest frequencies is hardly affected by closing the box because in this region the curves are almost flat. However this behaviour soon changes to eventually become linear against the higher frequencies when plotted on the doubly-logarithmic axes used here. To someone like me accustomed to dealing with filters in electronic circuits there was a temptation to reflect aspects of their characteristics in these curves. In particular, I fitted the results to the frequency response of a filter network which seemed to best describe the data. This enabled a 'corner frequency' (fc in the figure) to be identified at which the response was 3 decibels down on its maximum value, a common procedure in electronics. This frequency turned out to be about 160 Hz, a pitch which lies between D# and E in the tenor octave below middle C on an 8 foot stop. Although an average figure which will not necessarily apply to any particular swell box, it is useful as a working approximation. It means that the lowest frequency spanned by the curves in Figure 2 corresponds to about bottom C on a 32 foot stop (0.1fc), and the highest to the top note or so on a 2 foot stop (50fc). This covers virtually the entire pitch range of the pipe organ.
The 'intermediate' curve is interesting in that the amplitudes of the highest frequencies flatten out rather than continue to fall away as for the 'fully closed' condition. I am unsure why this is so, but the reason might be that the higher-pitched ranks (smaller pipes) are often planted closest to the swell shutters. This might imply that their sounds suffer least attenuation when the shutters begin to open. The mere fact that one can easily see such pipes behind the shutters might lend support to this - since they can be seen, there is obviously a relatively unobstructed line of sight to their radiating apertures (tops and mouths) along which sound can propagate directly out of the box without it having to bounce around and lose energy inside the box first.
A more complete set of curves including additional shutter positions is at Figure 3. Here a parameter k is used to identify each position, with k=0 meaning 'fully closed' and k=1 meaning 'fully open'. The k-values can be thought of as indicating the rotational angle of each swell shutter between its closed and open limits, or the position of the swell pedal itself. The incremental changes of shutter position between each pair of k-values are equal. From this family of curves one can see that the sound level jumps abruptly as the shutters move by a small amount from k=0 (fully closed) to k=0.1 (one tenth of their total movement). The effect is most pronounced at the highest frequencies, whereas towards the lower ones there is progressively less change. One can also see that the size of successive jumps at higher values of k gradually reduces as the shutters open further. This behaviour explains why swell boxes are most sensitive near to their fully closed position.
Figure 3. Generic swell box frequency response curves for many shutter positions (relative to 'fully open')
Three criteria were suggested earlier which might characterise a 'good' swell box. They are repeated below and will now be reviewed against the further analysis just given:
1. The overall attenuation of the sound when a swell box is closed should not be too much or too little.
2. There should be some frequency-dependent treble cut effect related to swell pedal position, rather than a simple volume control.
These will be discussed together since the subsequent analysis showed that they are interdependent.
It has been mentioned several times that the effect of a swell box is not simply a volume control which attenuates the sound equally at all frequencies. Instead, it imposes a different type of loudness variation in which the various frequencies experience different attenuations. The curves in Figure 3 suggest that the effect can be pronounced when a box is fully closed. The sound pressure levels of the lowest frequencies (around 30 Hz, the fundamental of a 16 foot pipe) are affected
less strongly than that of the highest ones (around 8 kHz, the top note of a 2 foot pipe)
which might vary by some 35 dB (a factor of 56). This disparity lessens as the swell shutters approach the fully open position.
Figure 3 demonstrates the sensitivity effect clearly. The curves show how the sound pressure level increases abruptly as the shutters open from the fully closed position by a small amount. This is most pronounced at the highest frequencies, whereas towards the lower ones there is progressively less change. Moreover, the size of successive jumps gradually reduces as the shutters open further. Whether this behaviour is advantageous or not is a matter of opinion. At least some organists like and expect it, perhaps through habituation from having lived with it all their lives. It is likely that the effect occurs particularly with swell boxes whose shutters close tightly to form an effectively soundproof joint towards the higher frequencies. In that situation residual sound leakage occurs mainly at the lower frequencies from the walls of the box itself which vibrate and re-radiate sound into the room.
All these remarks apply to wooden swell boxes rather than to those made of materials such as bricks or blocks. Although these can offer the dubious virtue of almost complete inaudibility when the shutters are closed, they do not exhibit the same frequency characteristics as those discussed above. By attenuating all frequencies in much the same way, they tend to over-control the volume, and in a less characterful manner.
1. The swell box shown in Figure 1 was made by J J Walker and illustrated in 'The Organ' by W G Alcock, Novello, 1913.
2. "Swell boxes and swell pedals - Part 2: Mechanisms", C E Pykett, 2023, an article on this website.
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