In the sound of a stopped pipe less harmonics are present than in the spectrum of a Principal. If a pipe is stopped at the open end, its pitch drops to half and the equally harmonics disappear from the spectrum.
The odd harmonics make the sound hollow. In a Holpijp = stopped pipe, the characteristic of the sound is more difficult to recognize than in a Principal. The scale allows a large dispersion in which the sound can move between around and clear with a ground tone that can be more or less strong. In this wide area, many sounds that bear the name Holpijp fit.
Especially for small organs it is tempting to choose a small scale, these pipes take up less space. The sound becomes more overtones with a small scale, but here it is undesirable and that is why we try to compensate for this with a higher cut up. The ground tone of a small pipe, however, has too little energy to develop enough weight and the ratio towards the harmonics does not give the sound a beautiful character.
It is also a good idea to choose a rather wide scale - but not too wide. A Stopped Diapason has relatively fewer overtones, but a nice sound needs a certain range of overtones. In stopped pipes there are odd harmonics, of which the third and fifth should be audible. If they are not audible, the stop will sound like a dull flute and will lack the characteristic features of a well sounding Stopped Diapason. I have spent a lot of time searching for the best scale for a Principal. In that stop the variations in tone are more clearly expressed. It is much more difficult to vary the tone of a Stopped Diapason, but nevertheless it is just as important to make the right choice, especially in a small organ where the Stopped Diapason is the most important stop or perhaps the only stop.
To judge if a pipe scale is too wide or too narrow you should have several pipes of different diameters for every pitch. I placed four complete Stopped Diapason ranks on a sound board. In the first place I found it important that every rank is uniformly voiced and also that it is balanced between bass and treble, as well as with the middle. If I thought that the sound of a particular pipe was not right, I could test it by playing a pipe of the same pitch in another rank.
Stopped Diapasons have no formant. The overtones are relatively weak and their number is limited. Thus is it more difficult to observe the influence of the overtones on the sound, which is why Stopped Diapasons can have a wide range of scales. Still, it is important to be critical, because little differences can influence the musical character of the sound. It is not sufficient to hear just a sound, but the organ builder must aim for a refined sound, refined in the balance between tonic and overtones.
It is very important that the cut-up is not too high; rather, start with too low a cut-up and increase it while you are voicing towards the intended sound. Too much tonic or too little quint-tierce sound makes the sound dull. An experienced voicer will observe that the useful range in scales is limited. Also important is the way the pipe starts speaking; the overtones must be audible at first, without chiff or spit. That also depends on the way the key is played, a sensitive tracker action in an organ makes it possible.
Of course, not everyone has several ranks of pipes of the same stop at one's disposal, but there is another method. Start with making pipes around c1 and work towards the bass and treble. Imagine in your mind’s ear, before touching a key, what sound you want to hear. If the sound tends to deviate from the sound concept, you can choose a scale for the next pipe which is a bit wider or perhaps smaller to see if it gives a sound that is a better match. In this way I discovered the most suitable scale for several stops.
