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Wind supply: treble or bass?


Clarabella8

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I wonder if I might call upon the expertise of the readers of this forum to help with a quick question?

 

I have a small chamber organ with a single rank of 4' wooden stopped pipes. I would like to make a new stand for it to replace the very utilitarian and ugly base on which it currently sits, and to do this I would ideally like to move the position at which the wind from the blower enters the soundboard. At the moment it comes in from below at the treble end; it would be possible to change this so that it enters at the rear, but I am wondering if it makes any significant difference if it were to be at the treble or bass end? There is presently a slight change in volume and tone (though not pitch) in the treble pipes if I play a chord with a significant number of bass pipes in it. Would this effect be worsened if the wind entered at the bass end?

 

Your thoughts would be much appreciated!

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As most often, I have no answer, but would like to share the interest in it, as this question was risen regarding the organ I'm serving at. The fact, that the wind enters at the treble end at some soundboards was seen as an issue, as the wind consuming bass seemed to be under-supplied. As improvement improvement it was thought not to simply change the entry side, but to create an additional entry.

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There are at least some organ builders on this forum (!), so far be it from me to suggest what you might do. I am a mere physicist. However you do deserve some sort of reply to your valid and interesting question.

 

As an experimentalist at heart rather than a theoretician, and when there are so many imponderables at issue (*), as here, quite honestly the best thing to do is simply to try it in my opinion. That's what I generally do rather than continue to agonise for ever. I realise this will probably involve you having to cut into the chest to make a new entry for the wind, but this could be done quite neatly. If it doesn't work, you could seal off the opening again with a blanking flange screwed over it against a leather gasket. But if it does work, you will have to do the same for your current aperture, so either way there will be visible evidence of the intervention. So in that sense you will not have lost anything.

 

Best wishes with it!

 

CEP

 

* The imponderables mainly centre around the fluid dynamics which determine how the wind enters the pipe during the first 50 milliseconds or so of its speech. This can be affected considerably by quite minor changes to the winding system, such as those you are contemplating, and they are next to impossible to model theoretically. They can result, for example, in audible changes to the attack transient the pipes emit - and these changes can be for better or worse. One cannot really know until one tries it for real. Then there might be changes to the steady state phase of speech, as you have already observed.

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As it's a small one rank organ I wonder if the pipes are arranged chromatically from bass to treble, chromatically one whole tone apart so that the tallest pipes are either side and smallest in the middle (or vice versa) or largely chromatically with a few bass pipes at the treble side? I thought the whole point of arranging pipes a tone apart was to ensure a more equal distirbution of long and short pipes across the width of the soundboard to prevent wind robbing at the extremes. But portative organs and other very small instruments that I've seen tend to just arrange by length from left to right. Almost certainly I guess you wouldn't be able to rearrange the position of the pipes to find out if it makes any difference though.

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Many thanks for the replies. The pipes are arranged in two rows, with basses to the left and trebles to the right, each pipe a tone higher than its neighbour.

 

It looks as though I will have to try it and see what happens, as Colin suggests. I'm no engineer, as you will have gathered, but at the back of my mind was the thought that maybe the wind enters at the treble end in order to give the treble pipes priority, as it were, for the new air coming in from the blower, in order to reduce the impact of the wind-hungry bass pipes on the trebles' speech. But I guess that it's more complicated than that in reality! If there's a risk of mucking up carefully done voicing (and I imagine that on very low pressure that risk is greater than normal) I think it'll be safest to arrange things so that the new entry point is as close to the present one as possible.

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The size (and sectional shape) of the trunk is more often the cause of the problem, rather than position.

 

DW

Thanks for that – my non-builder imagination keeps grinding, though. Is this about finding an equilibrium with a trunk admitting enough wind at a sufficient pressure, or is it about the trunk being too narrow in some instances? What’s the difference between a rectangular and a circular cross-section?

 

Thanks in advance,

Friedrich

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Thanks for that – my non-builder imagination keeps grinding, though. Is this about finding an equilibrium with a trunk admitting enough wind at a sufficient pressure, or is it about the trunk being too narrow in some instances? What’s the difference between a rectangular and a circular cross-section?

 

Thanks in advance,

Friedrich

 

As far as I know, I believe that rectangular cross-section is preferable for most situations. It avoids causing a vortex at junctions.

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I have an instrument which is very similar to Clarabella8's - one rank of stopped wooden pipes at 4' pitch arranged in two rows. There is no bellows and the wind comes in at the treble end, so I assume that this is the optimum position. In such an instrument, I would guess that there might be a danger of the big pipes robbing wind from the trebles if the wind came in at the bass end.

 

Incidentally, I bought my organ from a traditionalist RC priest, the late Fr. Ronald de Poe Silk, in Cambridge, in 1985. I think it was made from a kit, but I have no idea who made it. It is quite a well-travelled instrument, having lived in Orkney, Carrickfergus and Belfast before being loaded into a container and transported to Newfoundland (via Liverpool, Rotterdam, New York and Halifax NS, apparently).

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