Swell Boxes

[Choir Man]

Damian Beasley raised a very interesting question in another thread that first got me thinking and then, having over-thought, very confused.

Quote

With a swell division, there is therefore quite a volume of wind passing through. As it is often said that a nice tight swell is desirable, my simple question is - when it's shut, where does all the wind go?

How tightly do swell shutters close and, when closed, does this not restrict the flow of air out of the swell box, therefore raising the air pressure inside the box, which effectively reduces the pressure the pipes are speaking on which then leads to de-tuning?

DP

 

[Guest Geoff McMahon]

No swell box is ever anything near airtight, not remotely. A nice tight swell box merely refers to being acoustically tight, not air-tight!

 

John

[Damian Beasley-Suffolk]

Thanks. That puts much into context. Experto credo.

[pmb]

Audsley (Volume 2, p668), alluding to tightly-fitting swell shutters in a small box, recommends the provision of "a small trunk or conveyance from [the swell box] to some adjoining room" to avoid the effect described above. I believe, however, that Audsley is sometimes regarded as a dilettante, not an expert.

On the other hand, I believe I read an account of another organ where, owing to tightly-fitting shutters, the Swell division could not be played without opening the shutters a fraction. I wonder was it Maurice Forsyth-Grant's book? Or perhaps I am imagining it?

[Contrabombarde]

I seem to recall hearing about an organ that survived fire damage because the previous visiting organist had forgotten to leave the box open - there are advantages to having tightly fitting shutters! Can anyone remind me of the instrument in question?

On a related note (excuse the pun; the question relates to air pressure), if you play a note with no stops drawn, then release the note, the wind pressure in the channel below the pipes will equalise with that of the bellows. If you release the note and immediately draw a stop would the increased wind pressure directly under the pipe be sufficient for the pipe to speak transiently? On some organs I've seen what look like tiny holes drilled into the side of the windchest, would those be designed to equalise the air pressure, or would that lead to excessive wind leakage?

[Damian Beasley-Suffolk]

Windchests often have bleed holes for each channel, which should prevent this, as well as grooving to prevent ciphers and bleed unwanted air away. Mine just leaks, I think, but you don't notice the quiet hissing unless you're right next to it.

As for quick regulation, another story. As I mentioned in my original post, my house organ, which was conceived at the height of neo-baroquery, has a floating-base wind regulator built into the bottom of the windchest, with springs to maintain the desired pressure, fed by a separate blower. It's very compact, and very responsive. I have tried repeating notes as fast as I can to see if there are any transients, but practically speaking they are not noticeable.

The instrument also has a tremulant in the channel between the blower and the windchest, but it's completely useless. The regulator in the windchest acts so quickly that the only way I know that the tremulant is operational is that I can hear it chugging away and see the bottom of the windchest bouncing up and down in sympathy, because there is absolutely no effect at all on the pipe speech. In one sense, that's quite impressive. One can of course make a mechanical arrangement to inhibit the regulator's movement when the tremulant is active, but it really wasn't worth the effort and I don't really like tremulants which act on the whole instrument. Instead it was disconnected and the stop knob used to turn on a pedal bourdon which I added.

As for dilettantes - well, I know my place, and although there are many knowledgeable voices on this forum I'm sure our host chuckles from time to time as people like me  s l o w l y  turn enthusiasm into proper knowledge :-)

[John Robinson]

On 02/11/2017 at 19:45, pmb said:

Audsley (Volume 2, p668), alluding to tightly-fitting swell shutters in a small box, recommends the provision of "a small trunk or conveyance from [the swell box] to some adjoining room" to avoid the effect described above. I believe, however, that Audsley is sometimes regarded as a dilettante, not an expert.

Ha!  Exactly what I suggested!  I do have that volume, although I didn't read that passage - honest!

[Contrabombarde]

As an addendum to the Buckfast thread, I was minded to look at Ruffati's website which explains research they have done to create a much more effective "hyperdynamic" swell box with much better seals between the shutters. It also mentions a device in the box which somehow reduces the volume further when the box is shut, I wonder what that might be?

[John Robinson]

9 minutes ago, Contrabombarde said:

As an addendum to the Buckfast thread, I was minded to look at Ruffati's website which explains research they have done to create a much more effective "hyperdynamic" swell box with much better seals between the shutters. It also mentions a device in the box which somehow reduces the volume further when the box is shut, I wonder what that might be?

I don't know whether this has been done before, but could it be sound absorbent material on the backs of the shutters?  Of course, even when fully open that must still have some sound attenuating property.