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"Soft Bass"


Paul_H
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Whilst perusing NPOR  I came across this oddity:  https://www.npor.org.uk/NPORView.html?RI=P00147 
 

The Pedal features a 16' Bourdon and 16' Soft Bass; the notes say that the Soft Bass uses "same pipes as the Bourdon, but different wind pressure applied, supplied from a small auxiliary set of bellows"

 

Usually, reducing the wind pressure like this would affect the tuning so presumably they found a way to overcome it.  Anyone got experience of this?  Or come across it elsewhere?

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This is well known in Germany, as a crafty way of "extension" I suppose, although I don't know how common it is. The term used is windabschwächung, literally "wind weakening", and is done simply as you suggest, alternatively by two valves with different apertures. Presumably there is no need to play a bourdon and an echo bourdon at the same time, so nothing is lost. 

It probably does affect the tuning, but not much, as I have played with the wind pressures on the bourdon on my own house organ without needing to adjust the tuning.

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Yes I've half pulled bourdons before with effect. I first came across the situation on a gt to ped coupler which frequently slipped and when passing through gt 16 as a bass. I also knew another place where general cancel couldn't always raise enough energy to do a full job and experimented with part pressures for fun from time to time. 16 stopped was the only useful one to take public though. It is something I look to see if i can find in a mechanical stop draw where I can.

More quint and longer to settle on the slightly weaker note would describe the sound. Not anything like as useful as the proper pressure but sometimes can give a variety.

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My experiments were with the wind supply at the pipe feet. I have a single 16' bourdon, and the wind supply to each pipe is regulated by a simple sheet of wood with a hole the same diameter as the pipe foot, which can be slid in and out to reduce the aperture and with it, within certain limits, the volume. It's direct electric action, so half-pulling the stop isn't possible. I have done that, accidentally, on manual stops, but this really does affect the tuning. You might get a celeste effect over part of the manual compass doing this, but it's unreliable and only for fun when you're sure nobody else is listening.

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Changing the wind pressure will alter the tuning of a flue pipe, by an amount depending on the pitch of the note involved.  Typically a flue pipe will continue to speak over a range of wind pressure, going flat with decreasing pressure until it ceases to speak at all, and going sharp with increasing pressure until it flies to the octave (for an open pipe) or the 12th (for a stopped pipe).  Allowable frequency deviations are typically around plus or minus 1.5% from a nominal 'in tune' figure before the timbre as well as tuning start to suffer badly, though pipes will often continue to speak in some fashion beyond this range.

So for a pipe at 16 foot bottom C (33 Hz in round figures, assuming the organ is tuned to A440) the maximum permissible frequency variation in either direction before the pipe failed to speak properly would be about 0.5 Hz, resulting in up to one beat every two seconds with another 16 foot stop.  This is pretty slow and would probably be scarcely noticeable most of the time unless you were alert to the issue and listening for it.  But at middle C (262 Hz) the frequency variation might be up to about 4 Hz.  A pipe beating at 4 Hz with other 8 foot ranks would be very noticeable indeed and deemed to be unacceptably out of tune.  Things would get progressively worse going further up the keyboard and especially with higher pitched stops.

Therefore these figures confirm that you can perhaps get away with softening a bass pipe by reducing the pressure, whereas higher up the compass this would not be the case.

Hope-Jones experimented with this method of trying to get more than one power from a single pipe.  He tried applying it to his pedal organ Diaphones, and in the case of his Worcester cathedral organ (1896) there were supposed to be 'Diaphones at two powers'.  I think the selection was intended to be made depending on how far down you pressed the respective stop tab, which had a tactile detent which could be felt halfway down its travel according to one of his patents.  However to the best of my knowledge he never got it to work in the actual Worcester instrument, or it may not have been installed in the first place.

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