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Author Topic: Reed Block Chambers  (Read 1627 times)

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HallelujahAl

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Reed Block Chambers
« on: July 17, 2009, 12:31:23 PM »

Excuse my ignorance - but am in the process of replacing the valves on the treble side ogf the heligonka and can't help noticing that the chambers under the the top couple of reeds (you know, the notes that JK described being the ones that only a dog hating lunatic would actually use with any frequency) are bevelled or chamfered within the reed block - well - shaped differently anyway. Can anyone tell me why this is?
Hope the pics tell it better than I do.
AL
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Pete Dunk

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Re: Reed Block Chambers
« Reply #1 on: July 17, 2009, 12:40:54 PM »

As far as I know this is done to keep the chamber air volume down to avoid damaging the smaller reeds. The chamfer is there to give the reeds room to swing without hitting the back of the chamber.

My thoughts anyway ...
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Theo

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Re: Reed Block Chambers
« Reply #2 on: July 17, 2009, 02:36:14 PM »

I don't know about damaging the reeds?    The high reeds respond quicker and sound louder with a reduced chamber volume.   If the block of wood that reduces the chamber volume is removable then it's easy to verify that for yourself by removing the block from one reed and comparing it with a similar pitch reed with the block still in place.  For similar reasons sometimes the highest couple of reeds on each treble block are reversed so the rivets are at the top. 

I only vaguely understand the acoustics of why this is, but in practice it does make a difference.
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HallelujahAl

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Re: Reed Block Chambers
« Reply #3 on: July 17, 2009, 03:39:05 PM »

Quote
If the block of wood that reduces the chamber volume is removable then it's easy to verify that for yourself by removing the block from one reed and comparing it with a similar pitch reed with the block still in place.  For similar reasons sometimes the highest couple of reeds on each treble block are reversed so the rivets are at the top. 

The chamber block doesn't appear to be removable. But yes - the smaller end reeds are reversed as well on both reed blocks in the heligonka.
AL
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LJC

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Re: Reed Block Chambers
« Reply #4 on: July 17, 2009, 05:39:54 PM »

There is a really good article somewhere online about this - apparently with the full chamber volume you can get some sort of dampening from the frequency of the air moving and the reed and the size of the space. The author goes into the physics of it which were somewhat above my head without doing the maths. Ill have a look for it and post it up if I can find it.

My understanding of the reversed reeds was to allow a better airflow over the tip allowing it to speak quicker.
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triskel

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Re: Reed Block Chambers
« Reply #5 on: July 17, 2009, 06:57:28 PM »

The highest and lowest reeds tend to be problematic because they are made to a scale length that is practical, rather than ideal - otherwise the high reeds would be too small to be manageable, and the low ones too long, necessitating an oversized instrument to accomodate them. Hence the high reeds are filed very thin towards the tip, to raise the pitch, whilst the low ones are thinner at the root, or even with weighted tips, to lower them.

But, as a result of this, the high reeds need special treatment, including less air pressure, and various "tricks" are used, to try to get them to speak well. This is why you'll find they don't have valves on them, and sometimes there are venting holes in the chamber walls too - usually through the wax - to lower the pressure on them.
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