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Author Topic: Bellows pressure and tuning tables  (Read 715 times)

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Rog

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Bellows pressure and tuning tables
« on: October 04, 2019, 07:21:09 AM »

I’m sure this has been covered before...
Can someone explain or link to the relevant thread that explains the physics/reason why tuning table readings are different from readings of reeds in situ in the instrument?
My foot pedal operated table uses some full sized accordion bellows. It tunes reeds flat compared to when they go into the box (does this vary depending on the table or is this normal?). Bass reeds 10 cents, decreasing as the reeds get smaller, to about 5 cents around A4. This also varies depending on the instrument. I’ve seen discussions on here about manometers, for measuring pressure (which are cheap on eBay as it happens). If I wanted to alter the pressure on my tuning (peddle operated) table what’s the best way to do this?  Until recently it was pretty leaky and now that I’ve fixed the leaks the offset seems to have increased, so maybe I need a valve of some sort to alter the pressure?
For tuning unvalved reeds I have a flat wooden frame with slots cut (and padded with foam). I’ve found that shifting the slot to alter the air aperture below alters the pitch, so it’s possible to use a reference reed to position the slot, but it’s an inexact approach.
Some old pre electronic tuning tables had a set of reference reeds built in, and tuning was a matter of matching reeds to the reference reeds. I’m guessing these reeds were also flat to allow for sharpening?
Finally, a while ago I watched Paul Flannery use an air table (powered with a silent but v expensive air pump) and he tuned by comparing the pitch to a reference pitch (a bit like the old style tuning, rather than by looking at a reading on a tuner). I’m guessing he must have found a way to calibrate his tuning table, which is what I’m after really. All tuning tables give different readings and every time I make a new one (I have three at the moment) I have to calibrate it.

Theo

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Re: Bellows pressure and tuning tables
« Reply #1 on: October 04, 2019, 08:40:41 AM »

A tuning table is only a first step in the tuning process. Final tuning should be done in the instrument if you want a good result.  There are two approaches that work well.  In both the first step is to check the pitch with the blocks in the instrument, then either adjust the pitch in situ, or record the  pitch offset from the target pitch, then remove the reed block and shift the pitch by the amount of the offset.
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Rog

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Re: Bellows pressure and tuning tables
« Reply #2 on: October 04, 2019, 08:56:54 AM »

Yes, I agree, it’s the only way... esp piccolo reeds on an octave accordion. Interesting though that using this rig to tune ends of a melodeon give me same readings as when the box is assembled (it’s a small scale version of the one used in the Bugari factory).
https://photos.app.goo.gl/ocrYi5cdosxYYTRk8
...which leaves the physics of why readings outside are different... ;)
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