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PostPosted: Sun Jan 20, 2019 8:26 am 
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It's my impression that some guitars are super-sensitive to changes in humidity, with the action going up and down drastically as the humidity changes. Other guitars seem hardly affected at all. Why is that? Is it the particular kind of wood, or the particular combination of different kinds of woods, or the way the guitar is built (e.g. radii and bracing), or what?

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PostPosted: Sun Jan 20, 2019 12:30 pm 
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Joined: Fri Mar 03, 2006 7:09 pm
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Location: Hegins, Pa
great question and there are a few reasons

The first reason is that the RH differential of assemble to current conditions. If you braced the plates at say 55% rh and it is now 35% you have a 20% swing.Wood is able to expand and contract at some incredible rates. Some of the conifers will expand as much as 1/4 in a foot. So lets assume for this explanation that you were neutral at assembly and with the 20% swing the expansion was about 1/16 of an inch.
If you were not tied to bracing this would be not to much of an issue but since we are , the braces will not let the wood just expand , the plate will contract on the braces and pull the plate down. In extreme cases I have seen tops and backs pull the braces off and crack. To see how much that can change put a piece of wood between a clamp and tighten the clamp that 1/16 in you may be surprised how much that can bubble a top.


Another reason is the wood and how well it was dried and cured. Generally the older the wood the more stable. Also what is the sudden cause of the RH change ? Slow steady changes will have less effect that sudden shocking changes.

So when you brace a top or back work the RH down about 5 to 10% say 45% to glue up braces. The tops can expand with less damage than they can contract.

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PostPosted: Mon Jan 21, 2019 12:24 am 
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Joined: Sun Jun 25, 2017 3:58 pm
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Location: St. Louis area
Another note on humidity and tuning: an acoustic guitar neck is basically a hygrometer, two dissimilar woods laminated together. When the humidity changes, the hygrometer moves, like a pointer, and your guitar neck is the pointer. The strings will stay in tune relative to one another, but the entire pitch will change with humidity- guaranteed- when the neck bow changes.

Add to this that an unfinished fingerboard will absorb and donate moisture to the air at a different rate than the sealed/ finished neck. A builder buddy of mine posted a pic on FB of a hygrometer he made for his shop. He made it from 2 strips of scrap wood, mahogany neck scraps and rosewood from a finger board. He simply lamated the strips together. His hygrometer was about 20" long (surprise). It would move back and forth several inches, like a pointer, when the humidity in his shop changed. I was thinking about modeling the wood movement of my guitar, plotting humidity against the change of pitch in my guitar when it dawned on me, the neck indeed is a hygrometer, just like the one Bryan made for his shop.

I once built a plywood work bench along the wall of my basement, sandwiching the back edge against the wall between two furring strips. I attached 3x3 legs to the front and varnished the top. One corner of that plywood curled and lifted the corner almost 4 inches over several days, leg post and all, because I had sealed the top but not the bottom. That was 3/4" plywood. So based on that experience, water absorption rate difference between sealed and non sealed wood can cause it to curl up like a potato chip. Same thing goes with different woods, laminated together, such as a guitar neck ( compounded by the sealed neck and unsealed fingerboard). That my TCW...

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PostPosted: Mon Jan 21, 2019 5:27 am 
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Joined: Wed Aug 13, 2014 12:14 pm
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There hydrometer analogy is excellent.

So it should be be possible, by choosing appropriate woods, to build a guitar less susceptible to changes in humidity.
Is there a chart someplace that tells how different woods respond to moisture?

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When things are bad, try not to make them any worse, because it is quite likely they are bad enough already. - French Foreign Legion


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PostPosted: Mon Jan 21, 2019 7:34 am 
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Joined: Fri Mar 03, 2006 7:09 pm
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Location: Hegins, Pa
The neck is glued with the grain and not affected like the plates. Measure a neck one and you will see the movement is so slight the plates are the major mover. In 20 years I have not seen a neck move to the point of causing a big problem. That is because the cross grain hygrometer effect is on the top. There the wood is glued across the grain.
the neck also has support of the truss rod and the frets. I am not saying the neck won't move but seldom enough to cause any real notice.
also even though you have 2 woods glued together in a neck they are glued with the wood grain in the same direction so the expansion differential
is not going to have much effect

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Blues Creek Guitars Inc
Authorized CF Martin Repair Center
Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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