Official Luthiers Forum!

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You can't accurately measure moisture content of a soundboard.
Moisture meters do not work on thin timber.
So the procedure is to measure and adjust the Relative Humidity of the environment you are building in and allow the soundboard to acclimatise there.
Most folks aim for 45%RH
The meter you use MUST be accurate.

Yeah, it's unfortunate because the moisture content is what you really need to know.

You should have a standard procedure of either taking the wood from high humidity and letting it acclimate down to the control level, or dry it out and then acclimate up. The moisture content will be different depending on which way you go. I'd recommend drying it out and acclimating up.

40-45% RH is the general consensus, but I say go lower on cedar and redwood since their expansion rates are lower (wood-database.com lists humidity expansion rates for many woods). More like 25-30% RH when gluing braces. Gives that much more low range tolerance, without sacrificing high range tolerance since they only swell up about 2/3 as much as spruce from a given humidity increase. That is, spruce braced in 40% and then exposed to 80% should swell up about as much as redwood braced in 30% and exposed to 90%.

thanks Dennis ,...Do you think douglas fir tops would fall in the cedar and redwood specs ? I have some really nice tight grain pieces .

No, its expansion is slightly higher than spruce. And on that note, I should clarify that I was talking about western redcedar... Port Orford cedar (which is actually a kind of cypress) and even cedar of lebanon (which is a true cedar) are similar to spruce.

Alaskan yellow cedar (another cypress) is lower, though, plus it's less brittle than western redcedar, so anywhere from 30-45% would probably be good bracing RH for it.

Also, I need to go correct the numbers in my previous post... redwood and WRC are both 2.4% radial expansion (which is the one that matters for perfectly quartersawn wood), so actually closer to a 2/3 ratio with most of the spruces, which are 3.8% (European, red and engelmann). Sitka is higher at 4.3% but still not quite double 2.4%.

The typical 40-45% bracing RH is a good compromise if you want to use the same moisture content for everything. It's mainly just a few brittle, low expansion woods that really benefit from dryer bracing. Redwood, western redcedar, Indian rosewood, Brazilian rosewood, cocobolo. But they do need to be quartersawn, or you might have problems in high humidity from the higher swelling rate of flatsawn wood (usually about 1.5 to 2x as high).

I still need to do more research on high humidity limits to see what is the highest expansion rate that can safely go from 30% bracing RH to 90% environments. I'm fairly sure that anything below 3% can do it, but I'm not sure about 4% and higher. Using wider braces (more glue area) also may increase high humidity tolerance.

I think extrapolating too much using the shrinkage numbers listed is a bit problematic, if I correctly understand what the numbers mean and how wood moves once dried [which I very well may not]). My understanding is that those shrinkage numbers represent the movement (in both directions) of a timber from green to oven dry. Once a piece of wood has been dried, it will take on moisture and expand but not in the same way or to the same extent that it was when it was alive (green). Once a board is either kiln dried or air dried to, say 8% moisture content, it will never take on as much water as it had when it was cut even in a 99% humidity room. I THINK this is because it has lost moisture from inside the cells and between the fibers, but once the water is removed from inside the cells it doesn't want to go back in (I could have this all wrong). With that in mind, it is a leap of faith to say that two very different types of wood once properly dried will expand and contract in the ratios that the numbers provide indicate. By that I mean, one wood might move a lot from green to even dry but be much more reluctant to swell with RH swings, and another might shrink the same amount when drying but also like to swell and shrink with RH.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Good point. A more accurate statistic would be seeing how much it swells when going from oven dry to 95%RH or so. Could try to get all the way to 100%, but I'd worry about inaccuracy from going a bit over 100% and waterlogging the test pieces to different levels.

It's also possible that there's variation from piece to piece just like with density/Young's modulus/etc., even in perfectly straight quartersawn grain.

But for the most part, my experience is that the numbers are accurate to the way various woods behave.

They provide those numbers as reference so you have to assume there is value to them. I just don't know enough about the difference between movement during drying and movement after the wood is dried. I've hear a lot of stuff like wood X is tough to drive because it moves a lot but once properly dried. . . Things like that. This is, of course, all anecdotal. I bring ugly it up here because I have often wondered about this but never asked. Maybe someone here will have the facts.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.