Musings on humidity, wood and "duds" from a humidity geek

[Alan Carruth]

My violin making teacher used to say that there were two kinds of violinists: the ones who cussed makers in the summer and those who cussed them in the winter. Basically, your instrument is probably going to be happiest at the R.H. it was built at, or somewhere close to that. Other times of the year it might not be so good.

My understanding is that wood changes little between about 40-60% R.H., and much more outside of that range, although I don't have data on that myself. Since it's usually less of a problem to go down in R.H. on a completed instrument, rather than up, most of the makers who pay attention keep their shops in the low side of that range.

Woods vary a lot in the way they react to changes in R.H. Some woods, like Indian rosewood and mahogany, seem to be very stable, while others, like BRW, are not. This varies from piece to piece, and the cut matters a lot as well, with flat cut wood being very prone to warping and checking, while quartered wood is much more stable. I once talked with a fellow who was a wood buyer for Steinway, and his biggest issue was finding wide, quartered pieces of poplar for lids on grand pianos. If the wood is not perfectly quartered it will warp, and with that black mirror finish it really shows.

We run into problems because we glue different types of wood together; like ebony fingerboards on mahogany necks. Even if they were both reasonably stable by themselves, they're different, and that sets up stress along the glue line. We also glue cross grain pieces, like bridges and braces, onto wide tops and backs, which is a great way to get things to move around. Cabinet makers get very uncomfortable when they look at guitar designs, since they're taught to avoid that sort of thing. Given the stresses we have to deal with, and the fact that we have to build light to get sound, there's not much we can do about those 'bad' features, but you do have to accept that they contribute to instability.

It all gets very complicated. All wood will have some built-in stress just because of the way the tree grew. Over time, and with proper seasoning, this decreases, but slowly. Old wood is also less capable of taking up water from the air, due to the breakdown of the hemicellulose 'filler' that is the part of the structure that absorbs moisture. Again, this is very slow: about 1% per decade was the figure I've seen. Paradoxically, the typical careful luthier's shop is not a good place to season wood; seasoning involves lots of changes in moisture and temperature, which helps release stress and break down hemicellulose. Holding wood as 45% R.H. for years probably just gets it used to that humidity level. Sometimes it's hard to win.

[SongwriterFan]

My guitars definitely sound better near 40% RH than they do near 50% RH.

I try to keep my RH between 40% and 45%. A well-designed and appropriately sized (with variable speed) A/C or heat pump system is required to do that in the summer months (in Texas). A whole-house humidifier is the best way to accomplish it in winter (though a room humidifier will also work).

[zmf]

Originally Posted by Alan Carruth

We run into problems because we glue different types of wood together; like ebony fingerboards on mahogany necks. Even if they were both reasonably stable by themselves, they're different, and that sets up stress along the glue line. We also glue cross grain pieces, like bridges and braces, onto wide tops and backs, which is a great way to get things to move around. Cabinet makers get very uncomfortable when they look at guitar designs, since they're taught to avoid that sort of thing. Given the stresses we have to deal with, and the fact that we have to build light to get sound, there's not much we can do about those 'bad' features, but you do have to accept that they contribute to instability.

Thanks. Very interesting perspective. Living high in the Rockies, I'm now surprised I've managed to keep my guitars in good shape over the years.

I'm guessing there's no way I could acclimate them to an average 25% humidity.

[varve]

The question of optimal humidity does have a reasonably simple answer, imho.

Wood, maintained at a given relative humidity, will absorb moisture until it has a certain "equilibrium moisture content". In Spruce that has been pretty well-established ( see Bruce Hoadley's "Understanding Wood" for a great chapter on this. EMC is basically the percentage of weight represented by water). For example, relative humidity 45% will give rise to an EMC of 8% in non-torrefied spruce. 35% RH will give rise to 6% EMC, and so on.

So what? Well, a group of Tonewood experts working at Kyoto University in the 90s did some excellent work on acoustic properties of Spruce at different EMC's (ref, Obataya et al 1998). They found that stiffness is high, and acoustic damping reached a minimum, at about 6% EMC in Spruce. Above 8% EMC, stiffness takes a nosedive, and both density , and damping of the wood increases. So, that's the biggest reason why guitars sound so different at different relative humidities. And why they sound better on the dry side - hi stiffness, low density, and low damping are generally considered favorable characteristics in a soundboard. Too wet, and they sound like damp cardboard.

There's a lot of talk about torrefaction nowadays, and one of the things that torrefaction does is lower the EMC at a given relative humidity. So, take two guitars sitting at 45% relative humidity, let's say side-by-side in your living room, The non-torrefied one will have an EMC of 8 %, torrefied one perhaps five or 6%. All other things being equal, the torrefied top will have a bit more zip. There may be other things going on with torrefaction, but this simple fact may be the biggest advantage of the process.

[saxonblue]

I never was aware of humidity issues with guitars until joining this forum 3 years ago, then I began to worry.

So I bought a digital weatherstation with it's own inbuilt sensor & a remote one sitting outside the kitchen window. I've observed over that time the RH hovers between 45% - 55% for about 90% of the time year round so I don't worry anymore. Maybe I'm just in a fortunate location in that regard.

What is it now? 36%!!!! That's the lowest I've seen for ages! Better break the Martin out, got new strings on it too, it's gonna sound awesome. :-)

[fazool]

For the past week, I have slowly lowered my RH from 45-50% down to 35-40%.

I made one small truss rod adjustment and am letting things stabilize at their preferred RH%.

I already see a marked improvement. My action dropped 1/64 before the relief adjustment and now things are back to my preferred action and tone.

[fazool]

Here is a nice diagram from Gibson. I definitely drifted upward toward the top picture and am, now, slowly heading back toward normal.

[billder99]

Originally Posted by Fred Watson

... I was intrigued by your comment "Buy an all carbon fiber guitar and then don't worry, be happy."
Do carbon fiber guitars sound as good as wooden ones? I have never heard of an all carbon fiber guitar before.

Fred,

Spend a bit of time reading posts on the CF Forum here on AGF. Many of us CF owners are also wood guitar owners, and we love both for different reasons. But to answer your question, yes, a good CF guitar sounds surprisingly good... I liken the tone to really crisp, quality maple/spruce guitars... crystalline clarity, perfect balance.

[fazool]

I'v ebeen running 35% all week and just picked up my guitar and it was amazing (again!)

I'm so pleased its back to the way I love it.


I know that's a bit low for normal guitar RH% but after several years it seems optimal for this one.