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Hey all,

I have been resawing through the beginnings of my endless stack of wood. While most is back/sides stock - I do have some soundboard stock. While I suppose I theoretically knew there was a large variation in soundboard wood densities - spruce, cedar, redwood - it struck me more as I saw through them. Some of the spruce I have (particularly some really dense Sitka) runs harder than the some of my mahogany - almost like maple.... Some is so soft that it feels more like styrofoam.... Same for redwood. Some is really heavy and hard, some is strikingly light and styrofoamy. I have cedar billets that literally weigh less than half what other cedar billets weigh of the same size.

I suppose it is more obvious as I pick up the big chunks- I can feel the large difference in weight.

But it makes me think more about why we have to choose wood according to what we are trying to build and how we are going to build it. A bluegrass machine built for live aggressive outdoor play needs a more dense top for the headroom. A small body couch/indoor guitar needs a far less dense top or it will sound way too stuffy, muffled, and boxy.

I gotta say - it's kind of fun going thru it and thinking about what this or that piece would be good for.....

Thanks
John

It's all part of the game.

...and why it is so important to treat each piece of wood individually.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

So then... Thinking logically about this.....
How do you organize your soundboard wood?

Do you guys store wood by "species" aka red spruce, lutz, European, cedar, redwood, etc...
Or by supplier? By "grade"?

Or.... By density then some mix of the others...

Me personally... I had been organizing by supplier into spruce/not spruce then by whether the top bad a lot of medullary rays. Bearclaw got separated because I have very little of it.

Thinking now... Would it make more sense to separate by density then other stuff secondary? The assumption here is that a heavy/hard spruce, cedar, or redwood top may be useful in one sort of place where the significantly lighter stuff is generally useful some place else.

Thanks
John

Hmm... very interesting. I have just been categorizing by species and then considering the individual properties as I start my next instrument. I'll be watching this thread to see what others do.

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George :-)

I've measured the properties of a couple of hundred pieces of top wood, more or less, covering all the 'usual suspects' plus some stuff that most folks haven't tried yet. Just the other day I went through my notes and picked out the extreme cases of density variation of the ones that I had more than a few samples of (the 'usual' stuff), just to get an idea of the ranges for different species. So:

Species Specific gravity range Young's modulus along the grain (MegaPascals)

European .342 -.54 8800- 19,000

Sitka .38 -.53 8900 - 17,000

Engelmann .35 - .5 8200 - 16,000

Red ('Adirondack') .36 - .49 9500 - 15,000

Western Red Cedar .31 - .4 6,000 - 11,000


I have not tested as much Red spruce as the others, and that may account in part for the more restricted range. Again; these are the outliers in the distribution. For pounds/cubic foot, multiply the SpG by 62.5. For grams/cubic centimeter, or Kg/m^3, just move the decimal place over, so a SpG of .3 becomes 300 kg/m^3.

When I go looking for a top for a new project I start with the density. The Young's modulus along the grain tracks that pretty closely for the most part. I build to a stiffness target, and since stiffness goes as the cube of thickness, a low density top will tend to end up lighter than one with high density. Thus, for a Classical guitar or a more 'responsive' fingerstyle steel string I'll look first at the lower density tops. When I want more 'headroom' I'll move toward the denser stuff. Also, since the Area/mass ratio of the top tends to be higher for smaller sizes, there's less of a weight penalty for using a dense top for a small guitar than for a big one. I look at stiffness ratio along/across the grain too, using the ones with high cross stiffness on the wider platforms, such as Jumbos.

So far as I can see from the properties, spruce is pretty much all spruce when you control for density.

So....
Do you organize wood by density/Young's modulus first or species/origin first?

Realistically - the market is what it is.... A buyer who wants Adirondack may not be likely to really want Engleman instead - even if the properties of the Adirondack you have is not a good match and the Engleman in hand perfectly "matches" what he really wants in a guitar..

My guess is that the market being what it is - this sort of discussion is akin to suggesting that the fellow really needs an Osage orange back and sides rather than BRW......

Thanks

Obviously, if the customer specifies a species, I'll go with that, choosing the set that best matches up with my take on the desired sound and so on. But yes, for the most part, I look first for wood that has the properties I want, irrespective of the species. As far as I'm concerned, once you've controlled for density, it's all spruce.

Do you mean to say that if you could magically swap out a guitar built with say .38 of one species with another species that was .38, it would sound more or less the same, and that what we consider the generalized tone differences between the species really comes down to species density averages?

meddlingfol asked:
"Do you mean to say that if you could magically swap out a guitar built with say .38 of one species with another species that was .38, it would sound more or less the same, and that what we consider the generalized tone differences between the species really comes down to species density averages?"

If you stick to spruce, and work to the same thickness (assuming you're talking about the .38 as SpG), I believe so, so far as the species I've tested go. Western Red Cedar and Redwood follow the same rule relating stiffness along the grain to density, but they tend to have markedly lower damping, which may account for some of the tonal differences.

Although there's a lot of overlap in the density and stiffness, each species usually tends to be found within a smaller range than the extremes would suggest. Engelmann is usually the lest dense spruce, followed by Euro, then Sitka and Red, in my experience. If you make all of your spruce tops the same thickness over a large number of guitars the tone should tend to average out to reflect the species averages in density and stiffness. Thus you'd expect the Engelmann topped guitars to have more bass and be more responsive, since they'd be lighter in weight and lower in stiffness than the others, while the Red spruce would tend to the other end; less responsive with more 'headroom'. Since production shops do tend to keep the thickness pretty much the same, that might account for the usual ranking in sound by species. If you vary the thickness according to density and stiffness the tone can be much more uniform across species. I would expect that in a blind test nobody would be able to pick the species out in that case. That would be a hard experiment to do, if only because of the number of guitars you'd have to make with really tight quality control.

What Alan said.

The logical extension to this, of course, is to measure your wood's actual material properties and build to modal resonance targets. Do that well and it's pretty nigh impossible to end up with a "failed" guitar (assuming you get the playability aspects right and that it doesn't fall apart).

The bit that is very hard to control (because, amongst other things, there's no real good way of measuring it) is the damping in the wood. There's a tendency to think of damping as a single number, but really it's a frequency response curve; i.e. damping varies with frequency and I haven't yet found a way to measure that in a comparative way that doesn't involve cutting the wood to the same geometry every time. The difference between a very, very good guitar and a truly outstanding guitar hides in that wood damping curve (the wood's sound spectrum absorption).

The same concepts apply to back woods, too, except you can't just follow a density line because density and stiffness in hardwoods don't correlate like they do in softwoods.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor Gore wrote:
"The difference between a very, very good guitar and a truly outstanding guitar hides in that wood damping curve"

That's an interesting hypothesis, but I'd think it would be hard to confirm.

It seems to me that this is one of those cases where wood properties confer opportunities and set limits, but those need to be realized by proper construction; they're not necessarily automatic. I've seen plenty of guitars made from great wood, and some by good makers, that just didn't have the sound, and we all know that a good maker can get at least decent results from most woods. I suspect that the damping of the wood sets a limit for Q value that can't be exceeded, but it's possible to end up with a structure that has much higher damping than that of the wood by simply screwing something up. It's plausible that structure matters less as you go up in frequency, of course: when the vibrating areas get to be similar in size to the brace spacing it could be that wood properties come to dominate.

Anyway, thanks for another thought provoking post.

It depends on who the customer or player is and what they expect. This is marketing and pseudo science mixed. Players expectations change and some are very good at hearing certain frequency or ranges of frequencies. And then there is the perception of sound based on looks. Then there is loudness, volume and other dynamic variations. There is really no one formula to success. This is because, unless you are making a specific type of instrument like an orchestra violin or a classical guitar for stage performance, then you can make variations that might be perceived as good. Loud is often confused as good. Loud to the player is not always loud to the audience, or in a mix, etc.

My formula for success has changed but now is:
Use materials that are normal for a specific type of instrument.
Specify the weight and major characteristics of the wood only.
Use marketing terms that players can put neatly into categories such as "old growth", Michigan maple, Colorado spruce, bear claw, rosewood, brazilian, amazon. Or even green terms like "domestic wood", or reclaimed.
Then I keep notes regarding the physical properties like:
Radiation ratio, density, stiffness, damping characteristics. But I never use these terms to the dealers, or direct buyers.
Damping is the most important characteristic within the normal range of density of my materials. And as Trevor said and Alan always refers to, damping is variable along the spectrum of frequency range.

And to those who record information for us to use, thank you! Especially Alan, and Trevor. I would love to see Alan's notes on alternative woods, as this is getting important for us to use in the future.

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Ken