No science, just guess

[Fernando Esteves]

Hey folks, how are you?
On your own builds, specially acoustics, how much do you put in the planning the species of wood you will use, sound wise?
How much % of the guitar tone you think comes from the back and how much do you stress picking the species?

And soundboard, do you separate the subspecies of Spruce for tone or go for what is easier accessible to you?

[Barry Daniels]

Personally I like resonant woods for all parts. I want the wood to ring when I wrap my buckle on a node. So I tend to go with some type of rosewood or mahogany back & sides, and old spruce for the top (high stiffness / low weight). Any subdivision further makes little difference sonically, more a cosmetic issue. How you build with it is where the magic happens.

[Alan Carruth]

I feel that most of the sound is in the top, and what the B&S do is mostly related to density and stiffness, with, possibly, some influence from damping.

All woods vary a lot within species, which means there's a lot of overlap between species. Some mahogany is soft and low in density, with high damping, while other pieces can be almost like a rosewood. You have to go by the piece more than the species.

The same holds for top woods, but more so, since that's where the sound is. So far the closest I can come to a categorical statement is that redwood and WRC have lower damping than spruce, but even that is not always the case. I got some 'salvaged' redwood a few years back that had a tap tone like cardboard, and long-grain stiffness that was about half what it 'should' have been for a softwood of that density. You see this once in a while, and it's never a good thing.

IMO, spruce is spruce. Once you control for density and degree of quarter (how perpendicular the ring lines are to the surface, as seen on the end grain) it all works the same way so far as I can tell. I save the low density stuff for Classicals, in general; it makes a lighter top, and you need that with nylon strings.

Damping can be a can of worms. In theory it's fairly easy to measure, up to a point, but there are a couple of flies in the ointment. One has to do with the way you check it. The easy way is by simply holding it up and tapping, and usually that gets you a result that is 'close enough' without the need for other measurements. However, sometimes you'll get a piece that's lying to you.

A well defined tap tone pitch and long 'ring' are signs of low damping. When you tap a piece you're hearing all of the resonances that are active at the place where you're tapping, and not active at the place you're holding. Every piece of wood has lots of these resonances, and the pitches are determined by the size and shape of the piece and the wood properties. With so many possibilities it can happen sometimes that you'll get two different modes at the same pitch. If one of them is stationary at your holding point, and the other one wants to move some there, the second one will be highly damped. Since they're both at the same pitch they usually can swap energy easily, and you can end up with a 'dead' tap tone when the damping of the wood is not high at all. Generally altering the geometry of the piece, say, by cutting off some 'excess' length or width, can shift the relative pitches, and give a clearer picture, and tap tone. You can't always do that, of course.

The other issue is that we talk about 'the' damping of a piece of wood. It's not always so well defined. In particular, the damping factor of wood can vary with the frequency, and it's at least plausible that the damping could be low at the low pitch of a tap tone you can hear, but much higher in the the high range where (maybe) it matters more. The problem here is that it's hard to get readings over a wide frequency range from one piece of wood. Not only does that take time, but you also need to use samples of a lot of different sizes. Cutting into twenty pieces to get lots of readings makes it hard to build an instrument from the wood, and then there's the time it took...

The bottom line is that with time and experience you may be able to learn how to 'read' wood quite well by tapping and flexing it. Dr. Karl Roy, the long time head of the German state violin making school at Mittenwald, could rank wood samples by stiffness to within 3% by 'feel': about as close as most 'scientific' testing can get you. By that time he'd probably made hundreds of violins himself, and taught hundreds of students. Other tests, I'm told, show that very few people are nearly that good, let alone as good as they think they are.

That's why I use 'technical' measurements.