Density of wood

[Guitar Tom]

What are the differences in high density and low density in Engelmann spruce tops. I'm putting a parts list for the Tinozza AB. Guitar Tom

[naccoachbob]

He's building an acoustic bass. Here's the thread:
viewtopic.php?f=29&t=582
I just happened to remember that poste a few weeks back or so.

[Guitar Tom]

I found some Engelmann spruce That will be big enough for the top plate, but the seller said that it is from a low density log. I guess the wood is softer. I tried to get a response from to elaborate, but he didn't replied. My main concern is will it be strong enough to use. The bass is 18" wide at the lower bout and about 24" long. Guitar Tom

[Darryl Young]

Tom, you ask a good question and I wish I were experienced enough to answer it fully.

Per Al Carruth, there is a good correlation between stiffness and density in softwoods.........so a denser top is likely more stiff and a less dense top is less stiff. In my mind, it seems you need more stiffness in a wider body guitar to equal the strength/response of less stiffness in a smaller body guitar (assuming you want them to respond identically). So maybe a less dense board in a smaller body guitar and a denser board in a larger body guitar would be appropriate.

It complicates things more when you consider stiffness along the grain vs stiffness across the grain. There isn't as much variation in the length of different size bodies but considerable variation in the width of the various body sizes. So you might need cross grain stiffness to vary more than stiffness along the grain to match a soundboard to a particular body size.

It seems the same line of thinking would also apply to choosing back wood for a particular body size. If brazillian rosewood is the right stiffness for a dreadnaught, wouldn't you want a less stiff sample for a 00 to achieve the similar results? I don't know the answer to that question but it seems logical that you need less stiffness on the smaller body guitars to get equivalent results. You can achieve the appropriate stiffness by thinning the same wood more for a smaller body.......but would that give you identical results? Since the stiffness to mass ratio would change when you thin the same density/stiffness wood thinner, it seems you wouldn't get the same results.

Would appreciate any thoughts on this anyone would like to share.

[Guitar Tom]

I'm thinking that a less dense or softer top would be more damped. I believe the grain pattern would be wider that a dense top. Since I'm going towards an upright sound I would want it to be damped. Considering the upright upper harmonics start at 1600hz (the initial attack) that falls off quite quickly until around 600hz and the real information starts at around 200hz where there is very little decay. With that in mind I need to consider the structural integrity of the less dense top. Is there that much difference in the strength between the two? Guitar Tom

[Darryl Young]

I will repeat something I've read Al Caruth state. You can get to the same strength with either the high density top or the low density top........just leave the low density top thicker. To resist the pull of the strings, there is a minimum stiffness along the grain that must be maintained. If you brought both tops down to the minimum required stiffness......and they had exactly the same stiffness along the grain......then the denser top would be thinner and the less dense top thicker.......but the less dense top would be lighter (even though it's thicker).

The above discussion does not take into account how the cross grain stiffness is affected. I don't know if cross grain stiffness correlates to density, but I doubt it. My best guess is that you want your cross grain stiffness to correlate with the long grain stiffness so that the whole lower bout will move together to maximize the volume of air moved in the monopole mode. I'm sure there are a lot of ways to get there.........bracing design and tuning, picking the appropriate cross grain stiffness for a particular size body, varying the X-brace angle to provide more/less stiffness in the weak direction, etc., etc.