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I just finished deflection testing my soundboard. The only benchmark I could find (since this is my first time) was to use 5lbs of weight with the wood supported 18" apart. The idea is to end up with around .25" of deflection. After sanding my top a bit I ended up at .110" thick, which I think is fairly average for sitka? Anyway, just for learning purposes, I measured the density of the soundboard and came up with 479 kg/m3. The wood database website says spruce averages 425 kg/m3. So what I'm wondering is how out to lunch my measurements are? Do you find that spruce used for luthiery is denser than average? It would make sense to me since I've read that with spruce, density tracks fairly evenly with stiffness.

PS. I know that this is way overboard for a beginner builder, so no need to tell me that. I just figured, I have all the stuff for taking these measurements, so why not? It's just one more learning point. The subject of density vs stiffness is of interest to me because I plan on building a mandolin next, and there really can't be many benchmarks for mandolin plates. I'm thinking I can use the density of my mandolin top blank to decide what set of "numbers" I should carve the top to.

That's definitely on the denser side but by no means off the chart. I've had as low as 300 and some slightly above 500.

.110 is bang on factory thickness. So you've got a pretty good starting reference...

What does your Sitka top weigh at final thickness without braces?

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It will probably be alright.

It weighs 11.9 oz (.337kg). It is 590mm (23 1/4") by 426mm (16 3/4") The only hinky part is that I cut out the soundhole and installed a single herringbone rosette before measuring it. I put the cutout on the scale with the top, so it kind of depends on how the added weight of the rosette balances out with the spruce removed when I used my 1/4" bit to cut out the soundhole. If anything, I'd say the spruce weighs more than the rosette, but I'm calling it a wash. I was never planning to do any of these measurements or I would have done them all when everything was new and square.

It makes sense if it is a fairly dense piece. When I bought the top I didn't order the high grade, but I called the supplier and asked if he could send me a stiff top even if it had some cosmetic reason he couldn't call it a AAA. I ended up with crazy bearclaw, a kind of cool colour streak and what I think is a fairly stiff top for not too much cash. Since putting a washcoat of shellac on it, I've discovered a little runout in it. This could explain the average stiffness compared to the higher than average density.

Stiffness tracks with density with few exceptions. So when you asked for a stiff top in essence you were asking for a heavy top also.

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Jim Watts
http://jameswattsguitars.com

Heres a pic

I wonder why big factories don't use weight measurements to put different braces on guitars (or maybe they do). Seems like it would be pretty straighforward.

There is nothing wrong with dense spruce for tops, if you account for it. It just gives a different timbre.

Yes, it's another variable to use in tailoring the guitar to it's intended purpose.

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Jim Watts
http://jameswattsguitars.com

Stiffness (or, technically, Young's modulus along the grain) does track density surprisingly well.

A heavier top will tend to be less 'responsive', particularly to a light touch. On the other hand, it will most likely have more 'headroom' than a lighter one.