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Density, damping & stiffness in Rosewoods?

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Density, damping & stiffness in Rosewoods?

Postby Bret A Hedenkamp » Sat Oct 20, 2018 6:20 pm

I recently received a Madagascar Rosewood b/s set to make an OM guitar. I measured it to be 0.690 g/cm3, which is on the extremme low side for this wood. I researched density affects on stiffness and damping. I imagine there will be a bit more damping of the sound waves on this less dense MadRos?

1. Can I compensate for the relative softness by going thinner?
2. If so, is there any deflection info for backs as there is for mandolin and guitar tops?

Thanks all
Bret A Hedenkamp
 
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Joined: Mon Jun 25, 2018 6:12 pm

Re: Density, damping & stiffness in Rosewoods?

Postby Alan Carruth » Sun Oct 21, 2018 12:21 pm

So far, in measurements of lots of samples of various sorts of wood, I have found no strong correlation between density and damping. Damping tends to be a species characteristic, and, of course, it varies from one example to another. Rosewoods in general tend to have low damping, but BRW is usually lower in damping than IRW, with the best samples of Indian much like the worst Brazilian. I don't have enough data on Mad rose to be able to say much about it in that respect, although I suspect it's more like BRW than IRW.

"1. Can I compensate for the relative softness by going thinner?
2. If so, is there any deflection info for backs as there is for mandolin and guitar tops?"

I measure wood properties in terms of Young's modulus (E) along and across the grain, with the associated damping factors. I use those E values to calculate the 'correct' thickness. This is equivalent to deflection measurement, so I don't do those, and can't give you a target for that.

For softwoods there is a pretty good correlation between density and E along the grain. This doesn't hold as well for hardwoods, where there is more variation in the microscopic cell structure. Its harder to predict: you need to measure. Still, there is at least Keep in mind that stiffness, which is the real target here, goes as the cube of thickness, so a small thickness change can add a lot of stiffness.
Alan Carruth
 
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