Official Luthiers Forum!

Hi everyone. I heard about this forum from the AGF, and thought I'd drop by with a tone question for builders.
  
I've been keeping tabs on Peter's sinker redwood build and really liked the unique wood combination he used. Going along the unique combinations thought, I once pondered a rosewood guitar with a mahogany top. Has
anyone seen or built this combination before? If so, how did it sound (or how do you think it would sound)?
  
 

I have only seen all mahog, never like you said. Sorry I can't help. I thought someone else might have goined in by now.

Mahogany is denser than the softwoods that we usually use for tops, and would most likely end up making a heavier top if you build to the same stiffness. This would tend to cut down on the volume and treble response. This is one reason it's used for acoustic-electrics from time to time: the lowered response cuts down on feedback. Of course, you might be able to find a piece that's not as dense as most, and that would help.

What about using Luan- Philippine Mahogany? It has much the same look as Honduran or Cuban mahogany, but my experience is that it tends to be lighter and less stiff. I don't know if anyone has used it for guitar building- I've used it on classic mahogany Chris-Craft speedboats.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

[QUOTE=gozierdt] ... tends to be lighter and less stiff. [/QUOTE]

Wouldn't you need to go thicker to compensate for the lack of stiffness in these cases? It sounds like trying to match the stiffness of a spruce top would result in a much thicker soundboard.

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53% of all statistics are made up on the spot
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If it is thicker, it might not give good volume or response. I have a Martin
000-15 that is all mohogany, the soundboard doesnt apear to be much
thicker than a spruce. It sounds just wonderful I dont know if I personally
would use mohogany and rosewood(just the way mohogany finishes would
take away the awe of the rosewood finish imho), but maybe a koa
combination would be kinda cool.

WayneC:
He was comparing Phillipine with Honduras mahogany, I'm sure. The Pacific stuff does tend to be less dense and stiff than the American wood, but it's still denser than most spruce, I think.

The deal is that for most softwoods the stiffness along the grain and the density pretty well track. That is, a piece of spruce that is twice as dense as another also tends to have about twice as high a Young's modulus along the grain.

What we need in a guitar top when it's finished is a certain level of stiffness; enough to keep it from folding up too soon. We'd like to get that with as little weight as possible, in general, since more weight just reduces the amount of sound you can make.

The actual bending stiffness of any specific piece of wood of a given length and width will vary with the Young's modulus and the cube of the thickness. Doubling the Young's modulus for a given size gives twice the stiffness, but doubling the thickness makes it eight times as stiff.

If you double the density, and you want to keep the same weight as some reference piece, then you you can only make the new one half as thick. If the Young's modulus of the new wood was the same as the reference piece, it would only be 1/8 as stiff. If the Young's modulus is also doubled, along with the density, then it's 2x1/8=1/4 as stiff. If the new piece needs to have the same stiffness, it will need to be about 80% as thick as the reference piece, and it will then weigh about 1.6 times as much. That's why low density tends to trump high stiffness.

Allan,

This is a topic of interest to me as I'm making an all-mahogany guitar.

I'm going to consider the sitka vs. mahogany case. I wasn't certain how the stiffness compares, so I looked up some wood properties here. It looks like the elasticity of the sitka (@12% moisture) is 9900 MPa and mahogany (also @12%) is 10300 MPa. So it looks like the mahogany is stiffer than the sitka. If I understand this correctly, the Young modulus is telling me that a mahogany top will be stiffer than a sitka top of the same dimensions. If I want the mahogany top to have the same stiffness as the sitka, I can make it thinner.

The mahogany is more dense than the sitka (~0.45 vs 0.36). I still need to work out the math, but it seems that thinning the mahogany top to match the stiffness of the sitka top will still leave me with a top with more mass.

_________________
53% of all statistics are made up on the spot
http://driftwoodguitars.blogspot.com/

I've never measured any mahogany, except for a couple of sets of Cuban, which is more like a rosewood. I've got Sitka ranging in specific gravity from about .4 to .5, and Young's modulus along the grain from 12,000 to 14,000 mPa. I haven't tested all that many Sitka samples, so I would not be surprised in the range was somewhat wider. Also, I'll say that my testing methods seem to give results that are a little lower than most folks: I used them mostly for internal comparisons, so that's not usually a problem.

One solution for an 'all mahogany' guitar might be to cheat.... You can sometimes get cedro in wide, quartered planks. It looks very much like mahogany, but can be much less dense. As usual, it varies a lot, and you'd want to test it....

[QUOTE=gozierdt] What about using Luan- Philippine Mahogany? It has much the same look as Honduran or Cuban mahogany, but my experience is that it tends to be lighter and less stiff. I don't know if anyone has used it for guitar building- I've used it on classic mahogany Chris-Craft speedboats. [/QUOTE]

I've got some of this glued up, cut and ready to go for a back and side set. I hadn't thought of doing a top with it... Perhaps I will.

I've got quite a bit of this and it varies greatly from board to board as to weight/stiffness/color.