Timber selections

[Zandit75]

I'm hoping the very knowledgeable people here can answer a question for me.
I see plenty of guitars built out of one timber for the top, back and sides. Mahogany, Koa, etc etc.
I'm assuming there is a reason not all timbers can be used in this way, and would like to know what determines these reasons.
As I've mentioned before, I'm still looking to participate in a guitar building course, and I'm looking at having Black Heart Sassafras for the back and sides, but could this timber be used for the sound board as well??
Here are the technical attributed for the timber, hopefully someone here can make some sense out of it and advise.
http://tasmaniantonewoods.com/featur...ed-sassafrass/
Thanks in advance.

[dekutree64]

Yep, it ought to be good. The stiffness to weight ratio is what matters most. Damping is a factor as well, but more important for nylon than steel strings. I've never tapped on blackheart sassafras, so I'm not sure what its damping is like, but its Young's modulus (stiffness) and density look good http://www.wood-database.com/lumber-...art-sassafras/. A bit better than koa, actually.

There's no reason other than tradition to use a single wood for top/back/sides. For example, you could use sassafras top with rosewood back/sides. Or even rosewood back and mahogany sides The reason traditional styles are so common is not because they sound better, but because they sell better.

[printer2]

Density is the same as Walnut from the source I found, a few successful Walnut topped guitars have been made. Just needed to thin the top a little more to keep down the weight (bracing carries more of the load).

[nacluth]

Yes it can be done. All-hardwood guitars aren't typically as responsive as a softwood topped instruments, but they can definitely work as guitars. They have a different timbre to them, but if you know that going in, it shouldn't be a huge deal. Is this for your building course?

[Zandit75]

Originally Posted by nacluth

Yes it can be done. All-hardwood guitars aren't typically as responsive as a softwood topped instruments, but they can definitely work as guitars. They have a different timbre to them, but if you know that going in, it shouldn't be a huge deal. Is this for your building course?

Thanks for all the comments Guys, most appreciated.

Yes nacluth, this is for the course, and I was just curious to see if it could be done.
Here is some pictures of the guitars produced at the course(Not sure which ones are the Luthiers own built guitars, or which are his students) and I just thought it was such a shame that the beauty of the timber was not able to be shown off on the front of the guitar.








This particular guitar features a Cooba Pine top, hich has a lot of figuring anyway, and is my first choice for a top if I do the course, but it would certainly have a very large visual impact if the beauty of the BHS was shown off on the front.

[Alan Carruth]

Keep in mind that 'stiffness to weight' is a structural property, not a material one. You can't really talk about he stiffness to weight ratio of a material unless you specify how it's been used. The proper way to measure this stuff is by Young's modulus and density.

Young's modulus is the measure of how much force it takes to stretch a piece of material of a given size by a given amount. Because of the way stress in the piece distributed, it's a good measure of the bending stiffness of a given sized piece of material: two pieces of wood with the same Young's modulus will have about the same bending stiffness at a given size. Alternatively, you can use the Young's modulus to figure out how thick to make a piece of a given material to get the same bending stiffness as another piece that has a different Young's modulus.

If you measure the Young's modulus of a lot of different woods, you'll notice some interesting things. One is that there's a range of values, which is not surprising. What does surprise some folks, though, is that the range of values for hardwoods is not all that much different from the range for softwoods. What this means is that, in general, you'll need to make a hardwood top about the same thickness as a softwood one to get the same stiffness. As always, there are exceptions, and you really need to measure each piece of wood to be sure, since every species covers a range.

On the other hand, for the most part, hardwoods tend to be denser than softwoods. Again, there are exceptions: balsa is a hardwood that's often quite a bit less dense than any usual softwood, while Douglas fir can easily be as dense as many hardwoods. I've even got samples of European spruce and Engelmann spruce that overlap well into the 'hardwood' range in density.

Many of the 'usual' back and side woods, such as Indian rosewood, tend to be around twice as dense as an 'average' softwood, but to have a Young's modulus that's not any higher. Again, if you work the top to a certain stiffness, the EIR one will be about the same thickness as an average spruce one, but will weigh twice as much. Sometimes you can find a piece of something like mahogany that has a notably high ratio of Young's modulus to density, and there are hardwoods, such as cedro (and balsa), that can be right in the softwood range, but, for the most part, hardwood tops end up heavier than softwood ones at the same stiffness.

Adding weight cuts down on responsiveness, particularly in the treble range, because that's a matter of acceleration. There's only so much horsepower in a plucked string, and if you want acceleration from a small engine you have to keep the weight down. Power output (top speed) is also effected, of course, but because of the way average hearing works you may not notice that. The usual judgement is that guitars with hardwood tops sound 'sweet', but lack 'punch'.

[Zandit75]

Thank you Alan, most informative.
I actually understood that!!