soundboard woods...
Ernstjan
Just a little BIG question,
What about the soundboard wood?
I always thought WRC (Cedar) and Spruce were the only options. But I see
more and more instruments that have hardwood (for axample koa) used as tops
on guitars...
Don't they sound very shrill, stiff, high, etc...???
What about it?
EJ
alcarruth
enough to resist denting pretty well. The object is to make the top as
light as you can, but strong and stiff enough to resist the string
pull. Balsa is too soft to work well, and some other light, soft woods,
like willow, don't resist long term deformation. The spruces in general
are in the 'sweet spot': they tend to be light, stiff and tough.
There's quite a trange of spruces, from Englemann which is softer and
lighter, to Red that tends to be heavier and harder, but there's a lot
of overlap within the range as well. Western red cedar is sort of soft:
it's used because it's lighter and has good stiffness, but you have to
be more careful to not mark it. Redwood can be as light as cedar, and
harder, but it has some tendancy to split.
Most of the hardwoods are quite a bit more dense. You have to make them
tihnner to keep the weight down, but stiffness goes down faster than
weight as you thin things. Unless a hardwood has a really low density
and high elastic modulus it will usualy end up making a heavier top
than a softwood.
The reason you need to think about that is simply that you can't put in
much power with a plucked string. Small 'engine' -light 'car'.
Guitars with hardwood tops tend to be not so much 'tight' as 'small'
sounding. This is not universally true: some hardwoods are prettl low
in density, and all woods vary in properties. I've used some old walnut
that was nearly as light as spruce, and it's relative, butternut, is
even less dense. Some koa might be low enough in dnsity, and there are
probably others. But I've never run into maple that was anywhere near
in the right league.
Alan Carruth / Luthier
Stan Gosnell
@f14g2000cwb.googlegroups.com:
> Generally speaking you want to use a light, stiff wood that is hard
> enough to resist denting pretty well.
Alan, have you done anything with carbon fiber? Obviously way up there
in stiffness to weight ratio, but of course that's not the only quality
required. I know there has been some experimentation with it, but I
haven't kept up.
--
Regards,
Stan
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." B. Franklin
alcarruth
<<Alan, have you done anything with carbon fiber?>>
That's an interesting case, and illustrates the issues well. A little
physics as background:
The stiffness of a beam or plate will vary as the Young's modulus and
the cube of the height/thickness. We all know, for example, that a 2x4
up on it's side is a lot stiffer than the same piece of wood down flat,
and that's the rationale behind using tall narrow braces rather than
low wide ones. It works the same way for plates.
There are two things we have to think about when we make a guitar top:
sound and structure. The lighter the top is the better it is likely to
sound: it will certainly be louder, all else equal, and it will give us
more leeway to 'tune' it to get it to sound the way we want. But we
also have to pay attention to the torque load of the bridge: if the top
isn't stiff enough it will fold up too fast and that's no good either.
It turns out that bridge loading is the real limiting factor in guitar
top design.
Suppose we have a piece of spruce that weighs 20#/cubic foot, and we
know that it will make a top that is stiff enough when it's .120" thick
with 'normal' bracing. If we have a piece of rock maple that weighs
40#/cu. ft. it will have to be half as thick to have the same weight.
But, since the stiffness goes as the cube of the thickness that means
that if it had the same Youngs' modulus it would only be 1/8 as stiff.
Actually the Young's modulus of maple is something like twice that of
spruce (it more or less 'tracks' with the density in wood) so the maple
top would end up 1/4 as stiff as the spruce at the same weight.
>From this we can deduce a general rule: if we're comparing two
materials to use as tops the Young's modulus has to vary as the _cube_
of the ratio of the densities: if the material is twice as dense, it
has to have a Young's modulus eight times as high to achive the same
stiffness/weight ratio.
Wood is a very efficient material in general. It has evolved to have a
microstructure that gets a high stiffness for a reasonably low weight
and remains tough. In general it's hard to beat, and structures that
do so using other materials are usually fairly complex. For example,
welded tube and sheet aluminum light plane structures generally are not
any lighter overall than wooden ones. We think of alloy steel as very
strong, but you have to go some with it to beat wood, and the decision
to use one material or another is usually based on other
considerations.
CF has a very high Young's modulus, but it's also got a density close
to that of aluminum. (I loked up the numbers once, but don't remember
them) A solid CF top that weighs the same as a spruce plate will end up
quite thin, and if you thicken it to get the stiffness up it's 'way too
heavy. Used as a 'skin' on some sort of 'sandwich' structure it's OK,
and experiments with that have been carried out since the 70s. There
are issues with QC, and problems arising from the fact that the Young's
modulus of the skin is so high that it tends to break loose from the
substrate and peel off in ribbons. As usual, there's no free lunch.
Alan Carruth / Luthier
http://www.alcarruthluthier.com
Stan Gosnell
@z14g2000cwz.googlegroups.com:
> CF has a very high Young's modulus, but it's also got a density close
> to that of aluminum.
I didn't realize it was that dense. I guess the composite binder has a
lot to do with that. I've never worked with it.
> As usual, there's no free lunch.
Well, we knew that. ;-) It gets demonstrated every day.