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Hey guys, Im about to start a project with a walnut soundboard, and was curious how you guys treat your hardwood soundboards differently than softwoods. I.e. thickness, bracing, etc.


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I don’t. I treat them the same. I take them to the same stiffness as spruce...

I wouldn't use hard woods for a top. Unless you don't care about the tone, some people don't. Spruce and cedar are king. I've never played a hardwood top that I liked tonally. They are beautiful though. Someone correct me if I'm wrong.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Walnut, primavera, mahogany, Khaya, blackwood, and koa, and probably others, can make fine sounding instruments when used as a top wood. Just don’t expect it to sound like spruce. They will all generally have a darker, midrange forward kind of sound.

Generally they will need to be thicker than spruce to get the same stiffness. This is where deflection testing can be handy. I take my hardwood tops down to the same stiffness as my spruce tops. However thick it is afterwards is a byproduct. Because the top is of the same stiffness as a usual spruce top, there’s no need to do anything different with the bracing.

If you wanted to get fancy, you could make the top less stiff than a spruce top and beef up the bracing to maintain stiffness while saving weight, but without any reference numbers that work for you in your shop it’s probably best not to gamble.

So I can’t tell you how thick to make your top specifically. However, from lots of readings on the net here and there, the scuttlebutt is to make your hardwood tops ~20% thicker than your standard spruce top...

Hardwoods tend to have values of Young's modulus along the grain that are similar to softwoods, but at much greater density. You have to leave them about as thick as you wood a softwood top, but they end up weighing more. This cuts down on the sound output they can produce, and more so in the higher frequencies; hence the "darker, midrange forward kind of sound".

I only ever made one with a hardwood top; an acoustic bass guitar that was entirely made of walnut, on a 17" full Jumbo platform. It was intended to be used primarily plugged in, so the reduced acoustic output was not a problem, and the enhanced bass balance fit the compass.

Alternatively, you could go the other way. Making the soundboard smaller allows you to thin out without losing the stiffness, so top density is not so much of an issue. The ratio of surface area to mass tends to be higher on smaller boxes, and it's A/m that determines power output. Small boxes are also more 'treble balanced', so the reduction in highs from the added top mass is canceled out to some extent. Koa topped ukes can really cut it!

I recently finished an all mahogany guitar--the mahogany top started at about .10 inches, same as I do spruce. I don't know what the final thickness turned out to be, since I tap/flex/graduate it as I work it until it sounds like I want it to. Since this was my first mahogany topped guitar, I wasn't exactly sure what to listen for as I tapped it--it sounds very different from spruce.

It's my favorite guitar to date--loud, very punchy and balanced. Guess I got it right. As Ed mentioned, it's a darker sound than a spruce topped guitar, but It's become my all purpose player. It's a deep bodied L-00 sized guitar, so what Alan mentioned about smaller soundboards is probably at play here. My other favorite player is all koa. Hardwood tops can be great.

Dave

Thanks! Yes, I actually prefer hardwood topped guitars. My two current favorites are a Martin all mahogany dreadnought and a Martin all Ovangkol grand auditorium shape (my wife's and mine respectively). These big bodied hardwood topped guitars work really well for singing since they aren't competing so much with the voice for frequencies. I also love the darker Woody sound of them.


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I used a piece of (north American) sassafras for a soundboard on a "size 1" guitar. It is closer to spruce in density than it is to mahogany . I will use it again, and would recommend it to those who want a sound in between spruce and mahogany.

I've made a few mahognay top guitars and I think they have a very useful tone. They are different but they still sound great in thier own right. Like Ed said I treat them the same and measure to a target deflection.

To add on to what Alan said, and hopefully he will correct me if I am wrong, you could always make the top thinner, hence it will be weaker but then have about the same mass as a spruce top, and then beef up the spruce bracing to stiffen the top. I think you would loose some weight doing that.

Having said that though, the denser top is what gives a hardwood top it's uniqueness so maybe you don't want to try and make it sound like spruce... else jsut use spruce

Spruce is boring to look at though. If I could build one like the above and like the tone, I would never use spruce again.

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Hutch

Get the heck off the couch and go build a guitar!!!!

There's been a few questions and threads on this subject. Here's a reply I wrote a few years ago, and here's the guitar I was talking about.


The way a guitar vibrates, the sound it produces and its responsiveness is a function of BOTH the mass and the stiffness of the top. If you're looking for the responsiveness of a spruce guitar, you need similar mass and stiffness out of your hardwood soundboard. That was the approach I took on this guitar. If you match just stiffness to a spruce top, you will likely come out with something a lot heavier, with the tonal consequences others have described. If you match just the mass of a spruce top, you will likely have a bass biased, perhaps muddy, sounding guitar. And there's a continuum between the extremes. Managing the stiffness and mass gives you a lot of control over the sound your guitar produces. The other major factor is the intrinsic damping in the whole top structure (mainly in the panel) which really can only be managed by choice of materials. It is nigh on impossible to reduce the damping of high damping materials, though it can be distressingly simple to increase the damping in low damping materials.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

That's a beauty Trevor. Koa? Or some other acacia?

Wow! That's a work of art! I saw that you used K&K's. That's my plan as well. Thanks for the info, and the beautiful pictures.


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Thanks! Yes, it's Koa.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au