Hardwood top thickness

[hat]

When using a hardwood for the top, such as Walnut or Koa, where do you set the basic thickness? do you use the same .110 as for Spruce, or go a little lighter, or? What about bracing, same idea?

[Bruce Sexauer]

There are a number of variances in building philosophies which make it difficult to give a blanket answer to your question. I view the top as a structural membrane and not as the primary structure that resists the string tension, so it is easy for me. I simply flex and tap on the plate as I thin it until it all but loses its innate structural integrity. Hardwood being a very broad range, backward engineering tells me that my range of hardwood plate thickness runs from around .065" for a small guitar with very hard Koa to near spruce like dimensions of around .1" for Catalpa in a medium sized guitar. It takes a bit of experience make this kind of call and be consistently in the right ballpark. IMO, of course.

[hat]

Thanks Bruce. About the answer I was expecting I guess. I suppose there's also the relationship between the top, and the top bracing that comes into play. I guess it's just one of those things that you have to 'pay to play' so to speak. Perhaps erring on the side of a little too stiff with the top wood, then backing it down with lighter bracing is a decent compromise for a first effort with a hardwood top. I have some native Black Walnut ( quarter sawn, planed to 1/8" and air dried for 10 years or so) just waiting for the right project, now I think I know what it might be. I'm thinking a pre-war style 00, 12 fret slothead, maybe deep bodied ...hmmm......

[Alan Carruth]

As Bruce says, there's a range of opinion on this. I'm in the camp that looks on the top as a structural member rather than simply a membrane to move air. I'm trying for a certain 'balance' between the stiffness of the braces and the top, where each part carries it's share of the load. To that end I use a vibration test to measure the Young's modulus of the top wood along the grain and use that to determine the 'proper' thickness. Young's modulus predicts the stiffness at a given thickness. There are other ways to do the same thing, of course; some folks can do it by feel, and others use some sort of deflection test, going for a certain deflection under a standard load.

Anyway, what I've found in my measurements is that most hardwoods aren't much stiffer along the grain than many softwoods. Indian rosewood, for example, tends to have a Young's modulus along the grain that is similar to some of the denser spruces. In my system, then, I'd tend to leave an IRW top about the same thickness as a spruce one.

Hardwoods do tend to be a lot denser than softwoods, though. IRW runs about double the density of spruce, so the top would end up weighing almost twice as much. This tends to cost in responsiveness, treble, and overall power. OTOH, it gives you more sustain, and a 'sweeter' tone in some respects.

Generally speaking the penalty for using a dense top wood is less on smaller guitars, simply because they tend to be more treble balanced, and can be more lightly built than larger ones. Ukes commonly use hardwood tops, and work fine.

[Trevor Gore]

Any top, irrespective of its thickness, is going to act as a structure when glued to a brace assembly. To my way of thinking, to sound like the type of guitar we know, the top needs to be the same mass as a typical spruce top for the same size of guitar. I measure the Young's modulus and density of the top beforehand. The density allows me to calculate how thick to leave it (which is typically thinner and less structurally stiff than a spruce top of the same mass) and the Young's modulus allows me to calculate how stiff to make the bracing to get the total structural stiffness back where it needs to be. This puts the modal resonances and the monopole mobility both back in the right ball park. An example here.