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I know this question is very subjective but I also know that many of you more experienced builders can offer some technical and objective reasons for your choices. I have used Lutz and German Spruce but haven't built enough to get a definite feel for what makes the best classical top. What are the main differences between Lutz, German(called German but who really knows), and Caucasian Spruce? If you had to choose one, which would be your choice for the best Classical Guitar top and why? Thanks, and looking forward to your answers.

I forgot to include Engelmann in the choices. I have used Engelmann with good results. Lets Include Engelmann in the choices as well. Thanks

I would choose the top with the best stiffness to weight ratio regardless of species.
Others may differ.

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Jim Watts
http://jameswattsguitars.com

Thanks Jim. I know that in general is what to look for but does one of those species usually have that attribute more than the other species.

I save all of my lowest density tops for classicals, regardless of species. It turns out that the stiffness (actually the 'Young's modulus': a measure of potential stiffness) of softwoods along the grain pretty well tracks the density, and does so in the same way for all softwoods. An Engelmann top and one of, say, Sitka spruce, of the same density, will probably have the same long-grain stiffness at the same thickness. Since it's primarily long-grain stiffness that keeps the top from folding up under string tension, that's the controlling thing in how thin and light you can make the top.

The trick is that, all else equal, the stiffness will vary as the _cube_ of the thickness: making the top 10% thicker will make it about 25% stiffer. The relationship between Young's modulus and density is pretty much linear in the density ranges we usually see, so using a low density top and making it a bit thicker will end up giving you a lighter top.

Of course, different species of wood tend to have different average densities: Red ('Adirondack') spruce is usually denser than most, and Engelmann tends to have the lowest average density, but that's not a Law. There's a lot of variation within any species, and thus a great deal of overlap. I've gotten Red spruce tops that were as light as the lightest Engelmann, and Engelmann that was as dense as any Red.

I'll also note that the relationship between Young's modulus and density is not hard and fast: if you make a lot of measurements you'll find that about 2/3 of the samples will fall within 10% plus or minus of the predicted stiffness value based on density. That's pretty good for a natural material, but not so good that you can just 'set it and forget it' if you're looking for the best outcome. Ideally you'd measure the exact density and Young's modulus of each set of top wood you get, and pick the ones that have low density, and a higher than predicted stiffness. These measurements are not all that hard to do, but I haven't time to go into that right now.

Alan, How is density measured in the context presented above?

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I use little squares I cut from the off cuts and the circle that results from cutting the sound hole.

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Sorry Miguel, You use these small pieces for what? To measure density? If so how do you measure density?

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I have them at a specific thickness, say, around 1/8", I calculate area (circle: pi*radius^2, square sides^2), multiply by thickness to get volume, weight the pieces and divide the mass by the volume, then average the results to get an approximation to the average density. Not the most scientific of methods, I reckon...

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Why don't you measure density before they are cut to shape. Lot easier and no doubt more accurate.

The bigger the piece you start out with, the more accurate you're likely to be. I measure the length, width and thickness of a top half blank, and weigh it on a gram scale. Then I calculate the density (mass/cubic volume) from that. If the piece is rough sawn, or not very uniform in thickness, it's best to make it as smooth and uniform as possible without getting it too thin to use. The thickness measurement will be your least accurate one so the thicker the better: .004" error amounts to a lot larger percentage at .110" than it does at .140".

I work in SI units (kilograms/meters/seconds) so my numbers end up being kilograms per cubic meter. Top woods tend to run between 300-550 kg/m^3, or a specific gravity from .3 to .5. Since water weighs 62.5 lb/ft^3, the softwoods we use run between 18.75-34.375 lb/ft^3. If memory serves, the Young's modulus along the grain tends to be about 6000 MegaPascals at a density of 300 kg/m^3, and 15,000 at 550, and it's pretty much linear in between. This will get you within 10% of the right Young's modulus value about 2/3 of the time, which is better than guessing.

actually i did. i used to measure the density of the separate plates when i get them and of the top once thicknessed, but thought i would get more accurate dimension measurements from a perfect shape and a good tool (a circle, measure radius with a caliper) than from a big irregular board (it´s hard for me to make a board completely level) and a not so accurate tool (my bigger ruler is not that accurate, as it seems). i´m thick like that sometimes. but thanks for the correction.

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It has been pointed out to me that most luthiers do measure the stiffness of their wood by tapping and flexing, if nothing else, so that most builders are not 'simply guessing', and that my post implicating otherwise could be construed as an insult. It was, of course, not meant that way.

I've done my share of tapping and flexing over forty years of building, and I find that my work has become both better and more consistent since I started to rely less on intuition and more on objective measurements. The 'density' method is, of course, a short cut, and has its drawbacks, but I still feel that, for most people, it will give closer results than simply thicknessing wood according to the species, or relying on 'feel'. Naturally, there is no substitute for a good measurement, and if you're one of the folks who's better at that 'by feel' than I am, my hat's off. There are lots of good ways to do this, and the way that works best for you is the best way, whatever anybody else might think.

Print Alan C. posts
He is the living bible of guitar functions.

Jim Watts has the best advise for you question.

Mike

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Mike Collins

Thanks for all the replies guys. Alan thank you for your in depth explanation. It is a great place for me to start accumulating data. I have done everything by feel and sound up to this point but I know I can get more sound out of my builds and would like more consistancy. I have lucked out to this point and have had no complaints from my clients but I know in order to build top end instruments I need more experience and data which will equal consistency.
I knew my original question was way to broad but I thought it would be interesting to see what answers I might get. The wealth of knowledge available on this site never ceases to amaze me. Thanks again

Alan Carruth is one of the most valuable assets on the guitar forum front. No "yes and no" answers here, just thoughtful consideration based on years of dedication and experience. Thanks Alan.
Tom

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