Top/Back Thickness

[John Nephin2]

I realize my questions depend on the wood but in general what thickness do you normally aim for when preparing your top/back?
What would normally be considered to thin?

[Trevor Gore]

Generally, there is no simple answer to those questions.

It depends on the size and type of guitar you're making, what you want it to sound like, the bracing you use, the elastic properties of the wood you use and its density.

Top thicknesses range from less than 1mm to more than 3mm and back thicknesses from less than 2mm to more than 4mm for fairly well known guitar designs.

[Simon Magennis]

I go for something in the 2.2mm to 2.4mm range. If for some reason I made the back thinner than that i would probably just beef up the braces a little more. No particular science in it for me so far. (Unlike Trevor )
On some future guitar I am planning to go quite a bit thicker and heavier. But it may not happen for while.

[paul berry]

I recently read Cumpiano's article on his view of tap tuning, I now trust this man so do not intend to use this method in getting the correct thickness for my top

So just giving a heads up John

Do most people here also agree with Cumpiano's view?

[Barry Daniels]

No.

[Alan Carruth]

I can't speak for most people, but I don't agree with Cumpiano. In my opinion, the exact mass and stiffness distribution in the top are a big part of what shapes the sound. Wood varies a lot, and you can't get consistent tonal results by making all of your tops the same: if you could, all Martins or Taylors would sound exactly alike. Tap tuning is one window into looking at the way the top is working. Of course everything changes when you glue it to the rim: baking the souflee changes it from some soupy glop into a tasty dish, but that doesn't mean you don't pay attention to the recipe, and maybe adjust the ingredients before you bake it to get it to turn out right.

Similarly, there are ways of testing the top wood to find the appropriate thickness for the instrument you're making. This will vary with different sizes and models of guitar, as well as depending somewhat on the properties of the wood you've got. You might also want to use a different thickness depending on the type of music you're going to play on the instrument; a finger style guitar will tend to be built a little differently from one of the same model that's being used for country flat picking. I'd tend to use different wood, too. And you can't just go by the species: top wood within a given species can vary as much as plus or minus 20% in lengthwise stiffness at a given thickness, and the crosswise stiffness is much more variable. There's a lot of overlap between species too: some Engelmann spruce is harder and denser than some Red ('Adirondack'). There are reports, from people who have done the experiment, that most people are not nearly as good at feeling the difference as they think they are (not surprising), so that the only way to be really sure is to do the measurement. It's not hard, and there are threads all over the place about different ways to do it. One of them involves tapping...

We're dealing with designs that have been very highly optimized. The experience of Martin and Taylor shows that you don't have to test wood, or vary the way you build, to make reasonably good instruments. I think most hand makers aspire to do better than 'reasonably good', and try to be more consistent in their sound as well. Tap tuning, and the variants of it, as well as deflection testing and other methods, give you a means of fine tuning the end results.

[Chuck Morrison]

To be able to adjust the sound of an instrument one has to be able to adjust the stiffness of the soundboard (back and sides too). Tap tuning is a tool, a way to measure stiffness of a piece of wood by way of its resonant frequency. Similarly a caliper measures stiffness by way of it's thickness, area, volume, deflection and a scale measures stiffness by way of it's density. It's all related and each method gives a specific view of the overall picture. You can choose to go as deeply into it as you want. It's all just measuring the physical properties of the work piece.

[paul berry]

To be able to adjust the sound of an instrument one has to be able to adjust the stiffness of the soundboard (back and sides too). Tap tuning is a tool, a way to measure stiffness of a piece of wood by way of its resonant frequency. Similarly a caliper measures stiffness by way of it's thickness, area, volume, deflection and a scale measures stiffness by way of it's density. It's all related and each method gives a specific view of the overall picture. You can choose to go as deeply into it as you want. It's all just measuring the physical properties of the work piece.

The idea of weighing the soundboard had not occurred to me

It makes sense that weight is related to stiffness, sounds like weighing will benefit me in learning how to manipulate soundboards well

I would imagine a kitchen weighing scales will suffice for this measurement

[Chuck Morrison]

I would imagine a kitchen weighing scales will suffice for this measurement

Depending on what all you will be weighing (tops, braces, etc), a gram scale capable of a decimal or two would be good. Ounces don't hack it. Make sure you can get up to 300 grams or a bit more if you're going to weigh backs too. Then you'll probably want to be doing some deflection tests so as to correlate the weights you get with measured stiffness ....

[Alan Carruth]

Long-grain Young's modulus (E), which is a measure of potential stiffness, pretty well tracks density for softwoods. About 2/3 of the pieces I've checked have come within 10% of the E value you'd predict on the basis of density. The only thing that correlates with cross grain stiffness at all is the angle of the annual rings to the surface, with quartered wood having higher cross grain stiffness than other samples from the same tree that are not quartered. There's enough variation within each species that there's no simple way to predict what the actual value will be; you have to measure it.

If you don't have an accurate scale for weighing the wood, you can simply put the piece in water to see how high it floats. It's best to put it on end, to get an accurate reading. Of course, you need to work quickly and dry it off fast afterward. Some violin makers put the wood in a baggie, and that's OK if you don't trap any air.

[Simon Magennis]

I go for something in the 2.2mm to 2.4mm range. If for some reason I made the back thinner than that i would probably just beef up the braces a little more. No particular science in it for me so far. (Unlike Trevor )
On some future guitar I am planning to go quite a bit thicker and heavier. But it may not happen for while.

Re-reading, I realise I left something out here. My comment referred to the back. For the top I am mainly interested in how it sounds when I tap it as I thin if down and I generally go a bit thinner around the edges than in the centre. The last three I started were using the Blackshear Ramirez 1912 plan and I tried to get fairly close to the thicknesses in the drawing. Otherwise I guess the tops are usually in the 2.0mm - 2.2mm perhaps even dipping to 1.9mm in some spots around the edges.