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I have been looking more into these double top guitars and I was wondering what type of bracing is necessary for them. I am starting to plan my next project and I am leaning towards a Tom Bills style guitar. If I were to build one like that without a sound hole on the top I would be able to lose that extra chunk of wood supporting the Rosette. Also If I were to make a caul and glue the nomex and the skins together at the right radius would that maybe be enough structure to eliminate the need for bracing???

Robbie-
You could probably make a (double) top strong enough to eliminate the need for bracing. I think that the bracing in most guitars also works to 'break up' the soundboard into different-sized areas so that you get different resonant frequencies at work?

It would certainly be an interesting experiment. What sort of soundboard structure do the Bills guitars have?

Cheers
John

When built properly the double tops don't really need bracing for structural reasons. I still use bracing to control the sound and get the tap tones and Chladni modes at the frequencies I want. For a lot of info on double tops take a look at my Luthier Pages. Go to
http://dunwellguitar.com/
and click on the button for Double Tops and also the Luthier Pages. Both the guitar and bouzouki building pages show double top construction info.

Enjoy,
Alan D.

Thank you very much I found your website very informative, but it led me to some design questions. When you brace your guitar tops how do you come to the dimensions for the bracing? Excuse my ignorance, but in your case with a structurally sound top would you be able to use extremely reduced bracing to avoid added mass to the top and achieving the desired modes. Or did you find that in order to achieve those modes you had to keep a relatively similar size bracing that of a normal topped guitar? Also as far as weight reduction goes how much weight have you lost between a standard top and your double tops?

When I first started building double tops, lo those many years ago, I kept the bracing the same so that any changes in sound, etc I could attribute to the DT changes. I soon learned that this was overbracing.

Currently I use a 1/4" or less wide brace and I start with them a bit under 1/2" tall for the X. That is way too much, but I like to approach each double top/back and voice it slowly. I carve as much off the sides as I do the back and you can see in the photos on my web pages that the braces aren't much compared to a normal top. Further, I've been using red spruce for the braces but having had very good luck tone wise in the past with engelmann braced tops, so I plan to do some experimentation there as well. Current bracing pattern for my instruments is a 95-98 degree X with a single symmetric tone bar that gets double scalloped. No finger braces and a single cross brace under the fretboard. That one is also of 1/4" stock. You need to think "light" when bracing these tops, bracing can kill your Chladni modes real fast! Look at the info about the flying braces I used in the bouzouki, this can allow you even more freedom as the top no longer has to bear all the compression load.

So the short answer is that you can greatly reduce your bracing both structural resistance and in mass and still get the tone/modes desired. In fact you need to in order to get the tone/modes desired.

Not counting the bracing, you end up with a top that is about 1/3 less weight. Note that even though the guitar building pages show use of a rim of wood around the edges, I no longer do that. I only have hard parts in the center now as I show in the bouzouki pages. You probably save some more in the bracing but I have not measured that as a direct comparison to a solid top. So, there is an exercise for the student. Build two tops, one solid and one DT using wood from the same billet and see what the weight differences are both before and after bracing.

Laters,
Alan D.