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Larry,

I am in the throes of experimenting with triple-top sandwich tops with two thin spruce outer plates and a laser-cut thicker center plate.
I have one instrument built this way so far, but the sandwich was not quite thick enough - I had to add one cross brace just below the sound hole to stop the top from caving in above the bridge.

I have a second instrument nearly finished that has a thicker sandwich that shouldn't need any braces at all. Should be done in a few weeks. Look out for a posting from me on that....

As far as sound goes, based just on the first instrument, I'd have to say it's an acquired taste. It has fantastic sustain but sounds kind of "muddy" to me (compared with both a Taylor 814CE and a Martin Dred that I keep around for just such comparisons). I'm not convinced that I like it yet. When I get the second one done, I'll try and make an A/B/C sound comparision recording....

Dave F.

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Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

Tom, Not quite true, I think the a side bar that has been running through this theme, that would be braceless double tops. Double tops can get quite thick with out picking up much weight at all, as the core typically in the ball park of 2-3 lbs/ cu. ft. and the skins stay the same thickness. So you can have a top that can actually be too stiff and too light. This may be what happened on the guitar Ken McKay talked about, but I don't know if that was the issue or not. Maybe Ken will venture a guess.

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Jim Watts
http://jameswattsguitars.com

I call those gravity clamps.


Cool! I'll have to go for a lap around the local guitar stores and see if I can find some to sample. Hopefully a 00. I certainly would expect it to sound flabby on a dread.

What I'm most interested in is a 00 or smaller, with a very thick, low density cedar top so it hardly needs any bracing at all in the bridge area to achieve a fairly normal stiffness distribution. But still with a big upper transverse brace to prevent neck rotation, and fairly tall upper legs of the X to prevent soundhole cave-in.


I'll be looking forward to the results of that I hope it sounds good, but it will shatter my notions of soundboard design if it does... assuming it's even thickness all over, and has a soundhole in the usual place. If I were attempting such an experiment, I'd do a big side port instead of cutting any holes in the top. But then that would weaken the side, so make sure you have some really stiff back linings, or a nice sturdy triangle of back braces to transfer the neck's CF buttress force back to the headblock, without involving the sides.

Jim: Just trying to address original post. For sure solid top and honeycomb tops two different animals. My post was just meant to address solid tops.
Tom

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A person who has never made a mistake has never made anything!!!

I've only built one double top guitar and that one could probably have done just fine without bracing. It felt braced up once it came out of the clamping contraption. I braced it anyway but instead of my 7 fan Torres style I used 5 and very light ones. Next time I do it will probably use 3 or possible even none.

My take is that bracing is the control factor in a top, including double tops. Double tops tend to be muddy in the mid ranges. As a matter of fact, in listening to traditional guitars played against double tops, it's, usually, pretty easy to pick out the double tops even though you don't know the builder until after the listening test. There are exceptions. Some of the strongest guitars in those tests are often traditionally braced, solid tops. Also, Byers' style lattice bracing seems to remain pretty even and strong.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I don't think it was too light, but the stiffness was probably a little too high. That would be easy to do with further experiments.

My method was like this:
2.5mm spruce for top layer with soundhole cut. (Put a hard point of spruce with the thickenss of the core in soundhole area and a tab just under the bridge.
Glue core to it in a radius dish (cut out for hardpoint) with constant thickness of around 3-5mm (can't remember the density but probably light as possible) I used a composite pvc material but laser cut honeycomb wood might work or even nomex.
Once glued, flip the top and sand it on the same radius dish so the core is tapered and feathers to zero and the core thickness is gone where the plate glues to the rim.
Glue a veneer over the core on the underside to complete the sandwich.
Flip the top back over and thin out the spruce top skin so it creates some flex, like a speaker cone. This is a chance to effect the bass frequency somewhat.
Hope this makes sense.

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Ken

Torres did it.
Do you remember the article on restoring a Torres with no lower bout bracing that RE Brune' wrote several years ago? It would take a bit of searching to find, but it was very interesting.

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'Sandwich' tops are essentially a method of making a synthetic wood that's lower in density than normal wood, but otherwise has similar properties. Since a normal top without braces vibrates pretty much in the same way as a braced top, but at a lower frequency, sandwich construction should work pretty well without bracing. I've done a couple of tops that way, but had gluing issues on both. There's no good way to know whether you've got the bond between the core and the skins right until you load it for a while. I'm looking for something other than Nomex for the core, so I don't have to use epoxy or polyurethane.

I've done a copy of that very Guitar. It works! But then again so do Vihuelas and Baroque Guitars. No bracing on those either.