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Page 72 of the "Hand Made, Hand Played" book features a guitar built by Sergei de Jonge that uses bubinga for the back, sides, and top layer of the double top (the bottom being spruce). Does the double top allow you to use an otherwise less toneworthy wood for the show side as long as you have spruce or, perhaps, cedar as the bottom wood?
I like the idea of having the body all one wood and this would open up so many more choices if this is so.

I've only built one double top and I did the opposite. Tone worthy good quality cedar for the show side and lower grade cedar for the underside. But I am by no means an expert and will sit back and watch this thread.

Yes. It is a trade off though - looks for sound quality. Anytime you use something heavy like bubinga on the top it will not sound as nice or be as loud as a guitar built with a more traditional top wood. That being said, it can sound very good.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Oh woops, I missed the part about the bubinga top I thought you were refering to the quality of tonewood rather than species, yeah I'd think that would definitely impact the tone. The whole point of DT construction is to make the top as light as possible,,, I think?

I'd agree with that. Light and stiff is what a DT is all about, IMO. I can't imagine using a hardwood as a layer, but that's just me. The guy behind me in our table cluster at GFA makes all wood double tops with a Balsa inner layer. His name is Patrick Mailloux. Nice guitars. His tops weigh in in the 95 to 105 gram area. Pretty light.

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Waddy

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Sergei is a well-established and very, very fine builder. While double-tops aren't my thing, I would expect that he had his reasons for doing this. Why not contact him directly and ask what they were and if they worked?

It seems to me that with a cored, laminated top, the choice of woods is much less important than with solids. If one of the plates is a bit dense, as bubinga is relative to spruce or cedar, it would impact mass only little since it's so thin. Anyway, mass is only one factor in tone production and not the most important. Using a hardwood may, however, have a significant effect on stiffness both with- and cross- grain, which IS important to tone.

But I'd just ask Sergei.

Thanks, guys. I played a guitar with a Koa top not long ago and didn't care for the sound, though it looked deee-gorgeous. I was hoping these double tops would allow you to get the best of both worlds, but sounds like that's not the case.

Yeah, that DeJonge guitar is pretty crazy. I would be afraid to use wood with grain that squirrely, unless I was going for full lamination. But apparently it can be made to work, and who knows, maybe it felt more solid than it looks, and maybe he left the outer layer relatively thick for a double top (i.e. full top weight closer to that of a normal solid spruce top). I wonder what that guitar actually sounds like...

If you like the look of curly koa tops, I say build one and see how it sounds. Hardly the same territory as squirrely bubinga. Was that one you played a hand-built? A lot of the factory ones are duds. Just don't pretend it's spruce, and you should be fine I just built a solid rosewood top, and it sounds great. Koa should be a breeze

I love that! The curls behind the bridge totally look like they were placed there on purpose

As a builder of solid and double top classical guitars, I'll argue that it's even more critical when selecting the set for a double top outer soundboard than for a traditional solid top. My reasoning is that the final thickness of the skin is very small and typically the same from guitar to guitar. There is less material there to voice and no room to compensate in thickness for variations in stiffness and density. For these reasons, one needs to select highly consistent material for every guitar.

One indication that the outer soundboard counts for a lot is the well-known observation that the species of outer soundboard determines a large part of the tonal voice. That is, guitars with spruce outer skins tend to sound like spruce guitars, and guitars with cedar outer skins tend to sound like cedar guitars. I believe this can be explained by the notion that the majority of string energy is transferred from the bridge to the outer skin and that Nomex is not a resonant material, which probably acts something like an insulator.

Gary L, where did you get the data on the face skin being the dominant sound controller, I never hear that before? I'd like to see that to add to my own data sets. AFAIK the composite plate structure acts as a unit, at least the Chaladni modes seem to indicate that. I've built with dissimilar woods, but always spruce or redwood or cedar, the typical top woods. There is a distinct tonal difference with changes in the interior skin species, or if you skew the grain of the inside skin, but I hadn't noticed that it is the face skin that determines the result. The Nomex is Aramid which is just phenolic resin impregnated paper so I don't think it is an insulator, but a rigid component once it is glued in place between the skins.

All that being said, Darren, I still believe that you need to use the best "tonal" woods you can for the inside skin, but it can be "cosmetically" flawed. I don't use poor tonal woods for the inside skins, that just doesn't make sense to me. The cost is in the labour for a double top, the wood costs are small compared to that, particularly if you do your own resawing and can get more wood out of a billet or can get 1/4 sawn veneers.

Alan D.

Alan,
You raise great points, all of which make double tops a fascinating area for trying to better understand how sound and response are determined by material properties. I only build double top classicals so I defer to your vast experience with steel string guitars. I build three configurations: Spr/Nomex/Ced, Ced/Nomex/Spr, and Ced/Nomex/Ced. As I mentioned, in my experience, the outer skin always determines the ballpark of the voice in terms of “sprucey” or “cedary”. Randy Reynolds and Kenny Hill mention this phenomenon as well (American Lutherie Nos. 88 and 90, respectively). In listening to, and playing many classical double tops built by a number of builders, I find this to be consistent. To cite an interesting case study, the recent classical guitars of Ross Gutmeier are said to use a carbon fiber sheet as the inner skin (no wood inside). The recent cedar (outside) one I played last week had a typically cedar voice.

You are right in saying that the Chladni patterns of a Spr/Nomex/Ced sandwich are identical regardless of which face you take the reading. I’ve done that experiment and it demonstrates that the sandwich certainly moves as a unit. So how does one explain the outer skin tonal influence? A fascinating question! I hope the answer will provide more insights as to why double tops have a characteristic dynamic range, tonal response, and right hand response. I suspect there’s more to the magic of double tops than simply a reduced mass explanation, and it’s a great area for more acoustic research. Decreased internal friction? Increased speed of impulses through the soundboard? I hope others chime in with their thoughts and experiences.

FWIW, I am currently experimenting with what I call a "triple-top". The lams are spruce/spruce/spruce - I'm using some of those lovely 2A bargain tops from our pal Shane. The outer two are 0.050" solid and the inner is about 0.070" and has been "honeycombed" to look like Nomex (with solid areas for the bridge and around the soundhole, just like one would do with Nomex). I cut the hexagons out using my laser engraving machine - it took nearly 6 hours to cut! I glued up the top, using LMI Luthier's Glue, rollered on quite thin, in a 28' dish in a vacuum bag. It kept its dome shape nicely, is very stiff indeed, and taps with a nice sustained ring. I'm toying with the idea of using it with no bracing at all

The idea of doing this was to a) make it much easier to constuct, and b) reduce the weight, from what can be acheived using Nomex with epoxy glue. I'm not being very scientific about this since I don't have a "double-top" (with Nomex) to compare to (yet!). When I have some real numbers on the relative weights and stiffness I will post them. Also, I'll post some photos later tonight.

Should be interesting to see how it turns out. Has anyone already done this I wonder?

Cheers,
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"

Dave,
[Has anyone already done this I wonder?]
Yes, Frederich Holtier use to do this. Now he uses Nomex.

Dave,

The one double top I built I considered not bracing as well. It felt like a braced top in my hands. Normally I use 7 fans Torres style but on this one I used 5 and they were light. One day I will probably try another one. I wasn't exactly disappointed in my first one but I didn't get the Oooo Awwww that many people get from a DT guitar.

I've been thinking of trying one that is complete Nomex, that is no bridge patch or other wood patches. I wonder if that's been done before and if not then why not?

Hi Gary,

I looked up Frederich's guitars - he has a good website - but could find nothing about his methods/thinking or his progression from all-spruce to Nomex double-tops. Can you fill in with any details? How did he cut the honeycomb in the spruce - does he have a laser? I'm going to email him directly and see if he will help me understand his journey to where he's at today. I'd like to hear of anyones' experience with using a honeycombed piece of spruce (or balsa? or anything else for that matter) for the center layer. BTW, I'm doing this for steel string acoustic OLF-SJ style guitar to start with.

Cheers,
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"

I wondered about doing that too. By my thinking, once the honeycomb center (spruce or Nomex) is glued to the top/bottom layers, it effectively becomes thousands of tiny I-beams which are locked together by the hex structure and therefore can't bend/twist easily - that's what makes these tops so stiff in the XY plane. However, the string tension puts a lot of pressure on the Z direction (across the thickness of the top) where the honeycombing doesn't help. There's a good chance the string tension will crush the top if is left honeycombed under the bridge/bridgeplate area IMO. Hey, it's worth a try though!

I think around the soundhole it just makes aesthetic sense to have a solid center layer, and it's easier to inlay a rosette into a solid piece of spruce too. Do you agree?

Cheers,
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"

Hey, y'all. I did as Rick suggested. I contacted Sergei and he was kind enough to allow me to post his reply:

Hi Darren,

I've made 5 or 6 guitars with hardwood for the top layer of sandwich tops and they've typically sounded great! For me that's the whole point of sandwich tops--to allow me to have something exotic for the top and still get a good sound. I don't generally do sandwich spruce tops anymore.
That bubinga top guitar had a nice rich sound. I've never tried nomex for the middle layer of a sandwich top--usually balsa, spruce or cedar. On steel strings I've generally made the hardwood top layer .030", middle layer .055 and bottom layer .030. When I made spruce top steel strings with sandwich tops I kept the top layer about .045"--the other 2 layers the same.

Bye for now,

Sergei


How cool is that to get the man himself to let us in on his method!

Dave,
I suggest you email Frederich and ask him directly. He is extremely approachable and generous with his knowledge. This was the account he gave to me when we had tables near one another at the GFA two years ago.

Dave,
some helpful info can find here http://www.delcamp.us/viewtopic.php?f=1 ... 2&start=15

regards

Thanks for your follow up Gary. I'll review those issues you mention and give Randy a call. With the steel strings I've been leaving the tops a bit thick overall, something I'm now moving away from. I wonder if that is a factor in my failure to notice the top-skin dominant issue. Another thing I'm going towards, on a suggestion from Pit Juraschek from Germany, is to not run the bracing into the linings at all. The tops are quite strong enough to not need bracing for strength but do need it for control of tone, so this makes sense. It is easy to overbrace the double tops and my thoughts is that if you free up the perimeter bracing you are simulating the perimeter sanding which is not really an option or needed with double tops.

One of the puzzles to me with the DTs (no, not those DTs) is the projection business. They have an apparent tonal presence similar in the front and back rows of the auditorium. I don't understand that from a physics point of view, but it seems to be well documented, especially in the classical world. Simple volume measurements through out the auditorium don't support this, the volume still drops off as a function of the distance, so what is going on. Like you say, " A fascinating question!".

Laters,
Alan D.

Yeah it could be worth a try. I would probably inlay an edge around the sound hole for aesthetic purposes and maybe just fill the honeycomb in with epoxy for the rosette. I'd also keep a nice simple binding in case I need to remove the top It would be easier to inlay in solid wood but it would be a heck of a lot easier to just glue up full sheet of wood-nomex-wood.

I had started a thread here a while back about the idea of suppliers selling premade double tops. Most luthiers it seems tend to want to have control over it. I hardly buy premade anything but in the case of double tops I'd love to have access to premade tops just to experiment with and not have to deal with the glue ups.

Dave, did you ever get a response from Frederich? I'm curious to know why he is now using Nomex.

Also, if most of this is about weight. Couldn't you use a drill to hollow out ever so small circles on the bottom layer and glue the top layer over this thus negating the need for both Nomex and/or a middle sandwich?

Hi Darren,

I haven't found time to email Federich yet!! I'll get onto it soon, promise!

The asymtote of what you are proposing is just taking your standard spruce top and lightening it by drilling it out in places - that doesn't work, otherwise we'd all be doing it already, right? I'm sure that it's not just low weight that we're looking for here, but high structural stiffness WITH low weight.

The honeycomb (either Nomex or little laser-cut hexagon's in a layer of spruce like I'm doing) in the center of two thin sheets of spruce produces a very rigid structure. You can consider each wall of the hexagons to be a teeny-tiny I-beam, so when you add up all the hexagons and consider that every facet is joined at 60 degrees to another section, the net result is a VERY strong structure indeed. If you leave off the bottom layer of spruce, then the teeny-tiny I-beams become severely weakened - they become "T" structures which are nowhere near as strong as I-beams.

Cheers for now,
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"

I've been visualizing a top plate of normal thickness and upper bout bracing as normal but with a Nomex patch as the bracing. Or even fan braces made of spruce-Nomex-spruce.