Double Top Constr: Translating it into Lay Language

[printer2]

Something like this.

[asobi]

Dusting off an old thread becuase I'm genuinely curious. In the classical/nylon guitar world, the double-top construction seems to be a real "thing". Pioneered by the German luthier Matthias Dammann in the 80s, it seems, with other veteran and highly-regarded builders such as Michael Gee in the UK using this type of construction.

Wonder why it hasn't caught on as much in the steel string flat-top world, given some of the theoretical advantages in this thread. I'm going to guess it partly has to do with the difference in the string tension between steel v. nylon, but can stand to be educated by the experts!

[charles Tauber]

Originally Posted by asobi

Wonder why it hasn't caught on as much in the steel string flat-top world, given some of the theoretical advantages in this thread. I'm going to guess it partly has to do with the difference in the string tension between steel v. nylon, but can stand to be educated by the experts!

My understanding is that the attraction to a double top in classical guitars is that it gives the performer more volume and projection than a traditional classical guitar, allowing it to better fill an unamplified concert venue. The trade-off is a somewhat different tone, one that is often more lute or harpsichord like.

There are, of course, steel string guitars made with double tops, but filling an unamplified concert hall isn't usually a primary goal for steel string players.

More often than not, double-top construction uses a lattice bracing or other non-traditional bracing arrangement. That brings with it a somewhat different tone. For there to be wide-spread adoption of double-top construction in steel string guitars, there would need to be a perceived (relevant) advantage. That advantage would have to be increased stability, greater longevity or "better" tone. I'm not certain that any of those have been shown to be true for a double-top on steel string guitars. So, for now, anyway, they are largely seen as an interesting new technology/design element, but not terribly relevant. They are more work to make, and hence, more expensive for the consumer. The consumer would need a good reason to pay more for it.

[asobi]

Originally Posted by charles Tauber

More often than not, double-top construction uses a lattice bracing or other non-traditional bracing arrangement. That brings with it a somewhat different tone. For there to be wide-spread adoption of double-top construction in steel string guitars, there would need to be a perceived (relevant) advantage. That advantage would have to be increased stability, greater longevity or "better" tone. I'm not certain that any of those have been shown to be true for a double-top on steel string guitars. So, for now, anyway, they are largely seen as an interesting new technology/design element, but not terribly relevant. They are more work to make, and hence, more expensive for the consumer. The consumer would need a good reason to pay more for it.

Super helpful, thanks, Charles!

[Rosewood99]

Has anyone played any of the Eastman double top guitars?

[Dwight]

On the one I had the top surface was very thin and easily damaged. It was a very responsive guitar with lots of sustain.

[redir]

It's typically not as simple as taking two sheets and laminating them together with the Nomex. Usually the top is either built up with center pieces of spruce and Nomex or the top is routed out pockets. I built one in the ladder method. You take a regular top and rout out two D-Shaped sections in the lower bout leaving a full spruce section down the middle. Then you inlay the Nomex into the pocket and then laminate another spruce or cedar sheet on top and thickness everything so that it is very very thin. I forgot the specs but it's paper thin.

The Nomex sandwich top felt like a braced top it was so stiff. I braced it upa as usual and the guitar came out ok, not great but ok. Then I shaved some of the braces and it livened it up a bit but in general I would agree with Somogyi's assessment. You get what you get with a double top guitar and don't have a whole lot of control over it.

[printer2]

It is more work and we have the horsepower in steel strings that nylon does not. Not really a need for them.

[Scotso]

I have never heard of this for guitars. But my wife was a national team lwt rower many eons ago and still rows a single. So we have bought many boats for her over the years. This nomex sandwich is common in race boats as it is light. Usually it is a sandwich of carbon fiber on both sides or carbon fiber and Kevlar. But it is fragile. Denting is a huge prob as is delaminaton and softening over time(getting floppy, losing rigidity). Many North Am builders have moved away from nomex to other building techniques. Given my experience with the stuff I would be hard pressed to even consider it in a guitar .

[815C]

The first time I'd heard of a double top was when I was researching what guitar David Russel played as I loved his tone. It turned out to be a double top. I'm sure a lot of the tone is in his hands, but wow what a great sounding instrument.

[asobi]

Yeah, much more prevalent in the classical guitar luthiery. I was mainly prompted by the double-top guitars by Michael Gee in UK. One of the most phenomenal talents in the classical music world - Sean Shibe - now plays a Gee double-top. And the tone and the dynamic drama he drew out of his Gee in a recent Wigmore Hall recital (now taken off the website) was transcendent. One of the best solo guitar recitals I'd heard - he broke out his electric for Steve Reich for the second half!

[Alan Carruth]

The 'up' side of this technique is in the weight saving; the top plate itself can be about 40% lighter than a solid wood one of the same stiffness. When you don't have much power (which plucked strings don't), and especially when what you have lacks in high frequencies (which is true especially of nylon strings), that's a big advantage.

Many classical double tops seem to use more or less standard bracing. 'Lattice' bracing, pioneered by Smallman in Australia, uses alattice of carbon-balsa I-beam construction to take up the torque load of the bridge, with a thin (.6-.8 mm) membrane, often of redwood on Smallmans, on the top to move air. He also uses a number of other methods to 'keep the sound in the top'. The result, if the example I saw was typical, is something that has much in common with a resophonic sound, at least in a small room, but it certainly has power.

The Achilles heel of double tops is probably the glue line between the core and the skin. As Tim points out, there's not a lot of area involved, and no room for error. Tests that I've made with deliberately 'bad' glue areas in such a layup suggest that it's very difficult to detect such issues in wood structures. In aircraft this sort of structure is was not allowed until a method was devised (using an ultrasonic transducer) to locate such voids an repair them, but I'm not sure that would work in a wood skin, even if we could afford the equipment. I also found that long term vibration of a structure with such a void had the effect that Scotso noted: progressive delamination. Although I did not test to destruction I suspect that once such a void reaches an edge the failure would be catastrophic. Until then it can be very hard to detect. I'm told by one experienced repairman that this has happened on a few sandwich topped classicals that he's seen.

[justonwo]

You just have to play them. The theory doesn’t matter too much in the end. I’ve sent Ervin a number of emails over the years about the physics he cites in his book. Although well-intentioned, he’s gotten a number of things pretty wrong. Huygens wavelets comes to mind.

Despite the theories driving them, I have yet to play a double top that didn’t sound weak and spongey to me. New and different can be fun, but the old recipe works incredibly well.

[BEJ]

Originally Posted by billgennaro

This is an excerpt from Ervin Somogyi's great book "The Responsive Guitar":

"Double tops are based on the I-Beam principle in that the load-bearing work of any structural beam is done by its upper and lower layers only, and that the material in between (the “neutral axis”) exists merely to connect the stress-bearing top and bottom sections. A double top consists of two thin skins of spruce separated by a middle layer of balsa wood or very lightweight space-age honeycombed plastic. Such faces are very lightweight for their stiffness. The upside is that one obtains superior stiffness to weight material. The downside is that there’s no room whatever for sanding, shaping, voicing, thicknessing, or error in making the plate. What you initially make is what you get because you can’t shave, scrape, sand, tinker with, taper, or even easily repair a top once its made in this fashion."

Hope this helps.

The mention of balsa wood for the inner core makes me wonder if anyone has tried it. From my rc modeling days I know some balsa wood it harder than hell and very stiff, you just have to sort through a lot of it if possible to find these pieces.

Bruce,

[Carey]

The nylon-string double tops I've heard have *definitely* been louder than
average, but all had an unfocused, muddy sound in the trebles. The basses
have had a distinctive sound too; not necessarily bad, in that range, but I'm pretty sure I could could pick one out in a blind test. These were top
makers of these instruments, btw.