First Custom "Double Top" Ukulele ever??

Chuck, great questions and concerns! I've got a gig tonight and a family outing this weekend, so I won't be able to address this post respectfully probably until Monday or Tuesday...thanks...e.lo...
 
The final soundboard thickness came out to around 0.110” thick, with a weigh factor closer to that of a 0.080” soundboard. .

Holy smokes, that is a thick top. Even with a strength ratio of .080, I gotta agree with Chuck on this one. Maybe Steel Strings? I would imagine with the structure of Nomex and a DT, the ability to go thinner is there, possibly down to .050 or something like that - then that top would really move (Kasha who?).

Question: Why do the whole routing thing, instead of just a sandwich? If you really needed an edge for a glue joint, you could make a "frame" around the soundboard, and sandwich that as well - at least you know your .060 section stays .060. If not, binding would cover the edge anyway (like the soundhole). I don't mean to question your methods, just trying to get your advice as I'm questioning mine.

In any case, very interesting. Mind if I ask what type of Nomex, and where you get it from? I'm thinking a .030/.010/.020 (Spruce/Nomex/Spruce) sandwich, just for the fun of it - I'll throw it on one of MGM's thinlines, and do the back the same way.

Interesting stuff. - Aaron
 
Good. A while back one of the Guild of American Luthiers (GAL) publications had a great article about this. I'll see if I can dig it up in the time being. Have a good weekend.
 
Holy smokes, that is a thick top. Even with a strength ratio of .080, I gotta agree with Chuck on this one. Maybe Steel Strings? I would imagine with the structure of Nomex and a DT, the ability to go thinner is there, possibly down to .050 or something like that - then that top would really move (Kasha who?).

Question: Why do the whole routing thing, instead of just a sandwich? If you really needed an edge for a glue joint, you could make a "frame" around the soundboard, and sandwich that as well - at least you know your .060 section stays .060. If not, binding would cover the edge anyway (like the soundhole). I don't mean to question your methods, just trying to get your advice as I'm questioning mine.

In any case, very interesting. Mind if I ask what type of Nomex, and where you get it from? I'm thinking a .030/.010/.020 (Spruce/Nomex/Spruce) sandwich, just for the fun of it - I'll throw it on one of MGM's thinlines, and do the back the same way.

Interesting stuff. - Aaron

Aaron, the frame is also another method of this approach already being used. I chose the routing method as used by Dunwell. His was my guide through this process. Yes, the Nomex will allow for a thinner sandwich. The 0.060" is the thinness Nomex avaliable. However, once glued in place you could drum sand it down to whatever. Don't know if you would reap any benefits if it got too thin though. You can purchase it at http://www.lmii.com/CartTwo/Results.asp?searchtext=nomex

Like I pointed out, this was a prototype top to see if it would fly. Found out, it flies very well and the tone and projection (even with the bracing and thickness) is comparable to that of other solid top ukuleles I have built. So there is plenty of room to improve on the ability of the bridge to drive the top via brace sizing or removal and the thickness of the overall DTop.
P.S. Keep in mind that most of the thickness is filled with air. Also you need a solid wood structure surrounding the soundhole and bridge area, as well as, a small foundation around the outside profile of the top for support...elo..
 
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Good. A while back one of the Guild of American Luthiers (GAL) publications had a great article about this. I'll see if I can dig it up in the time being. Have a good weekend.


Chuck, here are some replies to your questions...

Q1: When gluing the Nomex into the routed area and then gluing the final layer of the sandwich on, how are you keeping glue from filling all the voids in the Nomex and turning it into a solid substrate?


I used an epoxy glue that I rolled out on wax paper on a flat surface, spread very thin. The cutout Nomex® is then laid into the epoxy for about 30 seconds so that only the honeycomb area’s are coated with glue. You want to keep the open (honeycomb) spaces free of glue throughout the total top gluing process. The Nomex® is then placed into the routed out Kiaat top piece. A piece of ½” glass is then placed on top of the combo. Note: the Nomex® is slightly higher than the router area to allow full pressure from the glass to press down on it. This needs to dry overnight. Once dries, the combo goes through the drum sander to bring the Nomex® thickness even with the top piece. To glue the bottom piece to the combo piece, you need to again roll out epoxy and laid the combo (top & Nomex®) piece into the epoxy for 30 seconds. Pry off the combo from the glue and place it on top of the bottom piece. Pressure is again applied over night to combine the top and bottom.

Q2: The final top thickness seems rather thick at .110". Are you getting any resilience or deflection in the top?

The top does seem thick as opposed to the final 0.066” solid tops that I have been making. However, a lot of this is trapped air chambers in the honeycomb spaces. Visually speaking this thickness is mostly unnoticeable to the eye from the soundhole. It looks quite nice with the banding of the hole. As for resilience or deflection in the top, I haven’t done my own tested for this. Therefore, I can only give you Dunwell’s test results on his Double Tops. He states, “The mode shapes for a given model of guitar are the same but where they occur and how clean they are is not. What I notice with the double tops is that the modes are very clean and narrow and the frequency at which they happen is very narrow, plus-minus only a few Hertz , and they are very strong, almost violent. Which all says that the plates are much more uniform than a similar solid plate. This is understandable since the Nomex has kind of homogenized out the wood variances.”

Q3: Is it necessary or even noticeable on such a tiny instrument?

This is what I am trying to find out. Only building such instruments will answer this question. I am only one uke into it so it’s way to soon to tell.

Q4: How does it sound when compared to a similar solid top build of yours?

Well, I never used Kiaat for a top before so I can't compare the results directly. Addition this was simply a prototype which allowed me to learn the process of a DT build on a ukulele scale.
But if I were pressed for an overview of the Kiaat’s tonal performance, I would have to say that string tone/volume are well balanced and that the volume level is similar to my other solid tops. The overall tonal spectrum (though clear and pleasant in nature) lies more so in the mid-range area. That is, it doesn’t have an ultra bright or boomy tone to it. Which is good in a way. But I am looking for more projection, more complex highs and a bit more bottom end.
Considering that I used an unlikely tone wood for the top, the results are were quite favorable. But these concerns are premature at this point due to it being my first DT build. It’s a bit like asking why the Wright Bros. first Kitty Hawk flight only lasted 120 feet.
To better address the tonal characteristics on my next DT build, I will changing out the wood; thin the overall thickness more; and remove/change the bracing pattern. I will also begin to set up and run some Chladni mode patterns on each top before I glue it to the body to get a visual on how these small tops will vibrate in comparision to the same solid top before I rout out and modify it into a DT.

I hope I have answered your questions in a clear manner. Thanks..e.lo..
 
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For everyones info, the American Lutherie article is in vol. 93, spring 2008. It is an article on Robert Ruck and has a good description on how he does composite tops. He resisted the idea for quite a while, but when the top players started demanding them, he went along. He uses three laminations of wood, instead of Nomex. I suspect that some of the appeal of this method is that the top does not need a long time to mature, as does a spruce top. That is why western red cedar tops have become popular with classical guitars.

Brad
 
What an interesting thread!! Wonder if anyone has tried or has any updates or newer opinions. Learnt something new about the world of tops.. thanks to this thread.
 
From my experience on double tops, or more specifically me watching others make classical guitars with double tops and asking them and listening myself to the results, all the work that goes into a double top doesn't yield better results.

It's similar to Kasha bracing- why glue on and carve 21 braces when it doesn't sound better than 3 fan braces??????

Also, a 1mm (.040") top wood skin is (and the same on the inside) is cutting it to fine for sand throughs straight into the nomex etc- one tiny scratch or dent and it's all over.

Having said all that, I know it's fun and exciting and very interesting to do such experiments and you should be applauded for exploring this and having a curious mind.

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