Randy Reynolds Healdsburg Guitar Festival seminar paper on double top guitar soundboard construction using thin tonewood outerlayers and and a nomex core, with discussion of construction methods [Picture] - created 04-08-2006

Reynolds, Randy - 04/12/2006.22:25:28

The original motivation for the Double Top was as a direct response to the Lattice braced guitars of Greg Smallman which pretty well challenged classical guitar paradigms (can one still use that word?) As an experiment to make sufficently strong lightweight soundboards the Nomex composite with two thin wood plates yielded a natural sounding instrument which the Smallman definately was not. The Double Top did not equal the lattice braced guitars in volume but did produce somewhat more power than conventional solid wood versions depending on the luthier of course.

While most of us value the slightly lighter weight of the Double Top in helping out with tone production, it is my hunch that there is less internal friction within the plate itself and this possibly helps increase the amplitude acheived under string energy.

Other advantages seem to be a soundboard that is less susceptable to humidity caused splits and also they may suffer less damage in accidents than their solid top cousins due to the "ripstop" nature of the Nomex. Just anecdotal evidence so far but encouraging nontheless.

The downside of Double Tops is that they are dependent on good construction processes as Al mentioned and of course they have only been around for about 11 years so standing the test of time is still underway.

By the way i did play a couple of Jim Watt's Double Tops in a guitar shop in Santa Fe last week and they are pretty terrific. Aside from being an expert in composites, Jim is a heck of a luthier.

Why a Double Top?

What has arrived as the primary reason for me to make Double Tops is the sound. These instruments seem to produce excellent balance from string to string and fret to fret over the entire fingerboard. To date I have never had a wolf note or a weak tone on any of these guitars. That alone is enough to keep offering them. In addition the sound is very natural, possessing a lot of presence....in other words, you just hear a lot more guitar as a player. finally, the tone is full with excellent character even in the upper treble register which is frequently a weakness of the nylon strung guitar.

The power coming from the Double Top can be very impressive and it seems to be able to generate really excellent volume without becoming harsh or edgy, or at least it might have somewhat more "headroom" than conventional soundboards. When you ask for more volume the guitar can deliver it with the result that the guitar finally does have some usable dynamic range. Most guitarists haven't developed the technique of varying loudness in their playing because there is very little volume difference between normal and loud in the conventional guitar as we have come to know it. The Double Top guitar will just make more sound when you dig into it and it can make useful noise with a very light touch as well.

I would comment that I have very little experience with steel string Double Top guitars and my description above relates primarily to nylon strung instruments. Charles Fox, who was on a panel with me at Healdsburg last year demonstrated a braceless Double Top steel string guitar (!) called the "Ergo" that suggests there is more to come in the Double Top story.

I don't want all your trade secrets, but it might be interesting to experiment a little. So one would buy a sheet of Nomex, some spruce, and some cedar, and have at it? Just plain old epoxy for the glue? or will it stick to other glues as well? I presume you need a method for maintaining uniform glue thickness such as a guitar version of a notched trowel? Thin both plates to about .6 mm and glue? Is vacuum clamping required or will lots of go bars do the trick? What is the non-nomex area under the bridge and soundhole areas and how are those thicknessed to match? Brace as if it were a regular top?

I'm always amazed at how open and willing to share info you pro builders are toward us amateurs.

Bill, I do have a paper which I presented at Healdsburg and I would be happy to post it somewhere at MIMF if that's acceptable. It isn't exactly a step by step procedure but it can get you going. The problem with answering all of your questions is that it is a bit beyond the scope of this thread. I have helped a number of luthiers get into doing Double Tops so I'm not trying to keep anything secret rather your interest is appreciated.

I just did email the file to Deb although it is not in a PDF format. I want to comment that it is not a step by step instruction guide but with some experience under your belt you should be able to sort things out. I am always available to help out if things are not clear.

I just strung up my first steelstring Double Top and it makes a similar difference there as it does with classicals. Very open, responsive, rich and well balanced plus all the other things one could want in a guitar. I have not made that many steel strings, perhaps 25 in all but this one is substantially better than anything I've made to date.

OK, it's now in the Library as a Word file, if folks want it to be available as a PDF I can still do that.

Hi Randy,

Thanks,

Enrico

There are two methods for the edge of the soundboard and I have gone back and forth. Mostly I do glue the Nomex all the way out to the edge and install purfs and binding normally. In this method the Nomex is exposed when you rout the ledges. Yes, I know but it works fine. The other method is to glue a 3/8" rim all around the edge and install the Nomex inside that so that when the ledges are routed you don't expose the Nomex edge. If you choose to use the router method of thicknessing then it is pretty easy to provide for the 3/8" edge around the outside. I hope this is clear enough

I do have pads under the bridge, around the rosette area and under the fingerboard. These pads are sanded to the same thickness as the Nomex and are there because the laminations assure solid gluing under the stresses in those areas and because there is minimal impact on sound. My friend Per Halgren in Sweden simply routs smaller pockets in the prime tone producing areas of the soundboard so he has a lot more solid wood left if that appeals to you.

You do not need vacuum to clamp the Double Top as you can use a go bar deck with a full size padded caul or any other method you have confidence in. Both David Schraam and Dake Traphagen have used this method or something similar as did Fritz Mueller for a while. Epoxy works with you here as it doesn't require a lot of pressure in the lay-up. Gernot Wagner told me that he finds that Polyurethane (Gorilla Glue) only needs clamping pressure halfway between epoxy and the PVA glues. I had always thought that it need quite bit more than PVA so use your own judgement here.

Randy, I'm still unclear about the domed version of the top layup.

I can see that the inside skin must be glued to the Nomex on a flat surface so that the resultant product can be run through a thickness sander.

If the top will be braced on a domed work surface, is this when you will also decide to glue the outer skin to the nomex/inner skin on a domed workboard?

What is the recommended 'lumber' for making up the vacuum press board. Would it be MDF, or melamine coated particle board? If particle board, do you paint the air channels on the board in order to get less leakage?

What is a recommended way of inserting the NPT thread into one of these boards? Does one drill an undersized hole and the let the flange self-tap?

I use a vacuum press as described in the LMI catalog. It has served me well for gluing the double tops

Henry,

I do the first and second layups in a 25' dome hollow form as it will still come with only a slight dome. The thickness sander will flatten it without problem as the plate wit the Nomex attached is very flexible at this stage.

"Vacuum Press Board" I assume you mean the vacuum frame. In my case I built a frame of pine and sealed it with Shellac. I took some care to make the joints very tight and also chalked suspicious looking areas. The valve was inserted in a bore chalked in with silicone seal. This was a prototype so I didn't spend a lot of time looking for the best materiels, however it worked very well anyway and I've now used for all the tops and backs on every guitar to come out of my shop for over five years. It looks like all of my jigs and fixtures....kind of dog eared. If I were to build another, I would look for better stuff such as the melamine you suggested.

As Per mentioned, the LMI idea works fine. I also use vacuum for bridge clamping and a number of other tasks in the shop.

I have always used a continuous duty pump. A reservoir system would be nice but I haven't found it to be something I want to invest time in. As I mentioned in my paper, make sure you find a pump with plenty of "headroom" or free cfi as it makes up for a lot of leaks.

Thanks Randy. You answered my questions and perhaps also cleared up a misconception about vacuum clamping.

My misinformed question regarding 'vacuum channels' stemmed from my idea that the workboard also needed to vacuum clamp the layup from the bottom. I see that it is really not necessary when using a vacuum frame such as yours.

I'm intrigued by the use of bicycle valves as vacuum fittings. Do people prefer the thicker Schrader valve over the Presta valve, and I'm curious as to what inner diameter tubing will work with either of these valves.

Also, when attaching such a bicycle valve contraption directly to the vacuum bag, and not the frame, what is the attachment technique. It seems as though the valve and a bit of the inner tube is rubber cemented to the vacuum bag. Is that about it?

There's also some good pictures of double top construction here:

http://www.trentonscott.com/deerheadHome.shtml

Click on soundboard.

I'm a bit confused as to what gets sanded in the thickness sander. Is it the wood or the nomex honeycomb?

Also, when using the router method to thickness a full thickness soundboard, are the internal pad and the outer ring left the full thickness sort of like a plauteau? Are the plauteaus (internal pads)left on the internal or outside skin?

Henry, I'm not sure what these valves are as I have always used the attachment I got from the vacuum bag vendor. It's pretty simple really and mine has worked flawlessly on the vacuum bag for many years. The frame uses a simple barbed fitting available from the same vendor.

Chris, Larry Breslin lives in the same town I do and I was able to show him my processes. Larry being a resourceful type has developed some traffic with the Double Top as evidenced by Trent's website.

I hope if you read the paper that the answers to your questions are in there but if not..........The first two plates are sanded to a design thickness (.6mm)as is the internal pad (the thickness of the Nomex you'll be using usually 1/8"). The pad is glued to the outer soundboard along with the Nomex. After curing they can be run through the sander once abain to establish the desired thickness less the second soundboard. The graduation of the soundboard is accomplished by block sanding the Nomex as you wish. Once you add the final soundboard, you are at the final thickness.

In either method you may choose to do the outer or inner soundboard first to glue the Nomex to it. I prefer to do the outer soundboard first as it is the most critical for being able to proof the adherance of the Nomex after gluing

In the router method, the pad and outside edges are left at the thickness of the soundboard less the final inner soundboard thickness which will be added after the Nomex and graduation are complete.

Here's a picture of a double top under construction showing how I deal with with the sound hole and binding areas. I think that this is pretty similar to what Randy does except he uses more core I believe.

Oops, forgot the pic.

here it is.

So Jim, how do you route or carve a pocket for the nomex in a plate that thin?

The thick piece of mdf there is actually a vacuum chuck which holds the plate flat while I route the pockets. Underneath the top in that picture is a an o-ring type seal for the vacuum, and a vacuum port on the other side. the seal is made from 1/8" dia rubber cord stock, neoprene I remember right.

Jim,

Do you have any vacuum holding the inner portions of the plate also? I'm assuming that the o-ring seal is around the perimeter. Also, what type of router bit are you using?

Hi Randy,

The vacuum chuck in the picture has a couple of locating holes in it as does the top, these are evident from the "ears" in the top. The top pins into the chuck and the template also pins into the chuck locating everything very accurately.

BTW, my building form has the same set of pin locations in it.

I bet I got some of this from you however:)