The Double Top soundboard allows the guitar to be more powerful while broadening the tonal palette achieving deeper basses and colorful trebles. For the player this becomes apparent when evaluating it with a conventional guitar in side by side listening comparisons. The differences are immediately apparent in favor of the Double Top and I have yet to find a player who wouldn't agree.
How loud is the Double Top? In a concert setting, I think the balanced tone can seem louder because. The advantage of the double top concept allows the luthier to change a number of combinations that are impossible in the solid top and this can contribute to more power. In general though the luthier cna make any guitar design louder by using fairly well known structural and design options. THe difficulty is in retaining quality tone and I feel that the double top has broader latitudes when asked for more volume. I think the best of the lattice braced Australian School guitars have more room filling power, however the Double Top is very close. In terms of quality sound however, the Double Top has an excellent natural sound following the expected characteristics of Cedar or Spruce that will speak musically to the audience. Finally, you as a player may find that the Double Top is more pleasant to play behind because you can hear the guitar well, especially with the acoustic port while the tone production is clear and well balanced. Color tends to warm/neutral and is readily available for musical interpretation.
Why does the Double Top work? It is conjecture on my part, but it seems apparent that the soundboard can reach higher amplitudes than a conventional soundboard because of less internal friction. I have encouraged technical acousticians to study this issue by supplying them with a variety of samples for audio analysis and will hopefully be able to offer a more cogent theory in the future.
Double Top Construction Notes
The Double Top soundboard is two thin .6mm soundboards with a Nomex honeycomb core glued with Polyurethane or Epoxy glue in a vacuum frame. Nomex is a resin impregnated Aramid fiber in a paper honeycomb form. Hexcel is a technical site where you can learn more. I have used 1/8" and 3/16" cell diameters purchased from George Spaar at Aerospace Composite Products. The 1/8" cell is most commonly used.
The picture at right shows the 1/8" (3mm) cell Nomex after it has been vacuum frame clamped and glued to the inner second .6mm skin with Thixotropic Epoxy glue. You can easily do quality assurance on your glue joint. Also when you sand it for any thickness graduation you may have in mind, the gluing quality comes self-evident.
Shown below is a device inspired by Luthier, Per Hallgren from Sweden where a router is used to remove materiel down to the .6mm thickness. The template shown above provides the pattern for the internal pads. The soundboard sits on a vacuum plenem and holds the soundboard very flat helping to assure accuracy.
The next step in making the double top soundboard is to glue on the outer soundboard, which is also .6mm thick or so. I like to provide a bit of sanding room for after the binding process, etc. You have to be careful with brace gluing as believe it or not you can get some pretty healthy imprinting.
Below is a picture showing how I cut the Nomex accurately. I use two identical templates made from particleboard. I place the Nomex between the plates and cut with a vertical slice with a sharp Exacto or shop knife. Nomex doesn't mark very clearly and it is a little difficult to shape. The templates solve this problem.
The hard points or internal pads are very simple and are used to assure positive gluing of stress points such as the bridge. Previous to the design shown above I have also used a Cedar or Spruce strip about 3" wide from the lower bout up to the upper bout, tapered in width to avoid stress risers along the grain. A rosette pad is included inside the two skins.
I use a vacuum frame to "clamp" the Nomex and internal hard points onto the inside skin. This is done flat. When cured, I make sure the assembly is all the same thickness by running it through the thickness sander to get the dimension I've decided on. This is why the preliminary Nomex thickness isn't all that critical. At this stage, I hand thickness the back Nomex plate to the graduation one would normally look for on a regular soundboard. In my case, I taper so that that the final finished soundboard tapers from 2.6mm at the center down to 2.0mm at the perimeter. I leave the upper bout at 2.6mm. I restore the clean, glued edge of the Nomex by sanding down in grits just as I do on wood. This way the Nomex isn't fuzzy and will likely make better glue joint. Then the soundboard is placed face down in my domed work surface and vacuum glued. Again, I apply the glue to the Nomex, not the Cedar or Spruce skin. I'm familiar with vacuum bagging/clamping, as I have been using it in other woodworking fields for about 20 years.
Wood Selection: I choose my soundboards just as I did previously and with as much care. I don't know if taking the top to about 1/32" makes a difference in how things work for sound, but I just assume they will.
Bracing: I Use my normal bracing, perhaps a bit lighter depending on where I want to finish up on the thickness of the lay-up. My first one was at 3mm and thereafter is 2.5mm tapering to 2.0mm due to some hand sanding. These are thicknesses typically found in conventional solid soundboards.
Durability and Repair: This is the obvious concern for any new development that hasn't stood the test of time. I have been making Double Tops since 2001 and haven't had any issues to challenge my confidence in its longevity. All the materials in use have estimated archival lifetimes of at least 80 years. During my research doing destruction testing I was amazed at how durable the soundboard would be as it took a good deal of abuse. I would liken the soundboard to ripstop nylon where the Nomex cell acts like an octagonal cleat in all directions at 3mm intervals. Because of the composite nature of the soundboard, I would expect it to withstand humidity issues better than solid soundboards as well.
I do guarantee the Double Tops along with the rest of the guitar for my lifetime as a working luthier.