Double top constructionDouble Top guitars were pioneered by German Luthiers Gernot Wagner and Mathias Damman. Initially, Gernot used Nomex as the core material for his laminations, while Mathias experimented with wood as the core. But by the mid 90's both were using Nomex for the core material, rather than wood. Lighter weight can be had using Nomex.
Dominelli Double Tops are built in this same fashion. They consist of a layer of Nomex (a honeycomb shaped polymer material developed for the aviation industry) sandwiched between two layers of spruce, cedar, or a combination of these two woods, for example spruce outer skin and cedar inner skin. Gernot Wagner has been been very generous, by giving me much advice on the technical aspects of how he laminates and braces his double top soundboards. By sharing his experience, it has saved me much time and experimentation in the development of my own double top sound. Thank-you Gernot!
The nature of the double top lamination makes for a stiff, durable soundboard, both with and across the grain of the wood. Because of this, less bracing is needed. This reduction in the need for heavy bracing reduces the overall weight of the soundboard even more than with traditional solid top construction.
Contrary to popular misconceptions (often the result of bad press by luthiers who do not make them) double tops are very durable. In all of my test glue-ups, the nomex tops were extremely resistant to cracking compared to a usual solid top. And if for some reason the nomex top does crack, for example, from a hit to the soundboard, the crack will not travel far because the nomex stops it from doing so.
In my workshop I have some samples of nomex glued to cedar, to show people how strong this type of lamination is. Nobody has been able to pull apart a cracked piece of 2mm thick cedar with nomex glued to it. The nomex will not pull apart; it is an incredibly tough material.
Here's a quick overview of the process:
This image shows the nomex inlaid into the soundboard, in this case master grade lutz spruce, chosen for its lightweight and stiffness. Water based glues cannot be used, as they would cause the thin skins of wood to curl like a potato chip instantly. Epoxy or polyurethane are the glues of choice here.
The second skin, shown here, will be glued onto the first lamination. Spruce or cedar can be used for either layer, depending on how I want the guitar to sound.
The laminations are glued up in a vacuum press, as shown here. This press was custom made for me, and features a neoprene membrane and a 3/8" thick aluminum platen. It does a great job; fast and user friendly.
No need for messing around with plastic vacuum bags, which tear, develop leaks and constantly need replacing.
This image shows the finished lamination. I took this picture with a 100 watt light shining through the soundboard. The nomex will not be visible in the finished guitar, under normal daylight. The result is a stiffer, lighter soundboard, which is now ready for bracing, and later, assembly.