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Featured Luthier: Fritz Mueller
Introducing Doubletopsby Fritz Mueller
This is an exciting era for classical guitarists and luthiers, as many new and very interesting innovations continue to be incorporated into their favorite instrument.
One of the latest - and perhaps most important in several decades - is a new style of top construction, commonly called doubletop, with significant improvements to the responsiveness and volume of the instrument.
Doubletops (or composite or sandwich tops, as they are also often called) already come in many configurations, but the defining feature is that the interior of the top contains numerous small air pockets. If you think of a hollow-core door, you will understand the basic structural concept of the doubletop; both consist of two relatively thin skins of material separated by a light, porous substance to maintain thickness and stiffness. But while few people will be concerned with the musical nature of a hollow-core door, composite construction has several qualities that make it very desirable for guitar tops. Chief among these is lightness. A doubletop can be 50% lighter than a traditional solid-wood top, and thus respond to the motion of the strings much more readily. For the sometimes anemic, usually underpowered classical guitar, the musical benefits are profound - greater volume and projection with a true dynamic range, much greater sustain, and vastly improved responsiveness to right-hand technique.
Luthiers have worked out several ways of creating the air pockets inside the doubletop. One method is to drill out the wood from the inside in a regular series of holes, and then enclose it with a second thinner layer glued to the inside. Another way is to make a grid of small strips of wood (perhaps of balsa, because of its lightness), and then enclose the strips in two thin outer layers. A third method, and certainly the most "high-tech" approach now used by many doubletop builders, is to enclose an ultra-light material called "honeycomb" in two thin skins of wood.
Honeycomb was developed in the aerospace industry, and has been used for many years in products such as skis and snow machines where its ability to be molded into light, stiff membranes is desirable. Honeycomb comes in many varieties, but is typically made from a paper-like synthetic formed into hexagonal cells. It is appropriately named, as it is very similar in appearance to the honeycomb in a beehive. By itself, honeycomb is flexible and soft, but when placed into a sandwich with (in the case of guitars) skins of wood, it forms a composite structure that is both stiff and remarkably light.
All luthiers, no matter what method of construction they use, manipulate the mass and stiffness of their materials in order to control the sound of their instruments. Slightly thinner here, slightly thicker there, add a bar here, carve the bar there - this is part of the luthier's daily routine. Stiffness is controlled chiefly by varying the thickness of the material, which in a doubletop means varying the thickness of the sandwich. Unlike a traditional top of solid wood, however, the honeycomb is so light that great differences in thickness (and stiffness) mean a change of only a few grams in weight. For a luthier accustomed to wrestling with the stiffness and weight trade-offs of solid wood construction, this is a gift from the gods.
Not only does composite construction produce a top that is light and stiff, it also produces one that flexes somewhat differently than one in solid wood construction. This aspect of doubletops needs much more research, but my impression (after constructing 30 + doubletops) is that the anisotropy of the top (the difference in stiffness along and across the grain) is greatly reduced, if not eliminated. Also, it appears to me that what I call the spot flexibility of the top - its ability to vibrate in small patches, while otherwise maintaining its overall stiffness - is significantly higher in a composite top than in a traditional solid wood instrument. Both of these qualities mean that a doubletop has a significantly greater ability to respond easily and quickly to a very broad range of notes and frequencies.
This increased flex capability is not without its downside, however, as achieving evenness and balance can be difficult in a doubletop. For example, early doubletops often suffered from wolf tones (notes that are unusually loud and, most often, short-lived). In my own beginnings with composite construction, it took several attempts before I learned how to, in effect- build stiffness back into the top to control its resonances adequately. For all the virtues of composite construction, it still requires the skill and understanding of an experienced luthier if its great advantages are to be realized.
It also needs to be said that in a doubletop, as in any instrument, the final sound is a product of all its parts, not simply the top. Back, sides, neck, bridge - all have a critical role, and the addition of a composite top is unlikely to make up for the inadequacies of an otherwise poor design.
To the best of my knowledge, the originator of doubletop construction in classical guitars is a German luthier, Matthias Dammann. Certainly Dammann's instruments are the best-known doubletops, and are now being used by players such as David Russell, Manuel Barrueco, Scott Tennant, and many others. The use of honeycomb in guitar tops, again to the best of my knowledge, was pioneered jointly by Dammann and another German luthier, Gernot Wagner. Dammann's and Wagner's instruments were the inspiration for my own work with composite construction, as they have been for many other luthiers.
Doubletop construction is quite literally taking off, with broad interest from players and over-the-top excitement by the luthiers who have ventured into this new medium. A list of doubletop builders will necessarily be incomplete, but here are a few of the individuals known to me. I would appreciate it if readers can help me with additional names. Matthias Dammann, Gernot Wagner, and Boguslaw Teryks in Germany. John Dick (Iowa), Randy Reynolds (Colorado), Kenny Hill (California), and Frederich Holtier (Ohio) in the United States. Fritz Mueller (myself) in Canada.
So what is the doubletop sound? Given the complexity of a guitar's acoustics, individual doubletop luthiers tend to create their own sound, the same as in traditional construction. Nevertheless, there are several qualities that I consider characteristic of a doubletop. Words are notoriously slippery when applied to sound qualities, but I will abandon caution and create a list to entice you to explore doubletops further. Here it is: loud, powerful, dynamic, sustaining, rich, warm, colorful, immediate, responsive, natural, beautiful.
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