Questions: Vaulted Back Guitar

by Sjaak Elmendorp

Originally published in American Lutherie #97, 2009

AG from the Internet asks:

Any tips on building a guitar with a vaulted back, such as the Baroque guitar in Plan #27? Little information is available on the precise shape of the back and effective ways of constructing it.

Sjaak Elmendorp of Nieuw-Vennep, The Netherlands replies:

After having made some steel string and classical guitars, I wanted to try something a little more involved. I bought a plan of a Baroque guitar with vaulted back, such as GAL Instrument Plan #27 drawn by Bruné. The plan provided only scarce information on the shape of the back, so as a novice to this field I was left to my own imagination. I must not be the only one, as there seemed to be a variety of schools of thought on the subject. Among professional builders there seems to be consensus that the guitars were constructed to have backs that have the same curvature across the width of the instrument, i.e., the cross sections resembled part of circles. From there on, it was a matter of combining the beautiful and ancient design with some modern mathematics.

Using the location of the back braces along the centerline as position indicators, the width of the back and height of sides and center of the back was taken from the plan at these points. A simple calculation in an Excel spreadsheet allowed the radius of curvature of the back to be calculated from these three data points at each back brace position. The shape of the cross sections of the back were calculated and printed. (Note: the formula for this calculation can be found in John Sevy’s article in AL#58 p. 42 and BRBAL5 p. 355.) I am happy to make the spreadsheet available to interested readers. It is available on the Extras page of the GAL website: www.luth.org. (Look for “magazine extras” under the “publications” menu.)

The cross sections were made out of plywood, to serve as a mold. The spruce braces were bent and attached with a few small short nails to the plywood. The ebony (10MM wide ) and maple (3MM wide) strips for the back were then cold bent into shape and glued on the braces using rubber bands and a few small clamps.

The sides, also consisting of alternating ebony and maple strips with reinforcing spruce braces to provide cross-directional strength, were made on a mold. A heat gun was used to bend the strips into shape.

It is now strung up and it looks, plays, and sounds correct. Although I see many areas for improvement, the back looks all right.