The Bottom Braces

The braces are a stabilizing element in the body. They help define the voice as well as physically re-inforcing the structure.

The braces are first marked out, using a ruler which has been bowed using a strong string as in the illustration. This yields a close approximation of a parabola. The parabola is an interesting engineering structure, as its mass diminishes outward from the center in exact proportion to the amount of a uniform load bearing at any distance from the center. Acoustically, this means no extra wood to slow down the energy, and maximum strength from the wood that is there.
Next, each brace (there are four in the hourglass, three in the teardrop model) is marked to length, rabbetted to accommodate the bottom lining, and carving proceeds. The aim is the produce a shape which is a parabola both longitudinally and laterally. We use miniature draw knives and scrapers.

Now the braces given a slight rounding on the bottom surface--1/32" sagitta is plenty--and tested together to have the same bottom profile. Then they are glued into the bottom of the body. These joint are critical. Of all musical instrument repairs, loose braces are the most preventable.