Monday, June 15, 2009

Restoring Tone

The braces on the top killed the tap tone. Shaping the braces restores much of the tone--not all, of course, because you can't expect to get all the tone back from an unbraced top that vibrates freely.

What we're looking for is a good balance between structural integrity and flexibility.

The process starts off with marking the minimum heights for the braces of the lower bout. There are minimums for the high spots and minimums for the low spots. I don't want to go lower than the minimums because that compromises the structural integrity. I should be able to hear a good tap tone before reaching that point, and that describes the goal--reaching a good tap tone while leaving as much wood on the braces as possible. Here's what it looks like before I start:

I start with the upper bout and work my way down to the lower bout, carving away with a very sharp chisel. The x-braces I want to just slide under the kerfing of the sides, while the finger braces--the four little ones on each side of the x-braces--and the tone bars--the two lowest one--will end about 3/16" from the kerfing. The upper transverse brace will also be tucked into the kerfing.

Here are the braces after the initial carving.

After each little bit of carving I tap it to hear the change in sound. Not much happens until I get to the lower bout, and then I can really start to hear it. Then I start by sanding each brace, tapping all the while. The changes are more subtle at this point.

Here are all the braces sanded to 100 grit. It sounds really good!

Still a lot of work yet to do. A little more sanding of the braces, then the whole thing, top and braces, get sanded to 320.

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