Wednesday, June 3, 2009

I Killed the Tap-Tone!

But I did it on purpose.

There are two concerns in building a guitar top. One obviously is musical--you want the top to vibrate like crazy creating sound waves which we hear as music. For this, you want the top to be as light and as thin as possible. As I thicknessed the top I kept tapping it with my knuckle, listening as it changed from a short, dull tonk to a ringing thuuuuuuuuummmmppppp that lasts for about two seconds and has low, middle, and high overtones. It also gave me a sheet-metal-like woof when held by the long edges and gently shaken.

However, once it was thin enough that these tones came out, I had taken it past the point of structural integrity. Add steel strings to the top at this point, and it would break apart probably as soon as I brought the string tension up to pitch. To restore the structural integrity I glued braces to the underside of the top. (I was going to say "back of the top" but that gets confusing.)

Now it just goes tonk again, but even more pitifully than before. The braces just kill the tone. The top is strong, but it ain't makin' much music. So now I have to restore the tone without sacrificing the strength, and I do that by shaping the braces, particularly those on the lower bout.

Before I do that I have to glue the bridge plate onto the top. This is a piece of hardwood--in this case rosewood--that is on the underside of the top directly under the bridge. Steel guitar strings have a little steel ball on the end of them that goes through the bridge to the underside of the top, and the strings are held in place by bridge pins. However, the spruce is a soft wood and the metal balls would quickly tear through the wood and start to wear down on the ebony bridge. So a hardwood bridge plate of rosewood or maple is glued under the bridge, and the ball-ends bear against this.

Here is the bridge plate after it's been cut to fit between the braces:

It gets glued on using the go-bars. And here is the top with all the braces glued on, ready for shaping:

The back isn't quite done either. It gets a center strip of spruce with the grain running width-ways. It's purpose is to reinforce the center glue joint. Sometimes one long strip is glued on before the back braces are glued. This requires cutting notches in the strip where the braces cross the center strip. I did that the last time. This time I tried gluing the braces on first, then cutting the strip into sections that fit snugly between the braces, and gluing them this way. Either way is acceptable, but I find I like this way better. Here is the center strip sections being glued:

Done! Isn't that zebrawood gorgeous? I sure hope it bends well.

And here are the back and the top, bracing complete, ready to be shaped.

Before I do that, I'm going to start the process of bending the sides. Each side is done separately and there is a couple of hours of downtime while I have to wait for the side to cool after bending, so I'm going to use the downtime to shape the braces. So it'll be bend one side, shape the back braces and start on the top braces, then bend the other side and finish shaping the top braces.

But more on that later.

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