tone bar design/distribution

[peter havriluk]

I've noticed that there's a general practice of putting two tone bars across the lower bout on an x-braced guitar.

What would be served by using three thinner tone bars instead? To simplify the question, how about installing the same volume of wood among three tone bars rather than two so that the mass remains constant, height remains constant.

Also, if using three tone bars, why would all three need to be the same height? The outer ones would be either closer to the 'x' brace or the rim and their vibration production would be different than the center one. Making the outer two lower than the center would trend toward a smoother gradation of the soundboard's vibrations? Or tapering the height away from the 'x' brace so the highest tone bar is closest to the 'x' and the lowest is closest to the rim?

I'm speculating about bracing on my scratchbuilt projects and I don't want to mimic Pennsylvania's finest guitar designs without considering differing design features. I suspect lots of the storebought bracing design is driven by production economies and sonic characteristics might be secondary, which limitations us highly overpaid scratchbuilders needn't worry ourselves about...

At any rate, I'd like to see if my musings make any sense to readers.

Thanks, folks.

[ken cierp]

Bracing including the bridge itself serve to stiffen the sound board and prevent along the grain cracks -- generally the stiffer the top more treble less bass. So it has nothing to do with appearences, unless you have the wherewithal, test equipment, and parameters to customize and predict outcome its best to copy what you know already works.

[John Parchem]

Stiffness and mass are 2 very important considerations for a top and often they need to be balanced against the other. A brace stiffness is proportional to the cube of its height. Where it is directly proportional to its width. So assume the braces are the same width three braces of the same mass as two braces has about half the total stiffness. If I was looking for half the stiffness I would keep the two braces at a reduced height or remove one tone bar. That way I could get the stiffness at a reduced mass. I am not sure what the advantage of three tone bars in that space would be.

I think many people have the tone bars get a bit lower as they go to the outside. I make my braces all taper when going from the center out toward the edges. If I have two tone bars the outside tone bar end up a bit lower than the inner tone bar.

[Steve Eubanks]

Bracing including the bridge itself serve to stiffen the sound board and prevent along the grain cracks

A brace stiffness is proportional to the cube of its height. Where it is directly proportional to its width.

If one wanted to give up some stiffness (i.e. more bass/less treble), but didn't want to lose crack protection, would that mean lower/wider braces would be preferable? Assuming the width of the braces would more effectively spread the crack protection without introducing as much stiffness.

-- se

[peter havriluk]

When stiffening sheet metal structures, the size of the unsupported areas has an effect on the stiffness (ashcan effect). Sheet metal ain't wood and I can't think of a way to draw structural parallels, but I think the idea of unsupported area crawls into the process somewhere, and three tone bars (same total mass, same height) would make for a less 'bouncy' structure than two. I think. Does this matter?

[John Parchem]

When stiffening sheet metal structures, the size of the unsupported areas has an effect on the stiffness (ashcan effect). Sheet metal ain't wood and I can't think of a way to draw structural parallels, but I think the idea of unsupported area crawls into the process somewhere, and three tone bars (same total mass, same height) would make for a less 'bouncy' structure than two. I think. Does this matter?

Isn't bouncy what makes the sound. The same total mass, same height still reduces the stiffness of the bracing in terms of the cross grain stiffness the tone bars provide. The unsupported spans are against the long grain spruce and the xbrace which is pretty stiff. I do not know sheet metal but I would guess the sheet is not a first order structural component relative to the braces. In a guitar, the top is a significant structural element.

[ken cierp]

I know John has the Gore books so I yield the floor to him regarding the math and science cause and effect. But the sheet metal comparison is out of place since the stiffness is the same in any direction, where as the stiffness of a wood top varies greatly cross grain vesus with the grain.