Why doesn’t the X in an X-braced arch top fall directly under the bridge?

[Dennis Duross]

Asking for a friend.

Seriously, if the three basic soundboard modes are up/down, left/right, and front/back, and the archtop is biased toward the up/down, would you not encourage the left/right by shifting the support to the center of the bridge?

[Barry Daniels]

A bridge there would have little stability for tipping forward and rocking sideways. It needs the support to be spread out.

Analogy: That would be like putting a single pier in the middle of a house.

[Dennis Duross]

Thanks for your reply, Barry. I have a vague memory of a builder who used a single brace, but maybe I’m mis-remembering. I guess that’s what started the wheels turning.

[Alan Carruth]

The legs of the X-brace do go under the bridge feet. The same is true of the two 'parallel' braces when that system is used.

Violins use a single 'bass bar' that run under the bass foot of the bridge and along the top, almost (but not quite) parallel to the center line. They have a sound post on the treble side, just behind the treble foot of the bridge. The sound post, which is wedged between the top and back, acts to 'nail down' the treble foot of the bridge,which can then act as a bell crank to convert the sideways motion and force of the bowed strings into a vertical force on the bass bar. The bass bar spreads that out to the whole bass side of the top. Although the post reduces the overall efficiency of the instrument it greatly increases the output in the low range, and tends to suppress the sound in the 'nasal' range around 1.5 kHz. It's often not a good idea to spend too much time talking about fiddles on a guitar group: they're much different systems.

On the guitar, flat top or arch top, the main driving force on the bridge comes from the 'transverse' pull of the strings as they move ' vertically' with respect to the plane of the soundboard. Flat tops get a little bit of 'color' (but not much power, if any)from the way the changing tension of the strings pulls the bridge toward the neck twice per cycle, but since the tailpiece takes up the tension load on an archtop the bridge doesn't rock in that way. Side to side forces, as on the fiddle, don't result in the production of appreciable sound because it's hard to move the bridge that way, and also because when one side rocks 'up' the other rocks 'down' and cancels the top displacement out. Adding a post, as in the violin, tends to reduce the output of the guitar too much, although it can be a useful technique to kill feedback when amplified, precisely for that reason.

As a general rule, you can assume that almost all of the simple and obvious things have been tried, and most likely more than once. Luthiers are always trying stuff, and once in a while something even works! When it does all of the other makers copy it, and it becomes part of the 'standard'. The stuff that doesn't work gets buried, and turns up once in a while when somebody cleans out a dusty closet full of old instruments. Nobody talks up their failures....

[Dennis Duross]

FYI, I found at least one reference to the single tone bar I was referring to above, in the early archtops of a builder named Bill Barker, from my hometown, Toledo, Ohio:

http://www.vintagearchtop.com/66_barker.htm