brace question on flat top guitar

[Brian Evans]

My apologies for a question that may seem a bit naive, but I do arch top guitars so I honestly don't understand this brace question. On almost all traditionally braced flat top guitars, ladder or X braced, there seems to be a very heavy brace across the upper bout, between the sound hole and the neck block. It looks to be almost 3/4" by 1/2" or larger, it spans the width of the guitar, and I don't understand why it's there. Acoustically it would seem so robust as to take the upper bout almost out of the sound producing equation. Maybe it's there to take up a rotational force from the neck block that might otherwise deform the top downwards?

Anyway, I am doing a lot of reading about bracing patterns, the what and why-fore, and I haven't come across this brace yet so I thought I would ask.

[Mark Swanson]

You are pretty much correct, that part of the top needs the strength from that brace, and that section also adds little to the acoustic sound production, so the big brace.

[Barry Daniels]

The upper transverse brace (UTB) receives a lot of downward force from the neckblock and especially the fretboard which sits right above it. An archtop has a floating fretboard due to the elevated extension so it does not need this brace. Also, the arched top has inherent stiffness which is mostly lacking in the thinner and flatter top on a flattop.

[Michael Lewis]

Whose idea was it to cut a hole right in the path of stress? Oh, wait, it was for gut strings that don't have much tension so it didn't matter much. But then steel strings were introduced, and they cause predictable damage. It's just architecture and physics. Someone should come up with a bracing pattern that overcomes all the stresses and still sounds good.

[Alan Carruth]

Earlier guitars had several transverse braces, as did lutes. X bracing and fan bracing in the lower bouts started to come in in the late 18th century, but they retained the UTB for strength. It doesn't really hurt the sound much, IMO.

Several folks have worked on brace designs that eliminate the UTB. The common element is some sort of diagonal bracing from the top of the block to the waist. Some use braces on the top, and others use flying braces, sometimes of CF, that go down to the side. There have also been guitars that had posts from the neck block to the tail block. These sorts of designs have been around for as long as I can remember. The fact that they have not caught on widely suggests that they don't amount to enough of an improvement to bother with; if they were all that good everybody would use them.

The sound hole location on guitars, just above the waist, which itself is a bit higher than the center of length of the box, actually has an acoustic function. It's fairly complex, but having a hole there tends to produce a set of internal air resonances that can contribute to the timbre of the guitar. For this to work you need to have the 'normal' sound hole, a more or less pronounced waist, and a top that can vibrate (I said it was complicated!). If you change any of those elements you end up with a different sound. Dreads often don't show these resonances, and it's one ingredient in their 'characteristic' sound.

The design of the guitar has evolved to fill a function. It's not 'perfect' as a structure, or acoustically, but it does it's job well enough. People are always trying to improve it, and once in a while somebody actually does, but that's pretty rare. Any time you make a major change it tends to alter the sound or playability in ways that move it further away from the desired qualities. Since we don't know everything we'd like to about how these things work, or what all the features do, it's hard to know what sorts of changes we can make that will actually be improvements. In the end, if what you want is a guitar, you have to make something that's pretty close tot he standard designs. There's nothing wrong with experiments, of course, but you do have to accept that most of them will fail.

[Jason Rodgers]

In the end, if what you want is a guitar, you have to make something that's pretty close to the standard designs. There's nothing wrong with experiments, of course, but you do have to accept that most of them will fail.

I think these two sentences sum up a lot of what you've been saying (sometimes over and over) on these forums over the past decade-and-a-half. Thank you, Alan!

[Arnt Rian]

If you do a lot of repairs on 'normal' steel string guitars, you will often see instruments where the top is sinking somewhere between the bridge and the neck block, it can happen both below and above the soundhole. Weak tops, insufficient or damaged braces are common problems. Sometimes, if the upper transverse brace cracks or comes unglued, the top will split along the neck block all the way to the soundhole, and the part that is glued to the fingerboard and neck block will shift into the soundhole, as the neck rotates in the direction of the string pull.

A bracing design that has become increasingly popular in recent years is the "A" brace above the sound hole, which consists of a pair of braces from the neck block, angled out on either side of the soundhole, towards the upper arms of the X-brace, crossing through (under) the upper transverse brace. It looks sort of like the letter A, hence the name, I suppose. I don't know if its as efficient as some of the designs Al mentions above, but its pretty easy to do, and it seems to make the area quite a bit more resistant to sinking and the dreaded neck block / fingerboard shift. Inletting the braces and tight joints are important.
Anyways, the whole area under the strings is a stressed area, a robust system for reinforcement is a good idea, just take your pick.

[Alan Carruth]

I've been using 'A' braces for quite a few years now, and they do work. I've seen a couple of mine dropped, with significant damage to the neck, but without the neck pushing in, as you would expect for that sort of thing.

One problem that comes up with the UTB is that the resistance to neck displacement depends on the integrity of the glue joint between the brace and the top. When I was doing more repairs I saw a few fairly new guitars where the top had split and the neck had shifted in where the UTB was actually still technically glued down. They used some sort of white glue, and the joint actually crept enough to allow for the displacement without actually breaking the glue line.

[Alain Lambert]

Where the A braces cross the UTB, do you notch the UTB so they go over or under? Is it not weakening the UTB?

[Bryan Bear]

The a braces go through the UTB which is notched to allow for a tight fit. The notches in the UTB are not on the topside of the brace, its full height is uninterrupted so it maintains most of its stiffness. Much like capping the open side of the X brace to regain stiffness of that brace. Or like drilling a hole in the center of a beam.

[Alan Carruth]

What Bryan said: the notches are on the compression side, and if things are reasonably tight it's not an issue. I use A braces that are no more than about 5/16" tall, and the UTB is a lot taller than that.