Why do we brace 'into' the sides of an acoustic guitar?

ikerstges · #1 10-02-2018, 08:38 AM

Hiall!

When closing the box, most common practise seems to notch the kerfing to enable cross braces and/or X-braces to fit into this notch and even into the sides (covered by the bindings in a following step in the process). I assume this is done for structural strength.

I understand that these braces have not only just a structural function, but should also support sound transmission in the back and top plates of the guitar. It seems to me that much of the vibration of these braces gets absorbed by their connection to the sides (and dissipates in heat).

Wouldn't it be better to let also these braces end with some space to the sides, just like many or most of the tonebars, and thus have their maximum part in the sound qualities of top and back of a guitar?

Cheers, Igor

Seagull S6 · #2 10-02-2018, 09:18 AM

Interesting thought. My uneducated opinion is that the back and side don't really contribute much to the sound but more so help focus the sound and overtones the top makes. It would be interesting to put a contact pickup on various parts of the body to better understand what the back and sides contribute to the overall sound.

Kinda like these bad boys which have the back and sides made out of some sort of Chorion like counter top material which was thought to contribute zero to the sound of the speaker other than focusing the front baffles sound and isolating the sound from the front and back of the baffle so that the vibrations don't combine and cancel out each other.

Compared to a speaker, a guitar could be considered to be somewhat of a modified closed baffle with a port.

Bob Womack · #3 10-02-2018, 10:27 AM

The top is a diaphragmatic resonator, designed to transmit and amplify the vibrations from the strings. The box is a tuned, ported resonator to do the same with a standing column of enclosed air. The top braces are designed to inhibit the top from vibrating too much and from succumbing to the tension from the strings and the pressures exerted by hand and arm while playing. They rely on the guitar sides for the anchor point to carry and distribute that load.

Bob

Rodger Knox · #4 10-02-2018, 10:46 AM

It is "typical" for braces above the soundboard to be let into the linings and braces below the soundhole to be feathered out, at least on the top. The upper bout is not that active acoustically, and the structural stiffness is important in that area.

Alan Carruth · #5 10-02-2018, 11:21 AM

The brace ends have to be supported for structural reasons. Most glues, and certainly the traditional hide glue, have poor peeling resistance. Suppose the top is glued down to the rim at the edge, and the brace is simply cut off at the end with no support. Any down load on the top will cause it to flex at the edge, but the brace, due to it's grain orientation and height, is stiffer. It won't move with the top, and will tend to peel loose. Once it starts it keeps going.

Back when I did repairs I ran into couple of Gibsons that suffered from this. Gibson used to use the nose of a belt sander to feather the brace ends down to next to nothing. When they put the back and top on they didn't bother to cut inlets for the brace ends, but relied on them to mash the liners down a bit when the plates were clamped on. The problem is that they used to use spruce for both the liners and braces, and sometimes the liners was the harder. The brace end would be what got crushed. This amounted to breaking it off at the edge of the liner.

At least twice that I can recall somebody came in with high action because the neck was tipping up. When I looked inside there was no upper transverse brace. I asked the customer if there had been a loose stick inside the guitar, and they confirmed that there had been, and they threw it away, not knowing where it came from. The repair involves notching out the liner to fit a UTB of the proper length, with some height at the ends. The brace is glued in, and brackets glued to the sides to support the ends.

In normal use the only braces that feel a downward force at the top braces in front of the bridge. Some makers will reduce the brace ends behind the bridge to zero, and not inlet them. Even the ones behind the bridge can get a down load from time to time, most notably from impacts, and this can peel the brace up unless it's very flexible for some distance in from the edge. I tend to inlet any brace that can't be made that flexible.

Yes, this does affect the sound. I figure a guitar that's falling apart won't usually sound too good, so I try to get the structure right and then get as much sound as I can.

murrmac123 · #6 10-02-2018, 04:43 PM

Quote:

Originally Posted by Alan Carruth

Some makers will reduce the brace ends behind the bridge to zero, and not inlet them.

I would have thought that from the point of view of getting the alignments right when gluing the top down, a more efficient approach would be to to mark out the location of the ends of the X-braces on the linings, cut the notches, and glue the top down using these notches as locating points.

The ends of the braces could then be chiseled/feathered down to zero thickness, leaving the sacrificial ends of the braces as an integral part of the linings.

Not being a builder myself, I assume that there are good reasons not to do it this way .. I look forward to hearing what these reasons might be.

Shuksan · #7 10-02-2018, 09:48 PM

Quote:

Originally Posted by murrmac123

I would have thought that from the point of view of getting the alignments right when gluing the top down, a more efficient approach would be to to mark out the location of the ends of the X-braces on the linings, cut the notches, and glue the top down using these notches as locating points.

The ends of the braces could then be chiseled/feathered down to zero thickness, leaving the sacrificial ends of the braces as an integral part of the linings.

Not being a builder myself, I assume that there are good reasons not to do it this way .. I look forward to hearing what these reasons might be.

That is basically how I do it. It's easy to do. I don't feather the brace ends down completely to zero though. They are still about 1/32" thick where they continue into the block let in to the lining. That is to address the concern Alan mentioned above about braces peeling off the underside of the top, particularly if the top takes a blow right above the brace end.

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mirwa · #8 10-02-2018, 10:32 PM

Quote:

Originally Posted by Shuksan

That is basically how I do it.

That is some nice attention to detail, even the shape of the braces along their length is incredible.
Steve

Alan Carruth · #9 10-03-2018, 12:51 PM

I actually inlet the ends of tone bars into the main X braces a little bit for the same reason.

redir · #10 10-03-2018, 01:45 PM

If I am going for a more responsive top instrument then I will feather to zero and then just glue the rim right over the paper thin ends, I guess like the Gibson style is. It's so thin that it doesn't crush it but I think it still gives a bit of peeling protection.

Alan Carruth · #11 10-04-2018, 10:36 AM

Part of the problem with the Gibsons was that there was actually a fair amount of thickness and slope at the end of the braces: the radius that the sander made was short, and they only came down to about 1/32" or a bit thicker at the ends.

As it happens, a fairly dense piece of spruce can have about the same Young's modulus along the grain as Indian rosewood or mahogany. Cross grain stiffness tends to be lower, of course, and varies a lot: let's say it's 1/6 of the long grain stiffness for a 'typical' back. Since the stiffness of a brace goes as the Young's modulus and the cube of the height, a cross brace about half as tall as the thickness of the back will have the same local thickness and resist bending along with the back. That's just a back of the envelope calculation, but it gives an idea of how low you have to go with the brace ends to avoid peeling.

murrmac123 · #12 10-04-2018, 04:15 PM

This whole "peeling " thing has me slightly perplexed.

I have never come across the phenomenon myself, but that proves nothing.

What would be really peachy would be if somebody could post a pic of a guitar top on which which the lower brace ends had been fully feathered, and which had nonetheless exhibited this "peeling" phenomenon.

redir · #13 10-04-2018, 09:22 PM

Perhaps 'peeling' is the wrong word, perhaps not. It's not like a banana. What I have seen doing repairs over the years it's more of a 'cracking' as in it just popped off due to some impact or something. But since it tends to be along the glue line I suppose peeling is a reasonable description, it just peels real fast in the event of an impact. Feathering to zero would remediate that in any case I think?

Alan Carruth · #14 10-05-2018, 10:53 AM

It's peeling in the sense that the glue line comes loose starting at one end and progressing along the length of the brace. It goes fast, and usually starts with an impact of some sort. The point is that most glues are not very good at resisting that sort of failure, so you need to design for it, either by allowing for enough flexibility in the brace to minimize the likelihood of it starting to peel up, or supporting it so that it can't

murrmac123 · #15 10-05-2018, 12:41 PM

To be honest, Alan, I cannot imagine such a scenario occurring if an instrument has been built with fresh Titebond and the braces have been properly clamped during construction. Glue starvation and inadequate clamping spring to mind.

Admittedly, I am extrapolating from my experience in furniture repair, rather than instrument repair, but I have never seen any "progressive" failure along the glue joint in a well constructed piece.

The impact normally creates an immediate local fracture, but doesn't cause any glue line separation... any wood failure is normally along the grain line rather than along the glue line.