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#1
09-26-2020, 08:08 AM
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Back Bracing Question
As far as I understand, the back of a guitar has only a minimal amount of forces exerted on it. It puzzles me that despite this pretty large (wide/tall) braces are commonly used.
Is there a reason behind this? or is it simply to mitigate cracks / splits? Furthermore with all the theories about a 'live' back as opposed to one that isn't, would 'smaller' bracing contribute to the back being 'live' or are there other considerations? Final question for you fine luthiers out there, which works better? I assume that certain wood species would play better one way or the other? Sorry for the plethora of questions, I am feeling inquisitive. Thanks, Nahil. 
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#2
09-26-2020, 08:24 AM
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Quote:
Taylor's "dirty little secret" of how they get guitars such as the GS Mini to sound so good is the use of a formed laminate back with NO bracing. It's something to think about when you evaluate what bracing does and how it contributes to rigidity and structural integrity.
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#3
09-26-2020, 08:32 AM
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 I am asking because I am just about to attach the back to the sides on my build but I keep thinking to myself that those back braces don't need to be that big. Im using Macassar Ebony and its plenty hard I don't 'feel' like it needs much bracing. I truly am loving trying to learn to build guitars...at every turn there are multiple considerations to be made and everything ties into everything!
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#4
09-26-2020, 10:02 AM
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How the back is intended to work, and the way it is braced, is a choice that you have to make. Many feel the back contributes little to the total voice of the guitar, just as many feel the materials used for back and sides contribute less than the abilities of the builder. I, perhaps in the minority, believe that the choice of back and sides material, and how it is assembled, can contribute substantially to the voice, if allowed.
I’d read thru the many lengthy build threads by Bruce Sexauer in the custom shop - you can see pictures of how he braces his backs, and from time to time he’ll comment on why he uses those designs, and what he intends to get from them. Plus, he builds almost entirely by hand, with a bare minimum of power tools, which sounds like a style that’d work for you - and personally, having now played a great many of his guitars, and owning a few, I can assure you that there is some extremely solid basis in what he believes works and how a great hand-made guitar should sound. Just keep in mind, what he does and how he does it looks very simple, but is the result of some 50 years of continuous practice. And that is probably what it takes -
__________________
More than a few Santa Cruz’s, a few Sexauers, a Patterson, a Larrivee, a Cumpiano, and a Klepper!!
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#5
09-26-2020, 10:19 AM
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Quote:
Thanks tadol, I appreciate the response. Reading Bruce's detailed build threads (and watching some videos of his has been great, there is no substitute for the years of skills and knowledge gained. I only joined this forum last month and it is the single best point of reference for all things guitar building! Amazing luthiers that are generous and take the time to detail their builds and respond to my many annoying questions!!
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#6
09-26-2020, 10:50 AM
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I've used several different back brace schemes over the decades. They do affect the sound in details, but the pattern seems to matter less overall than how well it's done in any given case.
The big discussion is between advocates of 'active' and 'reflective' backs. In theory an 'active' back helps to produce sound, while a 'reflective' back keeps the sound in the top. There's some truth in both positions, but, as usual, the extremes are not easy to defend. Certainly the top is better adapted to produce sound and project it to the audience. It's the only effective sound-producing part of the guitar that is directly driven by the strings, it faces the audience, it's lighter in weight than the back, and it's not up against your pudgy avoirdipois. It makes some sense to minimize and energy being wasted in making the back vibrate. On the other hand, the only guitars I've measured that had totally 'dead' backs in the range of frequencies we're interested in were Ovations, and I've never been fond of that sound. As it turns out, if you measure the way the top and back vibrate at different frequencies you can see that the back does tend to 'steal' energy from the top at it's resonant frequencies, and thus tends to reduce output except in one range of frequencies; the so-called 'bass reflex range'. Down at the lowest pitches the guitar acts like a 'bass reflex' speaker cabinet, with the lower bout of the top acting like the speaker and the sound hole as the 'port'. The 'Helmholtz' air resonance and the 'main top' resonance work together to produce a couple of peaks in the output, and these go a long way to define the timbre of the guitar, as well as producing most of the actual power output. In the range around the lower peak (say, G on the low E string) most of the energy is in the air moving in and out of the sound hole, but the top is also moving and producing sound. The problem is that the top is out of phase with the air; as the air moves 'out' of the sound hole, toward the audience, the top is moving 'in', so some of the air that came out of the hole gets 'short circuited' filling in where the top was. This reduces the output for the audience. If the back also has a vibration mode like the 'main top' resonance it will be largely driven by pressure changes in the box as the air flows in and out of the hole. Essentially, the back can act as a 'flywheel' to store some energy and feed it back to the air. This helps make up for the 'phase cancellation' going on between the air and the top, and increases the power of the 'air' resonance. The closer the pitch of the 'back' resonance is to that of the 'top' the more effective this is, but if they're too close it can also cause a 'wolf' note. Usually it works best if the 'main back' resonance is a bit higher in pitch than the 'main top' resonance. Thus having a back that is 'active' in the frequency range near the 'main top' resonance (roughly, the open G string) can augment the sound output of the guitar below that range. Again, most of the actual power the guitar puts out is in this range between the 'air' and 'top' resonances, so it's useful in terms of 'projection', at least. Above that low range back resonances tend to show up a 'dips' in the output. These can be useful in providing 'tone color' to the sound so long as they don't reduce the power output too much, IMO. Differences in the way backs work, due to mass, stiffness and damping, in this range help produce the 'characteristic' timbres of different back woods. These are 'small' effects as compared with the influence of the top, but not unimportant. All of this stuff has been worked out over literally centuries, and is embodied in the 'traditional' designs. I can tell you that, in my own case, I've worked through a number of 'improved' designs over the years, but, in the end, have come home to relatively small variations on those traditions. As has been said: "When the need is old, the old ways are probably the best (unless some new technique has been introduced), since it is unlikely that all of the designers of ten or twenty generations will have been fools". (David Pye) Last edited by Alan Carruth; 09-26-2020 at 10:56 AM.
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#7
09-26-2020, 12:10 PM
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Thanks for the detailed explanation Alan! That helps give me a bit of insight into the relationship between back and top and whats going on. Cheers! Nahil.
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