Live Backs

[Dave Bagwill]

The double back would allow even the standing flatpicker to get the fullest sound, right?

[John Link]

This is such an interesting conversation. Thank you all, but especially John, for putting the Gore/Gilet methodology into actual practice. And for Trevor's willingness to participate as he does.

Intuitively, the problem with a double back *could* be that the inner back is, well, inner. That is not the normal position for the back. What does that change?

Judy Collins (who was trained in the tradition of the piano) played a Martin dreadnought from the git go of her career. In one of the early videos she is standing with the guitar strapped on, yet she clearly understood to get the thing off her body for max output, and you can see her keeping it there for the duration of the song. It does not look comfortable, but neither is it awkward.

But when you look at just about any bluegrass picker, their gits are resting comfortably where they rest easiest. C&W, the same.

[Trevor Gore]

...most/many players sitting or playing standing up, crush the guitar body against their torso and that pretty much ( I would think) nullifies any effect the back is having on true output...

Having the back of the guitar against your body mainly affects how the back is damped, which certainly makes the guitar sound different if it has a live back (less so with a non-live back) but that doesn't turn it into a non-live back, just a damped live back.

The mobility of the back (along with many other things) affects the rigidity of the "vessel" and so has a significant affect on the the main air resonance and the top resonance as they all couple together.

As John P. points out, whether the back is damped or not and how the effect is used is up to the player.

Anecdote 1: Once upon a time I was working in a hospital operating theatre (long story...) and the surgeon demonstrated how a bone saw works. It's a saw blade that oscillates at high frequency, low amplitude. Put it against bone, wood or something of similar hardness and it cuts beautifully. Put is against flesh and it doesn't cut - it just wiggles the flesh about. This is an example of mechanical impedance (mis)matching. Put a vibrating edge (e.g. the bound corner of a guitar) against something soft (e.g. a leg) and not much happens, just a tiny bit of damping, maybe. Put it against something hard and you can get coupling which can change things significantly, depending on what you're coupling with and how well it couples. Worth thinking about when you consider how energy transfers from one bit of a guitar to another bit, or to something else.

Anecdote 2: At a guitar shop near here, the owner demonstrates his "posh" guitars by pulling them off the hangers behind the counter and then plays them to the customer whilst resting the treble side of the lower bout on the counter top. They sound a whole lot louder because he has the whole counter cabinet acting as a supplementary resonator (and, to a degree, is simulating the effect of high mass sides).

[John Link]

OK, this posed an interesting question to me, which I just attempted to answer both empirically and subjectively. I strummed an E minor chord vigorously (4 open strings, no buzzing) on two guitars I happened to have handy, once with the git free of damping and the other time damped against my plenty soft belly. I repeated several times and accepted the average, though there was not much variance to average, it was a case of a clear bell curve with few outliers.

90 year old OSCAR SCHMIDT LADDER BRACED (Just one top brace in money area, bridge pad is very small, does not extend to edge of bridge, probably maple, not spruce. 4 parallel ladder braces on one piece back. Light gauge strings.)
1. Damped: Chord sustained 6 seconds, sound contained fewer overtones than undamped.
2. Free of damping: Chord sustained 7 seconds, sound contained more overtones, sounded *perhaps* louder.

10 year old LINK DREADNOUGHT, X-BRACED (No scallops in top braces, conventional rosewood bridge plate. 4 ladder braces on thin 3 piece rosewood back, 2 in lower bout are deliberately not perpendicular to the center line. Medium gauge strings.)
1, Damped: Chord sustained 8 seconds, sound contained fewer overtones than undamped.
2, Free of damping: Chord sustained 10 seconds, sound contained more overtones, sounded *perhaps* louder.

[ken cierp]

This guy talks a lot about the sound-board before he gets into his back theory.

[John Link]

Interesting video. I got perverse pleasure from the part with the dreadful music that he holds the top up against the speaker to. In the end, I could not find much reasoning behind what he was saying, but would expect that top to do exceptionally well with emphasizing the treble.

The blackwood bowl back is very well crafted. It too sounded tight. Would love to hear the finished result.

[mike-p]

Hi all,

I haven't fully followed the theory on this thread but my experience as a player certainly makes me think i like a guitar with a live back, i like small mahogany guitars best, some larger mahogany guitars and not many rosewood ones except old Brazilian small bodies. Anyway, I'm planning my next build and want to build a nick lucas and i have a set of cuban mahogany, is this going to be too dense and rosewoody to get the woody lively tone that i like?

Thanks,

Mike