Live Backs

[John Parchem]

Trevor,

I do have question concerning a live back. In your build book you write that you use 3m radius for a live back and 5m on the non-live back. I was just wondering about the reason for the difference. I can see it would put the back under more tension as it is a smaller radius. Does that provide acoustic benefits. Also what would one lose if they did build a guitar with a live back thickness and bracing but with a 5m radius.

Thanks,
John

[Trevor Gore]

It is quite easy to make a back too heavy to move (low monopole mobility of the back) if some of the denser species of wood are used. When the live back was developed we thought that it would be beneficial to use thinner panels and higher curvatures to get a panel of the right stiffness but less mass and hence the 3m radius for live backs rather than our existing 5m radius dishes we'd been using prior.

A 5m radius provides a less stiff panel so it needs to be thicker or have more bracing to get the T(1,1)3 in the right place and then it gets rather immobile because of the higher mass. It's easier to make a non-live back with a 3m dome than a live back with a 5m dome and if you only want one radius dish the 3m is the one to go for. If you only have a 5m and don't want to get a 3m dish and want to try a live back, stick with lower density woods (e.g. hog, lower density Aus. blackwood, narra) rather than, say, bloodwood or a dense rosewood. It's tough to get a bloodwood back light enough to be live even with a 3m dome!

[John Parchem]

Thanks that helps.

On a current project. I have a East Indian Rosewood back that came out needing a thickness of 3 mm using your equations for the required frequency. That puts the plate right into the how to make a non-active back range. Would the idea be to thin that board maybe to 2.7 mm or so and put a 3 m radius on the back to increase the stiffness of the plate. Also I would leave the lower brace on the back close to full height in the center until I could tune it down to the right frequency after it was strung up.

Or is this a plate where it would be better to make a non active back?

[Trevor Gore]

3mm? Definitely out beyond the 3 sigma of my data!

Is this for the dread in your build thread?

If the Young's modulus is below ~10GPa and the density is higher than ~750kg/m^3 I'd say it will be a struggle to make it live at the right T(1,1)3 with the standard live back bracing plan.

There is, of course, the question about whether it ought to be live. If used for flat picking standing up you'd be better off with non-live. If for fingerpicking sitting, you be better off with a smaller body size and a live back. So without knowing more about how the guitar will be used, it's difficult to give further advice.

Would the idea be to thin that board maybe to 2.7 mm or so and put a 3 m radius on the back to increase the stiffness of the plate. Also I would leave the lower brace on the back close to full height in the center until I could tune it down to the right frequency after it was strung up.

To stand a better chance of it being live at the right frequency, yes, all the above.

Or is this a plate where it would be better to make a non active back?

Have a look at page 2-9 of the Design book, where you'll find the tap response of a pretty good 1953 D28. It's a non-live back. I don't have any data on this, but if someone argued that the main difference between a D18 and a D28 was that the D18 had a live back whilst the D28 didn't, I wouldn't have any trouble finding that plausible. (For example, all of the good J45s (hog back and sides) that we've measured have live backs). All this suggests that you need a lighter, stiffer (lower density, higher E) back panel than the one you have if you want a good shot at a live back.

If there is no specific requirement for a live back, go right ahead with non-live. It'll likely sound more D28ish if that's the sound you're after. Keep the top mass low with the right T(1,1)2 and you'll definitely have a banjo killer.

[John Parchem]

Thank you the really clear response. It is for the dred I am building. I switch back plates from the previous calculations I showed in my post. I included the information for this back.

I have a clear way forward with a non live back design as this guitar is being built to be played as a true banjo killer, mostly flat picked bluegrass. The guy who has me building it currently plays a mahogany Martin 00.

second back thickness.jpg (72.8 KiB) Viewed 2546 times

[ken cierp]

I certainly would have to read the book/s to understand the science here -- but are the samples tested free standing? I ask because 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 -- maybe I'm whacky but it seems to me the double or inner floating back is a concept that could be used to more constantly tailor a given instrument's sound qualities.

[John Parchem]

Ken, I agree with the player standing muffling the back and nullifying the value of a live back. In Trevor's post to my question he hit on the same issue: If used for flat picking standing up you'd be better off with non-live. If for fingerpicking sitting, you be better off with a smaller body size and a live back. . Classical guitarists use this muffling effect to change the sound they are shaping; while sitting they will keep the back mostly free and when they want to change the sound they will push the guitar away or pull it closer. My instructor spent quite a bit of time describing the intimate relationship necessary between player and the guitar. I have seen double backs in guitars I do not think they are whacky.