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’Double backs’ idea- been done?
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Author:  JohnAbercrombie [ Wed Oct 31, 2007 9:52 am ]
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There have been some comments about damping of guitar backs by players' bodies, especially if playing standing with the guitar on a strap. I understand many classical players take pains to make sure the guitar back is not contacted, to avoid damping.
(Not sure if damping is the correct term here- but you get the idea.)
Have there been any examples of experiments with 'double-back' guitars?
I'm not referring to laminated back guitars where something like yellow cedar is laminated inside the outer back wood, or to multiple-ply laminate backs like Smallman and others use.
By this I mean an outer (fairly rigid) back and an inner, acoustically 'live' back, separated by a small air gap.

Obviously there'd have to be a 'hole' through the inner back, or you would be building a wooden aneroid barometer.
Comments?
Links?
Theoretical info?

Thanks!

John
PS: Some would say that this is nothing but an excuse to lay in a double inventory of b+s wood...nothing could be further from the truth!
Author:  John How [ Wed Oct 31, 2007 9:57 am ]
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I believe Tim McKnight is doing that right now and has reported good success.
Author:  JohnAbercrombie [ Wed Oct 31, 2007 10:14 am ]
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[QUOTE=John How] I believe Tim McKnight is doing that right now and has reported good success.[/QUOTE]
Thanks, John.
I had a look at Tim's site as soon as you suggested his name- interesting stuff.
John
Author:  Pwoolson [ Wed Oct 31, 2007 10:27 am ]
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I'm doing it. They work great. Extremely rigid and responsive.
Author:  K.O. [ Wed Oct 31, 2007 10:53 am ]
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I works really well for dulcimers.
Author:  grumpy [ Wed Oct 31, 2007 11:03 am ]
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Double backs date back to at least the early part of the last century...nothing new under the sun

Correct. And often referred to as a false back.

Author:  burbank [ Wed Oct 31, 2007 11:21 am ]
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If my memory serves, it seems Martin made at least one, in the 20s, maybe? It had a conventional back, then piggy-backed on that was a sort second body, about 1" thick, set back about 3/4" from the main body, with a small soundhole on its "top". The reason was to get the "live" back off the player's belly. It's in the Longworth book, which I sold to stock up my WAS fund.
Author:  Mike Lindstrom [ Wed Oct 31, 2007 11:28 am ]
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I understand that early Selmer Maccaferris did too.  Called an internal resonator on the Maccaferri site.  Mine's been ripped out.  I've read most of them were, possibly because they weren't put in well and buzzed.  
Author:  Rick Turner [ Wed Oct 31, 2007 12:26 pm ]
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I have an 1890's Howe Orme with double double backs made of spruce suspended inside.   The Martins were made for Paramount in the late 1920s.   Some Spanish classicals have been made like this for decades.   Etc., etc.   As you can tell, it's always been a huge hit in the commercial marketplace...as in "so what?"
Author:  Peter J [ Wed Oct 31, 2007 1:05 pm ]
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John,

Manuel Contreras I built what he called a "Double Tapa" which was a second back suspended just above the bracing on the back of the guitar to improve projection.

He also built a doubled back shell which was intended to isolate the primary back and body of the guitar from the player. It clipped onto the body of the guitar at the heel and bottom by means of a friction fit. They are rare items now and probably in the hands of collectors. Here are a few photos of one that I saved for reference:

   





John Shelton, a classical and flamenco builder in Oregon has built a double bodied guitar for quite a while now. You can Google his name and search for "double body." His design adds a complete outer shell; sides and back, and spaces it from the primary body with small vertical pieces placed around the perimeter.

Regards,

Peter
Author:  Peter J [ Wed Oct 31, 2007 1:12 pm ]
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Here is a larger picture of the 3rd image of the Manuel Contreras back piece:



It appears that he used some cork or felt pads to isolate it from the body of the guitar.

Author:  RayH [ Thu Nov 01, 2007 2:17 am ]
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John,


You can see a double top and back being constructed here:


www.dunwellguitar.com


Just go to "luthier pages" then "Let's Build a Guitar"


He uses Nomex for both plates and is extremely cordial and helpful if you drop him an e-mail with a couple of questions. You can purchase Nomex sheets for one off's or experimental pieces at LMI.


 


Take Care,


Ray 
Author:  JohnAbercrombie [ Thu Nov 01, 2007 4:11 am ]
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Thanks for all the replies and information, everybody!
I do appreciate it.
I've printed out a bunch of stuff- I've got a plane trip coming up, so my carry-on bag will be full of printed info.

Cheers
John
Author:  John Mayes [ Thu Nov 01, 2007 10:54 am ]
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I know of an 1830's LaCote that had a false back. Same ideas with some
changes, but the idea has been around for ages.
Author:  erikbojerik [ Thu Nov 01, 2007 2:27 pm ]
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OK my impression (as opposed to experience) has been that the backs of
"flattop acoustics" are intended to be mainly reflectors, and that
archtop backs are much more in this category of being "tuned" to assist
the top in sound projection.  At least this is what I concluded after reading both Cumpiano and Benedetto.



Isn't that a traditional difference? (knowing that lots of folks are quite happy to buck tradition)
Author:  JohnAbercrombie [ Thu Nov 01, 2007 3:26 pm ]
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Erik (?)-
I think a lot of folks consider backs to be much more than reflectors in flat-tops- thus the ideas about the sound qualities of mahogany vs rosewood guitars, the mojo about Brazilian, etc.
Also, there was a recent comment about the fact that 'the builder who can't be named' often makes the backs of his guitars very thin (and resonant/lively?).
I think it is only folks like Smallman who have made the backs really rigid with multiple laminations who are treating them as mainly reflectors. (BTW, if Brazilian RW really does have extraordinary acoustic properties, it seems a waste to be using it in a 5-play lam like Smallman has.)
Like most ideas in the guitar world, get two people together and you will get three opinions on this topic!

Cheers
John
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