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I/m almost finished bracing a Sycamore back for a classical guitar using the methods in the gore book.Without any equipment , how would one go about tuning the back. This is my first gore/gilet style back.The back is abt 2.5mm , and after finish sanding abt 2.3mm using the bracing pattern from the OM/CL plans included in the book. The braces are spruce. Any ideas ?? thanks.

Without any equipment?
No Computer/I-Phone?
The method I understand from the book is to estimate the thicknesses required (for backs and tops) required by frequency testing the panels (one panel of each) in the 3 different directions, then build to the bracing plans and then box everything up.
Then frequencies for the top, back and body resonance can be tested, and adjusted (hopefully) from spectrum anaylysis. You would have to see/analyse the spectrums, or have something to determine the frequencies.
The only free panel testing from the book I remember was to estimate thicknesses required for top and back, although I have seen Brian Burns (website/blog?) and also someone on OLF (name escapes me) using a sort of body form with the braced plates clamped all round to it to investigate frequencies prior to boxing up.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

With some experience you can get the assembled back frequency into the right ballpark using 'free' plate tuning methods, but it's far from exact. Any change from what you've done in the past makes it harder to predict, so a new shape or bracing system is a new learning experience. In the end, if you're after some exact pitch, you'll probably need to reach inside and shave braces or some such.

Some years back I tracked Dana Bourgeois through his tap tuning process on a guitar at a GAL convention. I tested the modes of the assembled box before and after his tuning processto see what he had done. Basically, he started with a top that was a bit too heavy and high pitched, and shaved the edges to lower it's modes to get them to couple more strongly with those of the back and the air. All he used for 'instrumentation' was an educated finger, and his ears. The 'ears' part is easy to understand, the 'finger' thing a little less so.

His method is to tap the top in the bridge area, either in the center or where the end of the bridge would be (this is _before_ the bridge is put on). He also tapped in the lower bout on the center line between the bridge location and the lower edge. In each case he would tap and allow his finger to rest lightly on the top. What he was feeling for was a 'kick'. I'd always found his written descriptions of this confusing, but after seeing him do it, it became easier to understand.

Suppose you have the 'main back' resonance tuned to be something like a semitone higher in pitch than the top. When you tap the top it starts a compression wave going toward the back. When that wave hits the back deflects pretty strongly, since it's close to the top resonant pitch, and and returns a fairly strong wave toward the top. When that return wave hits the top you feel it 'kick'. Tapping in the lower bout between the bridge and the lower block checks the coupling of the top 'long dipole' mode with the 'A-1' air resonance. On Dreads it's possible for the top 'cross dipole' to be close enough in pitch to the crosswise air resonance in the lower bout, which may be the 'A-2', although I'm not absolutely sure of the terminology there. If nothing else, more coupling between the low-order modes tends to give you a more 'interesting' sound, ans probably increases efficiency as well.

The important thing is that Dana has figured out a reasonably reliable way to get good coupling going on his guitars without the use of any technology, and without even knowing about the resonant modes that are involved. His methods are not foolproof, in part because he doesn't really know what he's doing. For example, if the top 'cross dipole' mode is to low to begin with, no amount of thinning the edge will get it to couple with the 'cross' air mode. OTOH, thinning the sides in the area below the waist might help in that case, by increasing their compliance and lowering the air mode pitch. Dana relies on getting wood with high cross grain stiffness to make this work, but, again, he doesn't actually measure this, except by feel. This is one of those cases where 'tradition' could be helped by a little 'technology'.

Colin, have a cell fone and computer , so thats a start,Allan thanks for the detailed description of dana/s methods.I copied that voicing article, but frankly it went right over my head,I understand the semi-tones and the resonance part of your analysis, but I get lost when you start talking abt cross monopoles and dipoles.I should remember this stuff from high school as I have a ham licence.I have the gore /gilet books, but a lot of the theory goes way over my level of understanding.I need to make more instruments to get a better understanding of all these interrelated technical variables.I appreciate you taking the time to explain it.I get the construction methods in the build book.But have a very dificult time understanding the theory aspect of it.Will have to reread the theoritical aspects over again sitting next to my work bench.

Ernie, if you've followed the plans closely, leave the lower bout back brace full height across its width and you should come out close to where it needs to be. Best to be stiff rather than too floppy. Getting a live back right, on a classical guitar, requires precise placing of the resonances, which, for me at least, requires measurement tools. If you don't do the measurements, you are much more likely to get a better guitar by staying stiff/non-live. Classical guitars with backs that are too floppy really suffer from a lack of everything.

Actually, that reminds me of that great statement in the Grondona/Waldner book about the Torres "cardboard" guitar, a quote from Domingo Prat: "this guitar has an extraordinary sound, if perhaps a little muted, bland and low, as the author of the dictionary was able to confirm when he played it in the house of Tarrega"

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks trev , I did it exactly as you mentioned and followed the plans, and the back is now very stiff. I had originally used sycamore (buttonwood)braces, and for whatever reason, the back dried out and one of the braces cracked. Half the back was dried out and came undone.So I decided to do my first live back. Must say , it took a much longer than I had anticipated, but it came out fairly nicely. Thanks ernie