Double back discussion.

[Nick]

Very interesting, Nick i've seen posts on double backed flat tops but not on archtops. Is the inner back on the archtops arched or flat?
Cheers,,, Rod

I have no idea Rod, it was Dominic who posted about the double backed archtop Yeah I must admit I would be interested in hearing/seeing one but as I say, most top archtop players either play them on their knee or slanted away from themselves so would seem like alot of effort to go to for minimal gains (if any) on an archtop.

Nick if I may speculate.
Without taking the structural functions of top and back and sides out of play, inner backs will act very much the same as an undamped single back. The top will still be under compression between bridge and neck. Under tension between tail block and bridge. The effective back will still be under tension between neck block and tail. Albeit to a lesser degree.
Having learnt that reasonable 'wood' can be made from inexpensive veneers and fibre and glue, experimentation does not have to be toxic or expensive - but it is still time consuming.
Oooops - probably just set myself up for incoming

Speculate away Puff, that's why these discussions are always good & I'm always more than willing to review my ideas on things, I'm far from having all the clues on this luthiery caper! I've never been one for working things out on a board or having screeds of mathematical formula, I'm very much a design from idea, build the first one & tinker with it type of person.
You are correct that it is essentially the same as having a responsive single back & that is possible if played in the sitting position but hang a strap on the guitar & stand up & your responsive back is no longer such, it becomes a reflective back as the player's body damps out the back's movement. It would be possible to turn it back into a responsive back by merely tilting it away from you as you play (ala Archtop players & BB King ) but then makes it uncomfortable to wear as a corner digs into your ribs & uncomfortable to play as your fretting hand's wrist has to bend back further still. I also wanted to remove any bracing from the 'active' surface of the back just incase traditionally high bracing causes any funky wave paths inside the box, I don't know if any do occur (like I say, I'm more of a suck it & see kind of guy), but anything put in the way of a soundwave must have some effect on it, if I could remove a few of those interferences then it must be a more efficient instrument in some respects.

[Dominic]

Hey Rod, its not really a double back archtop, but it has a plate inside that works with the top and back to 'tune' the sound which kind of implies that the thing could be strung up but the top or back could still be removed prior to binding and final finish. I kind of like the look of violin style archtops, with no binding and with a small overhang top and bottom, and this would make it easier to do work inside the box after its complete. I'll see if I can find the article when I get home and post some pictures and other info about it. Looked very interesting.
Cheers
Dom

[Puff]

Dom I feel we have gone away from "double back" to what I might call an "interference plate".
The issues involved are genuine nose-bleed stuff.
The ask is for three plates to almost instantly find synergy across the range.
The need would be to tune the top to the interference and then the sum to the back, which would still need to be doubled to remove dampening

[Puff]

Hi Martin - actually it was you who alerted us to that internal plate link. Thanks.

[Kamusur]

Thanks for the link to Muratows site Puff, very 'heady' (brilliant?) stuff and it will certainly tolerate a few more reads.
Check out Rick Toones Neutral Tension Bridge (engineering reminsicent of a Bigsby) and also his latest offering. Perhaps there are some similar or parallel thought motives being generated here?

Steve

[Puff]

I have all the time in the world for Rick Toone as a thinker and luthier.
That said this Yankee thing of patenting concepts that have been in the common domain for Donkey's years gives me the shits.

[DarwinStrings]

Another question Nick, when you tune your top do you tune the inner back in relation to it and if so how do you go about it, like, what is the relationship.

Jim

[Kamusur]

I'm hearing you Puff

Steve

[Nick]

Another question Nick, when you tune your top do you tune the inner back in relation to it and if so how do you go about it, like, what is the relationship.

Jim

I tune it the same way as I tune an archtop back Jim, when both the inner back & top have been glued to the sides, I scrape away a small recurve at the edge of the inner back until I get an even response right across the plate (from the centre out to the rim) when I tap on the bridge position of the top. So it is a compatible relationship (or complimentary interferance if you want to call it that) in that the two plates work in unison.

[DarwinStrings]

Okay nice Nick, sounds very clever to me and also prompts my next question. Not that I aim to build double backs but up till now all my X braced guitars have been fairly ridged backs (you might say their main mode is much higher frequency than the top). I have almost finished my third balsa lattice and when that is done I am hanging up the balsa for a few years and starting to do some X bracing with much more live backs (main modes closer to the top) so what you are doing is helping me get a grip in that department. I had assumed that you had been tuning that inner back in some way but had to check so the next question is, once you are happy with the relationship between the inner back and top and then you add the outer back are you noticing much difference in the tapping of the top to when you had just the inner back in place?

Jim

[Nick]

Okay nice Nick, sounds very clever to me and also prompts my next question. Not that I aim to build double backs but up till now all my X braced guitars have been fairly ridged backs (you might say their main mode is much higher frequency than the top). I have almost finished my third balsa lattice and when that is done I am hanging up the balsa for a few years and starting to do some X bracing with much more live backs (main modes closer to the top) so what you are doing is helping me get a grip in that department. I had assumed that you had been tuning that inner back in some way but had to check so the next question is, once you are happy with the relationship between the inner back and top and then you add the outer back are you noticing much difference in the tapping of the top to when you had just the inner back in place?

Jim

You are right Jim, I don't tap 'tune' the back to any particular frequency ( say a 5th) to the top as some/most builders do but merely get it working in unison/conjunction with the top. In answer to your question, if I've got everything right (slots or hole sizes) then there is no difference in the 'tap tone' of the box or top after nailing the back on, as there should be free movement of air between the cavity & soundbox, so the inner back sees equal air pressure on each side of it, as if it was just a single back. If I were to make the holes/slots in the head & tailblocks (that allow the air movement between the two areas) small enough, then this would restrict the air flow & there would be a difference as the inner back has some restriction to free movement, this method could be used as a way of controling feedback or sustain but I would imagine you would need accurate data to know what effect different sized holes would have on the whole thing. To the full end of that scale are the plugs I made for the latest ones where the air can be blocked off altogether & the cavity acts as an air cushion. This as I see it, makes it become coupled more to the outer back then (which I tend to 'tune' more in the Kent Everett style of getting 'ringing' evenly in the 4 sectors between bracing rather than to a specific note).