Double back discussion.

[Dominic]

Nick, I have been pondering your double back some more and was wondering if you could use spruce for the inner back. I don't see the additional weight to the whole guitar being an issue (see smallman guitars) but a spruce inner back may give a more lively and responsive plate and since it is free from any structural responsibilities could work better with the lighter top. Any thoughts?
Dom

[Nick]

Nick, I have been pondering your double back some more and was wondering if you could use spruce for the inner back. I don't see the additional weight to the whole guitar being an issue (see smallman guitars) but a spruce inner back may give a more lively and responsive plate and since it is free from any structural responsibilities could work better with the lighter top. Any thoughts?
Dom

I can't remember his name but there was a guy in the states that was trying the spruce, I never saw the result (don't think he ever finished the guitar! if he did he didn't post it up) so can't comment on the results of that particular one. I can't see a reason for not using spruce, I just wonder about the complimentary interference, whether the inner back would overdrive the top (be too responsive?). I guess you could brace it so you could limit the amount it does this but the beauty of using other types of wood is that I can 'dial in' the amount the inner back responds to the top plate by the amount of recurve I carve in it's boundary. I also sit in the camp that the back works with the top in shaping a timbre for the guitar (not merely as a reflective surface), so carrying this idea further I should be able to almost shape the sound produced by the guitar. A harder back such as the maple I currently use, gives me a crisp bright, articulate sound, if I replaced it with say, mahogany, it should produce a more fuller mellower sound. Of course there are many other factors that shape the sound (such as bracing, top woods e.t.c) so the back is only one part of the chain, but to me, just as important a factor as say getting the top plate bracing right.
But that's not to exclude your Spruce idea, could be worth trying at some stage, would certainly remove a few ounces from the guitar's weight, although the Maccaferri was commented upon as "this feels very light" by it's owner. It's hard to say on Elsie because I used the heavier Maple necks.

I may copy this discussion over to the main forum as you initially said Dominic, could be an interesting discussion getting a few others input.

[Nick]

Dominic & I started discussing my use of double backs in the gallery section when I posted my build. To Dominic and myself (at least! ) it was getting interesting so I thought it would be a good idea to bring it into the main section, maybe a few others would have some valuable input or ideas.

[Dominic]

Looks its just the two of us Nick. There are so many possibilities when i think about it once the back that is in play acoustically does not have to perform any structural duties. For a start you could make the spruce back a tighter radius to stiffen it up to keep the back and top at different pitches while maintaining the weight loss and keep the braces underneath so you present a clean inner surface to aid reflection. Or flatten it more to get more bass response. You could also seal the air space to give you some resistance and use a much looser inner back. Or you could suspend the inner back from the very edges of the soudboard leaving a gap all around in order to pick up some of the direct energy from the top as it exits near the bound edge but also the inner air movement as a normal top does. There are so many possibilities. Can't wait to get back to more workshop and get into a few new projects.
Dom

[DarwinStrings]

How much of the inner back do you think you can hear. Like, with a active back rather than a rigid back I would have thought that most of the sound we hear from it comes off the outside of the back rather than being funnelled out of the sound hole.

Jim

Botanical names ROCK!!!

[Nick]

How much of the inner back do you think you can hear. Like, with a active back rather than a rigid back I would have thought that most of the sound we hear from it comes off the outside of the back rather than being funnelled out of the sound hole.

Jim

Botanical names ROCK!!!

To me Jim, I don't think you can hear the back specifically. My thinking is that it interacts with the top to shape an overall sound. Mahogany backed guitars will sound differently than Maple ones. A rigid back acts more as a reflector to push the soundwaves back to drive the top further & push some sound out of the hole (Most of a guitars sound comes off the face of the guitar not just out of the soundhole, stop the top vibrating while playing it & listen to the volume drop dramatically), I'm not sure if there would be any frequency cancelation going on or not but there could be? An active back reinforces the top's vibration by moving with it, I think Ervin Somogyi refers to it as 'complimentary interference'.
My original thinking & main idea behind having two backs was to eliminate the dampening effect the player has on it by having their stomach against the back . Most top archtop players tilt the guitar away from themselves to eliminate this & allow the back to vibrate freely otherwise the guitar sounds choked & lacks both volume & sustain (this is where I essentially got the idea!), the same happens on a flattop to a lesser extent. Having the double back I can rest my ample gut against the 'outer' back and still get a good amount of sustain, I haven't done extensive testing of this 'theory' but my ears seem to detect a longer sustain, I can have the guitar laying in it's padded case and it plays the same as if I was holding it in me grubby mits (not that playing a guitar in it's case is much use of course! ).

[DarwinStrings]

Hmmm looks like my poor writing skills are at it again. I didn't mean that most of the sound we hear from the guitar is off the back, I meant that when the back is playing a part in the sound, what we hear of the back is off the outside of the back not from out of the hole.

I can see now your point about the gut factor, you want a live back that interacts with the face but still be able to hold the back against your body.

I also realise your point about stopping the top from vibrating and you lower volume, when you stifle the top even if you are not in contact with a live back you also stop the backs movement as well as stopping bass reflex.

I thought the thinking behind a rigid back was to stop phase cancellation at higher frequencies therefore adding volume and or projection rather than as a reflector of sound waves.

Jim

[Puff]

It is certainly not just the two or three of you Dom - just need some more time to get things succinct

[kiwigeo]

My original thinking & main idea behind having two backs was to eliminate the dampening effect the player has on it by having their stomach against the back .

Has anyone done a study of the size of a players beer pot and the amount of dampening? Would be a perfect project for a government funded phD I reckon.

[Nick]

Has anyone done a study of the size of a players beer pot and the amount of dampening? Would be a perfect project for a government funded phD I reckon.

Larger the pot the more bass frequencies get 'absorbed' I'm guessing .

Yeah sorry Jim, after I posted I hoped the bit about soundholes didn't sound like I was trying to teach grandma how to suck eggs! But my brain just gets on a train of thought at times & I have to get things down otherwise I forget the whole point I'm trying to convey & loose the thread. Aint getting older great
I must be honest in that I haven't spent a great deal of time looking at rigid backs other than the advantage of the volume they create ( Thanks to Mr Smallman), you may well be right about the phase cancelation, just in my head I see a possibility of some frequency loss as certain frequencies clash on their way from the front to back & vice versa (hence some Classical offcianado's claiming Mr Smallman's guitars lack warmth & bottom end), whereas the active back I see working in unison with the top, but there must be a limit to just how active the back is otherwise with the back able to resonate too freely it may just get into a state of uncontrolled vibration (get into a feedback state with the top), which was what I was trying to get at with Dominic in my original post about bracing the spruce correctly or heavily enough . I may well be looking at that all wrong & more than willing to be corrected

[kiwigeo]

Has anyone done a study of the size of a players beer pot and the amount of dampening? Would be a perfect project for a government funded phD I reckon.

Larger the pot the more bass frequencies get 'absorbed' I'm guessing .

Would depend on he sort of beer used to cultivate the pot I reckon. I think a Monteiths Radler pot would impart a more flowery tone than say a DB draught pot which would result in a more down to earth and ordinary tone.

[Puff]

My original thinking & main idea behind having two backs was to eliminate the dampening effect the player has on it by having their stomach against the back .

Has anyone done a study of the size of a players beer pot and the amount of dampening? Would be a perfect project for a government funded phD I reckon.

Don't know if it was govt funded or made it to PHD but i do recall a discourse on roughly these lines which concluded that the size and shape of the pot were certainly factors but the sound and resonance was much more shaped by fluid levels within said pot

[Nick]

Would depend on the sort of beer used to cultivate the pot I reckon. I think a Monteiths Radler pot would impart a more flowery tone than say a DB draught pot which would result in a more down to earth and ordinary tone.

A DB pot would sound like exactly as it tasted...Shit! .

[kiwigeo]

My original thinking & main idea behind having two backs was to eliminate the dampening effect the player has on it by having their stomach against the back .

Has anyone done a study of the size of a players beer pot and the amount of dampening? Would be a perfect project for a government funded phD I reckon.

Don't know if it was govt funded or made it to PHD but i do recall a discourse on roughly these lines which concluded that the size and shape of the pot were certainly factors but the sound and resonance was much more shaped by fluid levels within said pot

I believe that PHD was completed by a bloke who is now one of New Zealand's leading Proctologists.

[Puff]

Some stuff here:-
http://www.youtube.com/watch?v=fAEQKmyB5Nk&NR=1
Dom you are probably way beyond this but it may help others. Erwin's sitting the box on his knee is essentially the same as the "gut" deadening that the double back has the ability to largely remove from the equation. As Nick has pointed out somewhere else the inside back can also profit from not having a pneumatic damper put on it by a sealed airspace between it and the outer back. That said; being able to 'tune' that airspace's dampening effect between very close to free flow of air in and out, and sealed will also add a dimension.
Essentially it all still comes down to synergy between the two operating plates.

[DarwinStrings]

I think you may have mentioned it before Nick but have you noticed much difference when you remove or insert that plug which couples/de-couples the cavities?

Jim

Life is good when you are amongst the wood.

[Dominic]

Good see this thread back on track.

Puff, I have seen this vid before but it does not make it any less interesting. Not having a gut and playing the guitar on an angle anyway while seated I don't get that deadening effect and I make what I think of as active backs. But for the more portly among us a DB may be a solution. But I still wonder if it is worth relieving the back of it structural duties so it can focus solely on working with the top to increase sustain and produce a more complex and interesting sound.

I read an article somewhere about a special archtop made of BRW and the builder put a suspended inner plate into it (but smaller than the back) that he tuned to get the sound he wanted. There is name for this but I can't recall it the moment and I am away from by books and mags so I don't have it handy. But if such a device actually works it would seem to be an area ripe for experimentation. BTW, I also have an idea for a very unconventional top which I'll share with you all once I have tried it out.

As Jim asked, I am also interested in the impact of the plug in the air cavity.

Cheers
Dom

[Puff]

There was this that I think Jim alerted us to.
http://www.woodworkforums.com/f98/my-ac ... gn-127179/

[Nick]

Well I haven't sat & had a good 'play around' to find the true effect of the plugs as yet. Initially the design intention was as a method of controlling possible feedback induced in the inner back under amplifier conditions (I hadn't done an amplified double back build before so wasn't sure how it would react under such conditions). I'm more than happy with the results I've gotten from the double back system in it's acoustic form, giving more sustain & I believe more volume for the soundbox size, but amplified and a guitar reacts differently of course. The onboard preamp has settings to control the normal feeback on the top plate but I wondered "what if the non restricted inner back starts to get driven by the amp speaker also?".
The intial testing I did with the 'plugs' (feedback busters? I really must come up with a more catchier name ) in the open position (allowing air to flow freely between the soundbox & the back cavity) while directly facing the amp's speaker. I deliberately got the guitar into feedback to find the sweetspot for the notch frequency & eliminated that then the guitar refused to feedback even at reasonable volume. Turn the plugs around (so that the passage of air into the back cavity is blocked turning it into effectively a single backed guitar)& no difference....no feedback, although the guitar's natural sustain did drop off & I could induce a drop in the acoustic volume by pressing my ample gut against the back & 'choking' it.
Conclusion: So I was thinking that these feedback busters were infact not really required & that I could have simply been overthinking this fact when I designed the guitar but as my endorser Bob Heinz pointed out to me when I showed it to him, I've only tested in one set of conditions (a medium sized semi reflective room & a small amp), who knows how it would behave maybe on a stage with foldbacks & other instruments competing to be heard on the same stage. The fact that I could turn the plugs around & get the guitar to behave like a 'normal' single backed guitar, meant that they are doing something...putting the guitar back into a known & controllable area where, if need be, a sound tech knows how to control such things or the onboard notch & phase switches can be used to reign in feedback. As far as being able to get these small plugs to have the desired effect goes I'd say they've been successful, as far as total control over any possible feedback from the freely resonating inner back goes, the jury's still out & would need more time to prove & more testing under differing conditions. Or maybe the condition will never exist? i.e the inner back will never get into a feedback state?!
There are otherways this could be controlled such as 'detuning' the inner back (making it less resonant, stiffer), using smaller holes in the end & neck blocks to slow the air movement in the cavity but then the more we do that, the more we head back toward just using a single plate for the back.

[Puff]

Dom:- "But I still wonder if it is worth relieving the back of it structural duties so it can focus solely on working with the top to increase sustain and produce a more complex and interesting sound."
I believe so and am also a fan of relieving the belly/top of as much structural duty as well.
A chassis from neck block to tail block, clear of the top and back. Could be straight flying buttresses between the outsides of near tops and bottoms of both blocks or laminated for tennis racquet/teardrop shape to also strengthen the lower bout perimeter top and bottom. With a relatively massive tail block, also clear of top and back, sustain may be enhanced? Much the same as adding mass to the headstock.
With a tailpiece and floating bridge - voila - all resonating plates are virtually free to do their thing.
With the DB concept the outer back can be the last fitting in the box build allowing fine tuning of the inner back to the top in close to 'real time'.
Tongue in cheek here - with appropriate valves one could also pressurise the DB cavity or rarify it

[Nick]

With the DB concept the outer back can be the last fitting in the box build allowing fine tuning of the inner back to the top in close to 'real time'.

That's the procedure I use Puff, I can tap tune the inner back to the top (I started my acoustic building by knocking up archtops which usues a similar technique so came at it from that angle) then 'close up the box' & glue the outer back on.

The whole double back thing, like most ideas, is not a new thing, I merely extended Mario Maccaferri's thinking when he made an 'inner soundbox' which was coupled to & suspended from, the top plate, completely free from the back & sides & I combined that with my experience of the archtop guitar in the recurve tuning & smooth inner face aspect. I would like to try using different woods for the inner back just to see how much difference the different woods have on the overall sound. Jobs for the future!

[Puff]

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

[rocket]

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

[DarwinStrings]

There was this that I think Jim alerted us to.
http://www.woodworkforums.com/f98/my-ac ... gn-127179/

Not me, first time I have seen that thread but it is a interesting build. Are you a member of that forum Puff?

Jim

[Puff]

Guess I must be Jim.
Was actually hoping you might have taken some weight from Rod's question - will get back to you on the other tonight,
Rod - putting a flat back on an archtop would be heresy even for this heretic. IMSO (S is for simple) the back always has more curvature than the belly whether it be inner or outer back. I may be a bit heretic in material but some design concepts are irrefutable.
For some idea of the interface required this may be useful http://zhurnal.lib.ru/m/muratow_s_w/violin_design.shtml
Exactly the same procedure can be used for any other instrument from uke to double bass. Making male and female molds as per this will set you up as well as anything I have ever come across.