I've been cogitating!

[Nick]

And yes, I have been warned about going blind, but I can't help it, I just have to keep doing it
I was thinking before (on the drive home from work) about back braces on a guitar, I know they vary in height between the differents types, the Selmer copies I make, for instance, have a height of 15mm (nearly 5/8ths for our American brothers) & I was wondering if these would have a noticed effect on the soundwaves in the box. Instead of theoretically, the waves coming straight back from the top and reflecting off in a straight pattern back towards the front. Would the bracing reflect some of the wave off at an angle causing 'soundwave collisions' within the box and some frequency cancelation? Then, as I do , I thought how best to solve this possibility. We cannot reduce the height "profile" too much or the back becomes weak, what if the braces were on the outside leaving the inside 'clean' and smooth. But then you run into the obvious problem of why would you want to play a guitar where you get stuck in the ribs by bits of wood stuck on the outside. Which, after all that schlep, brings me to my question!
Does anybody know or has perhaps experimented with a double backed guitar? I was thinking along the lines of having the 'real' back set inside the sides of the guitar and braced from the outside then having a small air gap between the braces and another outer back, so that it looks conventional from the outside.
If anybody has any thoughts/ input, links on my hair brained scheme it would be greatly received. I wouldn't go necessarily on a professional website's claims because they are trying to sell their guitars and can find any number of justifications for using this maybe gimicky idea.
I have a Mahogany B&S set and Spruce I bought years ago that have been waiting to be hammered into shape so it may just be a case of suck it and see, but I value your opinions so would be interested to hear your thoughts.

[jeffhigh]

Nick,
I believe there have been guitars made with double backs, with the intention of isolating the inner active back from the outer back layer to avoid dampening the back from contact with the players' body.
Not sure where the bracing was located.
A number of manufacurers (including Guild) have used braceless backs pressed and laminated into an arch

[DarwinStrings]

I have never built the type of back you are describing Nick but my understanding is that most of the sound comes off the front of the soundboard and Helmholtz resonance comes out of the hole.

There may be some phase cancellation going on from what you have described and also the deflection of sound by the braces would lose a bit of the sound energy to heat energy but it's my guess that is would not be worth worrying about. I think you would get more phase cancellation from the vibrating back and sides than what you have described.

Try a google on phase cancellation and you might find some worthwhile reading.

Having said that. Experiment is a great way to work things out and would love to hear how you went if you go ahead with it and if you think it made a positive difference to your guitars.

Jim

[graham mcdonald]

I wouldn't worry about the braces 'interfering' with the soundwaves. It doesn't work like that. What can be important is how stiff the back is and how it interacts with the air resonance and the lower couple of soundboard resonances. Folks like Al Caruth know lots more about this than I do, and Al especially thinks that the stiffness of the back can have a lot to do with the overall response of the instrument. He works with classical and steel string guitars and the Selmer type instruments are something of a different beast. Al has published quite widely and the articles he has written for American Lutherie are well worth reading. You can search a pdf at the GAL site (www.luth.org) and they can sell you any back copies you don't have.

At the same time, I don't think any of the guitar acoustics researchers ever take into account the damping factor of the human body wrapped around a guitar, and there have been numerous examples of double backs made in an attempt to take that out of the equation. None have been commercially successful as far as I know.

cheers

graham

[Allen]

At the Playmakers conference, Greg Smallman was showing us his back construction method of laminating several layers of veneer into a somewhat arch back construction that is incredibly stiff, and as far as I can tell, doesn't have braces. That's one option.

[Nick]

Thanks everybody for the input so far.
Thanks Graham for the Al Caruth connection, sounds like an interesting read. The fact you mention about human damping factor was one avenue I was coming from (the other being the soundwaves getting deflected off bracing), I know from my own carved archtops I've made that the volume can 'choke up' considerably if you pull the guitar in tight to your body whilst playing. Most of the top jazz archtop players will either sit whilst playing or hold the guitar at a slight angle away from the body if they're standing up just to eliminate this problem. Not quite so important these days with amplified axes though.
I now know that the original Maccaferri's had the soundbox in to achieve this very goal, he theorized that isolating the sound chamber from the player would increase the volume. From what I can see though and maybe why the idea didn't work, was that it was mounted to the soundboard. The mass of this box hanging off the prime driver must have dampened it to some extent (a detrimental one by all accounts 'cos they were all removed by their owners and volume immediately increased).
Yes Im not sure if this would be more gimic factor (which I'm erring toward now) than useful technique, sounding more like gimic factor but it might be fun just having a go, the wood I have is not top quality so apart from my time there's not alot of monetary value lost.

[John Maddison]

... Greg Smallman was showing us his method of laminating several layers of veneer ... that is incredibly stiff.

So stiff that he could stand on it!

[Nick]

... Greg Smallman was showing us his method of laminating several layers of veneer ... that is incredibly stiff.

So stiff that he could stand on it!

Haven't tried it , but having carved solid curly maple backs for both my archtops (5mm thick in the centre out to 3mm at the rim) I can quite believe it, the arch shape gives it incredible strength. I'm sure if I was game enough to try and stayed away from the centre seam it would hold my bulk, but I can see Laminations being that much stronger again.

[DarwinStrings]

You guys who heard Greg Smallman talking about the backs of his guitars, did he mention why he made them so stiff, was it because of phase cancellation?

Jim

[Hesh1956]

Tim McKnight offers the option to his clients of a double back. You might want to Google Tim and see if there are any details on his web site.

[Bob Connor]

You guys who heard Greg Smallman talking about the backs of his guitars, did he mention why he made them so stiff, was it because of phase cancellation?

Jim

It was for projection Jim.

[DarwinStrings]

Thanks Bob, 'projection' is that another way of saying volume?

Jim

[Bob Connor]

Not necessarily.

It means volume if you are listening from some distance in front of the instrument. Not from the perspective of the person playing the instrument.

[DarwinStrings]

So that's because the person playing the instrument is behind it. So you think what he was saying was he makes the back stiffer so that the volume is louder to the listener assuming the listener is in front of the instrument. You said "some distance" But I am still assuming the closer the listener is to the front of the guitar the louder it will sound to him or her.

Did Greg mention why this was so? Bob next time you run into him please ask him to join the forum so he can answer my questions.

Apologies for the hi-jack Nick, just seems like a good time to get some input into the Thin or thick back debate.

Jim

[Bob Connor]

I believe Kim gave him the URL of the forum and invited him along Jim.

Yes it is relative but I believe what he is trying to achieve is to project the instrument to the end of a concert hall so that an unamplified classical performer will have some chance of being heard. Particularly if in a concerto setting.

Rick Turner uses the same approach with his steel string guitars.

I tend to go for a flexible back to envelope the player in the sound of the instrument. If you want projection a pick up will provide that.

In the classical world the pickup is not so much of an option (probably due to tradition). However the guitarist that was playing a Smallman at Playmakers did have a pickup installed (although it wasn't Greg that installed it)

Both valid ways of doing things. It just depends on what you are trying to achieve.

[John Maddison]

If anyone wants a copy of the DVD of Greg Smallman presenting his ideas, with classical guitarist Graig Ogden doing the playing, at Playmakers 2009 (followed by Harry Fleishman spruiking his assymetrical acoustic designs) just send a PM with your mailing address.

POST SCRIPT ... 6 November 2009
I made a terrible mistake by sending copies of Greg's talk on DVD to five mebers of this Forum, without his prior permission. See my lengthy post at this thread. Thanks

[DarwinStrings]

Yes I agree they are all valid, I would just like a few more why's answered. The idea of acoustic volume appeals to me as I do like that unplugged sound from a steelstring and other unplugged instrument are loud. I do think that thin may have just a tad more flavor in the sound.

Jim

[Kim]

Whilst at Playmakers09 I did in fact ask Greg Smallman to drop in and join us at the ANZLF. Bob was standing close by and quickly wrote down the URL on one of his McGuitar cards. As I passed the card to Greg I explained that the ANZLF was a stringed instrument forum based right here in Australia.

Greg looked surprised and asked in which state this forum was located? I told him the server was located in Victoria and explained that he need only go to the web address written on the back of the card and he could join in. I told Greg it would mean a great deal to so many if he could find the time to join us here and share his thoughts with us.

Greg smiled and ask if what was written on the back of the card was a website? I said yes. Greg said he may have to take a look sometime but does not go on the Internet very much.

I found Greg Smallman to be very friendly, polite and approachable. A modest man who is more than happy to share his boundless knowledge and expertise with those eager to learn. At the same time I found Greg to be a man who appears somewhat unimpressed with digital technologies, but at the same time, far too graceful to ever express so in conversation if he thought it may offend or embarrass you.

Sadly for us, I would not expect him to be joining us any time soon.

Cheers

Kim

[John Steele]

Paul Hostetter has a bit of info regarding the Selmar/Maccaferri guitars with internal resonaters that were semi double backed. http://www.lutherie.net/resonate.html
I remember seeing pictures of an old prototype (1940ish?) Gibson with a double back & sides. Kind of a box-in-a-box with sound 'vents' around the perimeter.

[Nick]

Apologies for the hi-jack Nick, just seems like a good time to get some input into the Thin or thick back debate.
Jim

No Worries Jim, all information debated is useful info as far as I'm concerned, so if another o/t element of this craft pops up in one of my topics I love to look and learn

might I add that this is a reason I come to this particular forum, somebody comes up with an idea or question and nobody bags them or puts them down as a crazy. I have noticed on another forum that there tends to be a bit of looking down the nose attitude going on.All comments posted her have been useful so a big hats off to all ANZLF forum members and Bob for hosting a great forum :serg

[Allen]

In the GAL magazine that I just received, Michael Dunn is featured, and some of his models have the internal soundbox. He talks about developing the idea over many many guitars, some being very mediocre, to the point now where they work very well. It's a very interesting read, and I'd highly recommend a subscription to anyone interested in building stringed instruments.

[Nick]

In the GAL magazine that I just received, Michael Dunn is featured, and some of his models have the internal soundbox. He talks about developing the idea over many many guitars, some being very mediocre, to the point now where they work very well. It's a very interesting read, and I'd highly recommend a subscription to anyone interested in building stringed instruments.

Thanks for passing that on Allen, I think I've signed up to GAL Apparently they don't send out email confirmations once you've signed up and there has been no charge against my credit card or latest mag so I may have to email them to see I am actually signed up! The articles look very informative and the mags look like keepers.

[Clancy]

Hang in there Nick, the mag will come.
Same thing happened to me last year.
Took forever.
No confirmation of having subcribed.
Then one day the postie delivered my presents.
Nose to the page for the next few weeks!

[Allen]

Same with mine. The back issues of that year came in one go. Didn't pull my head up from the pages except to sip on the beer.

[Nick]

Emailed them & it seemed they had record of my subscription/order saved on the puter but hadn't actioned it. Most apologetic but all sorted now Looking forward to reading the Micheal Dunn article, I think he uses a system similar to the Maccaferri sounbox but obviously with refinements that modern technology allows & his own 'touch' added to it.