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I've been lent a book Talking Wood a guitar makers diary by Andy Manson a famous by UK standards luthier, and to quote "For some years I thought that the flat top guitar should be strutted in such a way that the leverage of the bridge was resisted by stiffness around it, and the sensitivity would come from thinning the board towards the edges. It does work, but really it's not very special like that. Guitars made in this way tended to be rather quiet and not very rich intone. I wanted excess. Lots of volume, bags of tone and sustain into next week. So then I thought - what I'm after is the opposite of what I'm getting. Let's try then to do the opposite and see what we get. Keep the struts firm towards the edge and light around the bridge. Radical! If you throw a stone in a pond with shallow banks, the ripples more or less disappear. Now try a swimming pool - they bounce right back, I've found recently, by using the classic Martin x-brace, but keeping it really quite light, and getting the stiffness around the edges by adding quite small struts, the effect is generally a lot more lively than by controlling it using just the basic pattern." Dunno what you feel about that statement?

Now on my first guitar I lucked out on tone, I over sanded it cause of a poor rosette and ended up 2.4mm in the middle and 2.8mm at the edges before joining to the rims and final sanding this ended up in a tone I love and strive for I. I've just started on the book above and came across that paragraph and wondered if i should stick to something that's worked for me even though different. How do you feel about this guys approach?

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Formerly JJH

I learn more from my mistakes than my successes

Since building my first and doing some research I've started to wonder how important inletting all the finger braces and tone bars into the sides is to the crisp rich sound of the old Martins. Thinning around the perimeter sounds good on paper because it will allow more movement of the top as a whole but I also wonder what the actual effects are on the sound of the guitar.

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Jeremy Douglas

This is how its done on the 'MArie Amntoinette' Baroque guitar plans, and how they did it in the old days. The little struts go out about 1/2 inch, about 10 a side and then they used paper to hold the top on. It seems like this would be the same effect, but the thinning is around the edges up to .87 which is the same as now give or take for a 1.00 top .

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Actually, I know Michael Bashkin(bashkinguitars.com) does his bracing this way, with an X brace, and 3 tone bars radiating like a spoke from the back edge of the bridge plate. This tone bars are peaked at the edges and thinned near the bridges. I can truly say, that his guitars are some of THE most responsive guitars I've ever played. So yes, it can be done, and works. But my guess is that it took him 100 guitars to perfect it. For the hobby builder to try this and succeed, I'd guess you'd be lucky to get it to work as well as it does for him. There are so many other factors that affect his tone. This being only one of them. You should test it, and see what happens. Can't hurt!

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Tracy
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I like the theory of more movement around the bridge and stiff at the edges for better tone.That's the way a drum is designed and works quite well.So this all does make since to me.I like the idea of stiffness around the edges to increase sustain and give more tone to the top.Mabey a thicker lining at the top would help to stiffen the rim.

Know you know why some folks have made a switch to double sides and stiff linings.

Hmmmmmmmmmmmmmmmm, might be something there eh?

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Hi, John. I love your analogy of stones thrown into a shallow pond and then into a swimming pool. I don't know what others will make of that, but I think it's an insight that might lead you and a few others into some very interesting discoveries. Of course, you might run into some "bumps" along the way. That's to be expected. I'm just encouraging you to experiment. Please let us know where your experimentation leads you.
Patrick

Someone please explain how a deep swimming pool equates to a stiffer perimeter on a top

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The most effective way for the top to produce sound is to move as a loudspeaker cone, in and out of the body. We call this the 'main top' resonant mode. Of course, unlike a speaker cone, a guitar top does need to have enough bending stiffness to resist the static load of the strings over the long term, and this means that we can't use the sort of 'soft' edge that a speaker does. Still it's possible to think of the top as a sort of piston, moving in and out. If you know how the top is actually moving you can figure out an 'equivalent piston' area and mass for this mode; how big and how heavy the top would have to be if it was, in fact, moving as a lflat piston, to have the same sort of output as it really does.

In an article in the Catgut Society 'Journal' in May of '02 Bernard Richardson, one of the more prominant reasearchers in the field, talks a lot about this. He makes the point that the stiffness and mass distribution in the top have a lot to do with the equaivalent piston area and mass: you could have two tops with the same weight and stiffness as measured by bridge deflection, that would have very different output. In particular, he shows that reducing the stiffness in the center of the top is more effective at increasing the piston area than reducing it at the edges. This seems counterintuitive, but it make sense when you look at what's happeneing. More to the point, it's what the author quoted found. This is, of course, exactly what Martin has been doing for years with scalloped bracing. This profile increases the output of the 'main top' mode, and enhances the bass range in particular.

Stiffening around the edges would be expected to raise the frequencies of the two modes that usually show up next as you go up in frequency; the 'long dipole' and 'cross dipole'. Neither of these is particularly effective in itself in producing sound, but the 'long dipole' often does engance the output of the guitar through coupling with one of the internal air modes. Neither the long dipole nor the 'A1' air mode by itself is a good sound producer, but together they can have a significant output at some pitches.

The ineffectiveness of the 'cross dipole' in itself can have an effect on the sound. It's generally the next higher pitched mode above the 'main top' resonance, and, because it's not very effective by comparison, if it steals any energy from the main top mode the output of the guitar in that range is likely to be reduced from what it could have been. The closer in pitch the cross dipole is to the main top mode, the more energy it will steal, so moving it upward in pitch tends to increase the output of the guitar in the low and mid range. Since there's a lot of bending going on in the 'wings' of the top, outward from the bridge ends, for the cross dipole, stiffening those areas up can broaden the main top peak in the output spectrum. Usually the effect of this is to give a 'fuller' or 'more solid' sound.

Again tradtional scalloped bracing, with the X brace height low at the bridge and high in the wings, lowers the main top resonant pitch and increase the output, AND it raises the pitch and lowers the amplitude of the cross dipole. It's a double whammy: more output and a broader band for the main top peak.

There's no free lunch. The lower mass and stiffness in the center of a scalloped top gives lower impedance, so the top is more easily over driven. There is also a structural risk: you DO need some stiffness there, and reducing it too much can drastically reduce the life expectancy of the guitar top. It's perfectly safe if you know what you're doing, but you do have to know.

Good stuff! But I'm a little confused. I use a traditional Martin scalloping pattern but it seemed that when I stopped inletting the lower limbs of the X, moved the outboard peaks in a little, and started thinning the edges of the top in the lower bout the sound improved quite a bit to my ear. Am I hearing that thinning the edges may not be that great an idea in some cases?
Thanks
Terry

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Terrence: if I'm reading what Al wrote correctly, he's saying scalloping is more effective than thinning, not that thinning the top around the edges won't work. Everything being equal, therefore, thinning around the perimeter is simply another tool in the box to 'loosen' the top a little, and can be used in conjunction with scalloping.

I really do appreciate the reference to the book -- there was a ton of good information in that 1 short quote...

And then....
Leave it to Al to explain what is going on... and inadvertently answer one of the questions I have about a recent build...

Top is Quite light -- it was floppy and cardboardy crossgrain at 0.125" -- and I took it to 0.100" thick.

Bracing is an aggressively tapered version straight out of Cumpiano...
The main X is 3/4" tall and right around 5/16" wide -- tapering down to about 1/8" at the linings..... so it perfectly fits the model of "Stiff in the middle"
.....

and the sound is exactly what is described between Andy Manson's Quote and Al's description of what a stiff centered guitar would sound like....

What I noticed about it...

Tone is basically what you would describe as "Guitar" -- Nothing out of the ordinary.
Response is fairly balanced -- not too trebley, not to bassy.
Volume seems good.
I haven't been able to overdrive it yet... The louder you strum, the louder it plays... all the way up to "ROCKSTAR!!!!" I think it actually *sounds* better when you really get into the thing and play LOUD... like the tone is better....

and... the sound is much better with Mediums than with Light strings.

All of this surprises me because the upper X legs telegraph a little bit.
But... apparently this is totally consistent with this bracing scheme...

Sounds like it is time to go in and shave the X legs a little bit.... add see what happens...

Thanks

John

Keep us informed please John

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Formerly JJH

I learn more from my mistakes than my successes

In the end I emailed the author and he gave me his phone number, we had a good chat about how he selects his woods, as he hand pick his tops and they are all roughly the same stiffness, he said he sands them to just under 3mm around 2.9mm he then scrapes the inside face, he doesn't like sandpaper he said learn to use a scraper and you'll get a better finish and save money! He then takes an off cut from an old sound board and uses that to reinforce the sound hole, does the rosette, then braces it see link below

http://www.andymanson.co.uk/photos-group-90.html

This shows his process he told me to repeat my current build where the centre was 2.2mm and the edges 2.8mm to see if I can repeat my sound rather than trying to imitate his with extra braces round the edge, and you just have to do an experiment with water in a shallow edged bowl and a washing up bowl drop something in to see the ripples and you can see his reasoning, but talking with him he now has over 1000 instruments under his belt so maybe not repeatable to the likes of me!

Also talking to him he's moved away from hide glue now and for the last 10 years has been using titebond, though did say his favorite animal glue when he was using it was rabbit skin glue I though that was for gold leafing though, he's also used pearl glue with 100% success. He said if the joints are perfect the glue line is so small it wouldn't make any difference tonally.

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Formerly JJH

I learn more from my mistakes than my successes

John,

Thanks for posting the follow up info with Andy.
I really appreciate the info.

I was curious whether be built things "lighter" in the center or "heavier" on the outside.... and then whether it was done through bracing or through soundboard graduation or both. I think it looks like it is "heavier" on the outside... but I can't really tell for sure.

Now... I guess my question... from a design perspective.... If you don't want a tremendous belly ... you can't lighten things up too much in the middle... unless you can find a smart way to help distribute the load away from the X and evenly into other braces....

And then the next problem -- I am thinking that if you thin quite a bit around the edges, and in the center too... the whole thing moves too easily and the "Main Air" frequency drops too low (Around E and lower) even though it is structurally sound....

But.... How to do this? I am currently pondering on this.
Al posted earlier that this is basically what scalloping braces does
and what thinning the soundboard just south of the bridge does.......
Is the dumb answer just "Scallop your Braces" -- but choose the scallop locations carefully?

Thanks

John