Soundboard Radius

[Tod Gilding]

Hi All, Can anyone enlighten me on the effect that a different radius on a soundboard would have ?

For example if I was to build two identical guitars, with the only difference between the two being the soundboard radius, what difference could I expect ?

[DarwinStrings]

The more you radius something the stiffer it gets, so that would be a outcome given that every thing else is the same like thickness, original wood stiffness and bracing. Hopefully the engineer mob will correct me if I am wrong here.

Jim

[Puff]

IMHO - In the long-grain scheme of things yes but cross grain will be less.

[seeaxe]

Thats interesting - why do you think would cross grain stiffness be less? I wouldnt have thought it would change if the sound board is curved around the main axis of the axe....

[Puff]

Place dunny roll on end and stand on it. Topple dunny roll and catch up with it by stamping on its side.

[Kim]

Given that the top is very thin softwood, I am no longer convinced that after the initial bend and the wood being given a few days to settle in, that there is any variation in stiffness whatsoever over a flat top. I use to be in the camp that believed that the forced form would increase stiffness, but the more I think about it, the less I believe that is actually the case in the long term.

But then that is just my thoughts and to each his own. One thing I will add is that a radius in the top does provide a buffer allowing it to withstand greater swings in relative humidity before it splits in the dry times, and that alone makes it very much worth the effort. As for any tonal advantage, some of the best sounding guitars around are true flat tops so it just depends what the maker does with the rest.

Cheers

Kim

[Tod Gilding]

Kim,
I Think I am with you on this one,I have worked as a professional firefighter for the past 16 years but prior to that I was a licenced builder and draftsman who worked with structural engineers on every job, I have gained a lot of knowledge in structural engineering and I am struggling to understand how carving a brace to a radius could help to stiffen a soundboard to a great degree,if the brace was stressed (having an inward force applied as to give the brace an upward arc) this would provide a soundboard that would withstand downward pressure, but not in an upward direction) as a novice luthier I am struggling to understand how this could be a good thing,as I believe the soundboard should be free to vibrate in both directions, concave and convex, please dont take this the wrong way as I know the traditional methods work, I am just struggling to understand How.

[Tod Gilding]

[Place dunny roll on end and stand on it. Topple dunny roll and catch up with it by stamping on its side.


][/quote]

Puff , Can I get an ounce of what your smokin

[kiwigeo]

A stressed top sounds better than one that's not stressed. My first build was a flat top OM, my second build was a classical with a 25' radius top. Both guitars had sitka tops but the classical sounds miles better than the OM.

I work my tops according to what I call the John Holmes rule.......ie. a top must be light and STIFF.

[Lillian]

Are you sure you want to use that standard Martin. From what I just read, almost stiff would be an accurate description of that standard.

[Kim]

That is hardly a comparison Martin. A steel string OM v a nylon string classical both of which had sitka tops. What of the bracing? What of the back, sides and over all dimensions?....What of the hundred other ingredient that matter? Where is the science supporting the notion that a 2mm thick piece of spruce bent into a 25' radius will retain greater stiffness than the same piece of wood not bent into a radius??....I know on the surface it 'seems' like that would be the case, and that may well prove correct should the material being tested be something with the long term resistance to bending and structural integrity of carbon fibre or perhaps spring steel. But we are talking 'very' large celled softwood here. Surely the cells on the underside of the top would simply compress and those on top would elongate and the lignin which hold it all together would conform in a very short while much like it had been heat bent. Once that happened any added tension in the short term must be lost and with it goes any added stiffness. As I see it, in the long term, such a top simply becomes the same wood in every way as that left without a radius except it has room to contract further during periods of low humidity.

Must add again that there really are some fantastic sounding 'true' flat tops out there. Larivee is an example, my advice to anyone currently taking it for granted that a radius topped guitar is somehow tonally superior to a flat top is to get hold of a good one and have a play before you jump to that conclusion. As is often the case with our craft there are many ways of doing things, not always better ways, just different. i.e. thick bracing thin top, thick top thin bracing..Q: which is better?? A:The best balanced of the two course.

Cheers

Kim

[Dave White]

Kim,
I Think I am with you on this one,I have worked as a professional firefighter for the past 16 years but prior to that I was a licenced builder and draftsman who worked with structural engineers on every job, I have gained a lot of knowledge in structural engineering and I am struggling to understand how carving a brace to a radius could help to stiffen a soundboard to a great degree,if the brace was stressed (having an inward force applied as to give the brace an upward arc) this would provide a soundboard that would withstand downward pressure, but not in an upward direction) as a novice luthier I am struggling to understand how this could be a good thing,as I believe the soundboard should be free to vibrate in both directions, concave and convex, please dont take this the wrong way as I know the traditional methods work, I am just struggling to understand How.

Tod,

Forget the structural engineering a take a quick course in Quantum Mechanics - imho the stuff that happens there is of much more relevance to guitar making, unless you don't want your guitar to fall down

[Dave White]

Are you sure you want to use that standard Martin. From what I just read, almost stiff would be an accurate description of that standard.

Lillian,

In that case you just brush some liquid viagra on the top

Seriously though - what does "almost stiff" mean?

[Dave White]

Didn't we have this discussion on a recent thread here ?

[seeaxe]

Hi All, Can anyone enlighten me on the effect that a different radius on a soundboard would have ?

For example if I was to build two identical guitars, with the only difference between the two being the soundboard radius, what difference could I expect ?

Tods original question - what difference would more or less radius make?

I agree with Jim - more radius = more stiffness but only in the longitudinal direction - the analogy is a piece of flat plate, and the same size plate bent into a channel or C section - the latter is way stiffer. In the other direction - across the soundboard - more curvature makes no difference to stiffness.

If you go too far though you must eventually make the top so stiff that it's too stiff and detrimentally affects the sound, so I guess over time people have found the sorts of radii we conventionally use work best.

My conclusion is that if you wanted to build a very light guitar, you could maybe provide the equivalent stiffness by introducing more curvature - this would provide more resistance against the overturning effect of the bridge. You could of course do the same thing by having no curvature and beefier bracing.

If you have enough time and zoot, I guess you can experiment and find out, but as Kim says you would have to make sure everything else is exactly the same. I believe a few people test their soundboards to compare stiffness before they put them on the body

Good luck anyway

[Tod Gilding]

Thanks for the replies so far, it seems that I am looking at the soundboard radius in the wrong way, (I think I'm getting It)

[Alfred J]

The conventional wisdom, according to the voicing DVDs I studied, is that less radius helps to give comparatively more bass. I was using 33ft radius on mini jumbos (40cm across the lower bout). Then I made a 30 ft radius dish and I'm giving that a try in my latest attempt at building the holy grail. There are a lot of other factors that come into play, though.

[Allen]

I don't worry about what the exact radius is as it changes so quickly and drastically with swings in humidity. I can build one with 25' radius and it might be almost flat one day and about 10' radius another day. Of course if you're lucky enough to live in one of those places were the RH stays a perfect and constant 40% every day all year round, then you may be able to build an instrument that you could evaluate that way.

[Dave White]

I've been trying to get my tiny brain around this one so maybe Puff and Richard can help me out here. The dunny roll analogy doesn't help me much as it seems to me to be more about how tubes behave - cut the dunny roll down the length, put it one end and then stand on it, that's more like a free plate of a guitar top.

So tops have different stifnesses longitudinally (with the grain, long axis) and laterally (cross grain, short axis) and typically the longitudnal stifnees is a lot more than lateral stiffness. So are you saying that if I arch along the longitudinal axis (simulating ladder bracing) then I actually increase the ratio of longitudinal to lateral stiffness even more? If I want to bring them more into balance then I should arch around the lateral axis (braces going head block to tail block? Also if I use a spherical doming (using a radius dish) then I increase both lateral and longitudinal stiffness by the same amount and keep the ratio the same? If this is the case then Howard Klepper has things all wrong. This is from his website:

"“My system of arching is different from most. I used to dome my tops and backs, but now I arch laterally (from side to side), but not longitudinally (from neck to tail). The shape I use is not quite circular; it is called a spline curve. The reasons for using my system are somewhat technical, but I can explain them without using numbers or equations. Wood is far more flexible across its grain than along its grain. You can feel this easily if you pick up a piece of thin wood such as a guitar top and flex it. The longitudinal stiffness in spruce (Young's modulus) may be as much as 30-40 times greater than the latitudinal stiffness. This is perhaps the most significant of the ways in which wood is what engineers call an anisotropic material--it is not the same in all directions. One effect of this is that sound travels faster through wood longitudinally than it does laterally. A major purpose of the bracing in a guitar is to increase strength and stiffness in the weaker latitudinal direction.

Builders of cellos and violins have found that the best sounding instruments are made from boards that are proportionally stiffer across the grain. One cello maker I know of will not use a piece of spruce that has a ratio of longitudinal to latitudinal stiffness greater than 15 to one. Doming a guitar top to a spherical shape raises both longitudinal and latitudinal stiffness. It does not improve the ratio, and the amount of latitudinal stiffness that can be imparted is limited by the stress added by simultaneously flexing the wood in its longitudinal direction. If you pick up a thin piece of wood and flex it, you will readily see that it is more easily flexed in either one of these two directions than in both directions at once. Arching the top of a guitar in the lateral direction only is an opportunity to raise the stiffness in the direction in which it is lacking--across the grain--and to do so more than in a spherical doming system, because the wood can take a higher arch if it is flexed latitudinally only. I believe this helps the sound by causing the top to vibrate more evenly in all directions--decreasing its anisotropy. It adds more stiffness in the direction in which it is most needed, and allows for lighter bracing without sacrificing strength. Last, I use a spline curve for the arch because it is the curve that a thin panel such as a guitar top will naturally follow when flexed. This means that by using this curve the panel gains the most stiffness in proportion to the internal stress that the flexing induces. I use this arching system for both tops and backs.”

Enquiring luthier left brains need to know

[kiwigeo]

That is hardly a comparison Martin. A steel string OM v a nylon string classical both of which had sitka tops. What of the bracing? What of the back, sides and over all dimensions?....What of the hundred other ingredient that matter? Where is the science supporting the notion that a 2mm thick piece of spruce bent into a 25' radius will retain greater stiffness than the same piece of wood not bent into a radius??....I know on the surface it 'seems' like that would be the case, and that may well prove correct should the material being tested be something with the long term resistance to bending and structural integrity of carbon fibre or perhaps spring steel.

Fair enough comments. I do actually have two early classicals which have different degrees of arching on the tops. The tops are Sitka from the same batch of tops (same tree most likely). Bracing is identical (vanilla Torres fan bracing) as is top thicknessing. The instrument with the tighter arch sounds miles better than the other one.

A definition of stiffness - my understanding of the term is the resistance of a material to deformation.

I believe arching affects stiffness of a top. Take two pieces of unbraced spruce, bend one piece into an arch and leave the other piece unbent. Push your finger into the top and see which one deforms.....it will be the unarched top. Perhaps not very scientific but you get the picture.

[Puff]

Even taking all the internal bracing out of the argument we still do not have a free plate if it is constrained by the ribs/sides.
From strung up get-go an arched/domed top is under compression for most of its resonating life while a flat top is under tension. Recurve has a degree of both.

[seeaxe]

Dave, my thoughts are:

I would agree longtidinal stiffness of the top is much larger than cross grain stiffness.
Ladder bracing was probably developed a the simplest way to match or equalise the stiffness in the two directions

But I've read the quote a few times now and I am not at all sure what Mr Klepper is doing. He says he arches laterally but not longitudinally - that will increase the stiffness in the longitudinal direction. I believe that modern steel strings have adopted this because of the much higher loads the strings put into the soundboard - we need more oomph or the sound board bellies and distorts.

The second paragraph impiles he bends the soundboard the other way - confusing. If he does that then he has bent it in both directions = domed.

Don't Macaferris have a major bend acorss the sound board?? That would be bent the way Mr Klepper implies it should be. Are they curved the other way too?? Nick??

Anyway, if all you need to do is to make your sound board equally stiff in both directions, you can do that a lot easier by adjusting the bracing than with spline curves. Use Martins method - make it and test it. Put the braced sound board on battens at the head and tail and load it with a set weight - see how much it deflects (this is what I am calling longitudinal stifness btw) . Now move the supports to the sides and do the same thing (lateral stiffness) - "adjust" the bracing until you get the same amount of deflection for the same load. You will then have equal stiffness in both directions. I am not sure that will make the guitar sound any better though!

The other reason for arching is as many have observed to give the top some room to move with humidity swings.

I would agree to the a stiffer top will have less bass than a less stiff one. So putting more curvature into the top should have that effect.

Interesting thread!

[Puff]

Can we bring this back to basics and I will try to avoid analogies Dave
Take your standard twenty-litre cooking oil drum and cut it off two inches up from the bottom to make an approximation of a banjo head - flat top. Take index finger and give it a bong.
Pop it on the bowl lathe or potters wheel and with a hardwood stick spin it into a dome like a resonator cone. Take index finger and give it a bing.
Hey - if I played with the depth of the sides of these, and varied the radius of the dome I might have the makings of a fair drum kit. Love the sound, reminds me of Nelson's Bar, English Harbour, Antigua, circa '75

[Nick]

Don't Macaferris have a major bend acorss the sound board?? That would be bent the way Mr Klepper implies it should be. Are they curved the other way too?? Nick??

You rang?
Yes Macca's have an extremely interesting & truly 3 dimensional top. They have the 'definite' bend behind the saddle (the pliage), they also have two different lateral radius's along the top's length. So the top starts from the neck having a 7 foot radius down past the soundhole then it opens up to 12 foot by the saddle . I also bend in the longitudinal direction also (just off the top of my shiny dome, I don't have my notes with me & my memories shot ) to a radius close to 25 foot, so it is truly a 3D shape or barrel. It was based on what Mario Maccaferri knew best at the time....Italian Mandolins.
Along with the top thickness of 2mm & the ladder bracing, it is thought that that (the barrel shaped top) is what gives the Macca's their typically cutting/thin gypsy sound. They also have a fair bit of projection, especially the small oval holed versioned Selmers.

[Dave White]

I would agree longtidinal stiffness of the top is much larger than cross grain stiffness.
Ladder bracing was probably developed a the simplest way to match or equalise the stiffness in the two directions

Richard now I'm really confused. Perhaps we are confusing ourselves with each ohers definitions and terminology of lateral and longitudinal arching. By lateral arching I mean that the top is straight along the line from neck to tail block and drops away on each side - ie the curve goes laterally across the top (across the grain) but it is arched around the longitudinal axis of the top (with the grain). This is what Howard does and where his "spline curve" goes. Arching is part of the process and the bracing completes it. From your quote above we all seem to be in harmonious agreement that this will increase the cross grain stiffness and reduce the long/cross grain ratio of stiffness.

Given that most guitar arching is either like the above or on a spherical shape, then they will inrease at least cross grain stiffness and in some cases long grain stiffness as well. I don't know of any that would just increase long grain stiffness - even the Macaferri and mandolin pliages have lateral arching as well.

Hence my confusion with your statement "I agree with Jim - more radius = more stiffness but only in the longitudinal direction". Is this lateral arching as in my definition above (and as Howard uses the term) and do you mean an increase in cross grain stiffness? If so then we were in violent agreement all along and as usual language got in the way. If not we definitely need to explore the Quantum world further