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top and back arcs
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Author:  Rod True [ Wed Sep 21, 2005 11:02 am ]
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Thought this might be of some interest to everyone here. This is a question directed to Bill Cumpiano and his reply from an other forum from a couple of months ago.

First the question.

Mr, Cumpiano, I originally posted this question in the work bench, but perhaps just asking you would be a better place to start. There are quite a bit of discussions going on regarding what radius is used on the back or top of our guitars. I have a good deal of information on building and have to date done a good number of instruments. ( finishing had previously been the big pain). I started building a using a 16' radius on the back with a flat top. I have been through various iterations of radius combos over the years mainly with the top in the attempt to get a better neck angle. I have changed the back from time to time with no particular reasoning in mind . Just that others has done different radii so I thought I would give a different radius a try. Although with the tighter radius, I from time to time, had an issue with the back getting a dimple just forward of the end block. . I have arrived at this juncture doing 28’ Back & top. The reason for the top is I really like the neck angle it hits the bridge just the way I want it to. The back is 28’ just because I think I was too lazy to change the dish one day. All this to say, is the any data out there or just general opinion on the origination and or the why of the guitar back/top radius. I like many others started doing the radius because that’s what I was instructed to do.

Now the reply

I spent the first half of my career ruminating about "which" radius was optimum for both top and back. Then I received enlightenment from the Three Stooges.

I think it hit me when I heard Curly say, "I'm trying to think but nothing happens". I realized that the more I tried to fathom the matter, the more my head hurt. The question bred nothing but more questions. And the sky never opened up and the voice of God never bellowed The Answer.

Yes, like you, I was taught to curve the top and back braces, but I took it simply as a given. It was only until later in my career when I thought it imperative to discover how much was optimum.

But I got over all that.

This is how I succeeded in putting my "mind" to rest.

I learned that acoustically the back is NOT a sound "reflector". That is, save for all but a few frequencies, the sound goes RIGHT THROUGH the back as if it wasn't there--just like you hear the bedsprings in an adjacent cheap-motel-room squeeking right through the thin wall into your own room. The back's function, sonically, is to enclose the soundbox, which traps air inside, which gets sloshed in and out of the soundhole, which creates a high-pressure "hockey puck" of high density air in the soundhole, which vibrates energetically, creating concentric sound pressure waves whose long wavelengths aren't restricted by the physical size of the guitar and therefore which produces the entire low- and lower-midrange response of the guitar.

That's what the back contributes to the sound. It encloses the air inside the soundbox. It turns the soundbox into a...soundbox. It doesn't "focus" the sound like your satellite TV dish. The discovery threw the idea of an optimum reflector-shape into the wastepaper basket.

What I also learned is that you can double or triple the stiffness of a thin plate of anything: paper, plastic, wood, by simply...curving it. So I've concluded that the practical logic to arching or doming or curving (however you want to call it) the top or back is to increase it's structural integrity under stress.

And indeed, the string tension is suffused all over the guitar, like if it was soaked with water, through and through. Everything, it turns out is trying to distort in the easiest way it can, just to allow the nut come closer to the saddle. The sum effect is for the guitar, the entire guitar, to slowly bow away from the strings, each component contributing to the distortion as only it can.

In order for the guitar to bow away from the strings, the weakest component of the structure will distort first, and most. The weakest component is not the neck or the fingerboard, but its the back and top plates (the sides, being dramatically curved are extremely rigid, notwithstanding they are even thinner than the top and back). So how do you stiffen the plates sufficient to oppose the stress adequately?

Well, you could stiffen it with wooden bars across it. But the bars (braces) limit the top's vibrations. Think of the banjo, where the plate's structure is achieved not with braces but by tension. A guitar without braces would tend to sound...more like a banjo. Loud. Sqwuaky. But we can't "stretch" a wooden plate to get it to resist tension loads. We have to brace it with wooden bars. The taller and larger the bars, the greater the constriction, the greater the limitation of its modes of movement.

However if you use smaller bars and curve them so they both support the plate and impart a curve to it, you can get away with thinner, lighter bars... resulting in the same support as larger bars on a uncurved, flat plate.

So my rule about curving the plates is: just...curve the plates. Or, put in other words. the plates should just be...not flat.

Well then, if a little is good, is a lot more a lot better? Well, yes and no. Depends on what "better" means. What examples do we have of instruments with radically curved plates? Well, arched top guitars. What's the consequence of guitars with greatly arched top and back plates? Well, play an arch top. Short sustain, a lot of fundamental, narrowed harmonic structure to the tone (i.e., a less complex tone, with fewer overtones). This extreme predicts to me what direction the tone approaches as you arch the top with an increasingly greater curve. Simply put, at some point the top goes PAST the point of optimum structure, as you increase the curvature, and towards a marked change in the guitar's tone--as the plates become stiffer than need be to just support the tension stresses. Since I personally dislike the tone of archtop guitars, I don't go in that direction.

I simply choose just make my top and back plates...not flat. When I resolved to do that, my head stopped hurting.

Period.


William Cumpiano
Author:  RussellR [ Wed Sep 21, 2005 11:12 am ]
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Thanks Rod

Thats an interesting piece
Author:  tippie53 [ Wed Sep 21, 2005 11:28 am ]
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    Often things are way over thought. There is a point that Mr Cupiano did not touch upon. See when you glue 2 disimilar woods together you creat a hygrometer , or a device that reacts to relative humidity in the air.
     So with the curvature of the braces this does something else that Cupiano overlooked. The strength issue was mentioned but this arch also allows for movement of the wood during humidity changes. I think in the early days before kiln dring and such the trade new of this process and planned for it.
    Guilds in the olden days were very secretive and I read this many years ago when I wasn't yet into guitars. I wish I can remember the writer but it makes perfect sense. look what happens to a plate when moisture is added or taken away , the top moves , if the top where flat the chances of splitting would increase alot.
   So while Mr cupiano tried to make his paradyme fit , the common sense of the old masters was allready there. Please think not that I am trying to make Bill look bad as he is a master and a most sharing man , but often we think things to fit the way we want and often overlook the true meaning.
   
Author:  Dave White [ Wed Sep 21, 2005 9:46 pm ]
Post subject: 
[QUOTE=Rod True]
"Well then, if a little is good, is a lot more a lot better? Well, yes and no. Depends on what "better" means. What examples do we have of instruments with radically curved plates? Well, arched top guitars. What's the consequence of guitars with greatly arched top and back plates? Well, play an arch top. Short sustain, a lot of fundamental, narrowed harmonic structure to the tone (i.e., a less complex tone, with fewer overtones). This extreme predicts to me what direction the tone approaches as you arch the top with an increasingly greater curve. Simply put, at some point the top goes PAST the point of optimum structure, as you increase the curvature, and towards a marked change in the guitar's tone--as the plates become stiffer than need be to just support the tension stresses. Since I personally dislike the tone of archtop guitars, I don't go in that direction.
William Cumpiano
[/QUOTE]

I tried very hard to bite my lip and skip past this post but couldn't, so forgive me in advance if I get a little "excited" in this response.

First let me declare a couple of "prejudices". First I have a HUGE, HUGE respect for Bill Cumpiano - his and Jon Natleson's book has been a huge support and reference point for my initial guitar making. Second, I make guitars that have big curvatures in the tops and backs - my back braces are shaped to a 15' radius and most of the top braces to a 16' radius (note I say curved rather than spherical/domed as the surfaces of my tops and backs are not spherical).

What makes me see "the red rag" is when big name builders who have great influence on other builders (whether they realise it or not) make bold statements that are based on limited (or in some cases when you ask and push - no) direct experience of their own building. These then become the given wisdom that is promulagated and become "facts". The above quote is a classic, and if I was not being polite (which I am trying very hard to be Lance!!) I would say that Bill was talking out of the anatomical equivalent of a guitar's end pin hole. In fairness he says things like "predicts to me" but these sort of qualifications get lost in the folklore and "Chinese Whispers" that come after.

If my guitars had the tonal attributes he attributes to archtops then I wouldn't be building them. Also if Bill had ever seen, played or heard Martin Simpson play a Sobell guitar (Stefan's guitars are a great inspiration for me) then to say that "Short sustain, a lot of fundamental, narrowed harmonic structure to the tone (i.e., a less complex tone, with fewer overtones)" beggars belief. Interestingly Bill missed out the "big" archtop thing - great projection!

Also what about the guitars made by the Larson brothers and Howe-Orme?

A carved archtop guitar top and one that is a flat plate with big curvature induced by bracing are "very" different beasts. As part of an integrated guitar design, arching - as well as providing strength, gives projection, help with trebles (thing of the change in tone as you tighten a drumskin)and I believe (combined with the bracing) great balance,sustain and complex overtones.

Big and non spherical curvature makes guitar construction a little more "tricky" as you can't use domed rims when gluing on bracing or for sanding the side profiles and the whole issue of neck relationship with the body becomes a lot more "interesting".

I would hate to think that builders who contemplated experimenting with arching would be put off by reading things like this.

My own 2c view would be that arching was done for strength and for guitars that could deal better with humidity fluctuations, and that top radiusing was kept small as dealing with necks, bridges etc becomes a lot more complex.

I suspect that Alan Carruth might have something to say about Bill's view on the role of the back in a guitars sound.

Hey - I feel a lot better now
Author:  Colin S [ Thu Sep 22, 2005 3:13 am ]
Post subject: 
Hear, Hear Dave As a professional scientist of 30+ years, I do sometimes get just a little bit fed up with the trend towards 'kitchen science' that sometimes gets expressed by a number of the better known builders. Often based on very little controlled experimentation.

If my PhD students produced unsupported bunkem like that they'd soon be shown the error of their ways!

Yes I've got his book, and it WAS inspirational, but that doesn't make him right.

Colin
Author:  Shane Neifer [ Thu Sep 22, 2005 3:40 am ]
Post subject: 
Dave,

I have to agree with all of your comments, I had the same feeling. The most striking comment was comapring an archtop to a flattop guitar. It is almost like comparing a banjo to mandolin! But!!! I spent a few hours with Bill earlier this year to mostly see his shop and talk about some of the wood that he buys from me on occasion. He is a commsumate educator and is passionate in his desire to share his experiences. I feel that he is, though, a little crippled in the position he finds himself in. He is asked his opinion often, and the "teacher" in him directs him to answer using the knowledge base he has, his years of experience. Bill builds all of his guitars as one offs, using a hot pipe, no side bending forms, no radiused dishes, no CNC machines almost exactly as he describes in his book. I actually asked him if he still uses his cork edged workboards to create the arch of his tops and he took me into a back room where there are a STACK of cork edges. So when he is asked (and can afford the time) he will offer his opinions based on the work he knows and does. We must all realize the service he has provided to the guitar building world through his relationship with Natelson and book they wrote. But still, he is just builder and his comments should be taken as measure of your own beliefs. Do they re-enforce your thoughts or cause you to reconsider what you are doing or are they so different from what you do that they make you look just a bit more critically at your processes with the result of just a bit more attention to your own details? Either way, his comments serve a good purpose and at the very least are a good start to a great debate!

My $0.01 cents worth!

ShaneShane Neifer38617.5880555556
Author:  Howard Klepper [ Thu Sep 22, 2005 4:49 am ]
Post subject: 
I like Bill Cumpiano's non-dogmatic approach to most issues. That being
said, I second everything in Dave White's post. Bill's comments about
archtops were dogmatic and misleading. The best that can be said is that
he did note that his dislike of archtops is personal.

I arch laterally only, BTW (Dave--I distinguish arching from doming;
probably the same distinction you are making re curving and doming). I
do it to a spline curve, so it is not identified by a radius, but I use a pretty
high arch. This enables me to give the wood more stiffness in the
direction in which it lacks it, while not inducing an undue amount of
stress in the wood. Take a flat sheet of anything and compare the ease
with which it is flexed in one dimension with the difficulty of trying to flex
it in two dimensions at once, into a dome, and you will see what I mean
about undue stress. With wood, the unnecessary stress from doming is
multiplied by the far greater stiffness it has longitudinally. My guitars are
characteristically strong in their midrange and treble fundamentals all the
way up the neck, including the plain strings, where most flattops get thin
and plinky sounding. They also have long sustain. Works for me.Howard Klepper38617.5862731481
Author:  Dave White [ Thu Sep 22, 2005 5:35 am ]
Post subject: 
Shane,

As I said I have - still have - huge, huge respect and admiration for Bill. On my website I describe him as the "midwife" of so many guitar builders via that book, his website and desire to communicate/educate. The whole guitar making area is an amzing free sharing of ideas, techniques and innovations that is pretty unique - very liitle of the "secret hoarding" you find in other areas. Also people like Frank Ford and frets.com and forums soch as the OLF. I also love and applaud "mavericks" in all areas - I am definitely one myself by nature. By the way, I brace my guitars on a go-bar deck with little bits of cork arranged in appropriate places to get the shape!!

Howard,

Yes - I was just trying to find words to indicate surfaces that are far from flat, but are not spherical or part of a spherical surface (the "Music of the Spheres" or a load of balls?). Your technique is interesting and sounds similar to Stefan's. If you look at his guitars they are like the surface of a cylinder front to back. I agree with you about the lateral thing, which is why I love and hunt out bearclaw European spuce as this generally has very good lateral stiffness. I think of the shape my tops become as more "pre-tensioned" rather than stressed, rather like the skin of a tunable bhodran, and via the bracing shape most of the curvature is supporting lateral "stiffness" (this is also why I like X braced backs). And I agree with you what arching can do - "great sustain" is the second thing people say about my guitars after they have said "boy that's loud".
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