Soundboards: Arched vs Flat.

[pavliku]

I have two books which I have been using to make my guitar; Irving Sloane's Classic Guitar Construction is one, Jim Williams' A Guitar Makers Manual is another.
Mr Sloane shows the use of an arched soundboard, while Mr Williams uses a flat soundboard.
What are the advantages/disadvantages of each method?
I would be very interested in any opinions you may have.
Thanks, Paul.

[Kim]

Arching the soundboard allows more protection against splitting due to a fall in humidity, this is about the only real difference.

More specifically, wood, all wood but especially large celled soft woods like spruce, expand and contracts quite a bit 'across' the grain as moisture in the air rises and falls. A domed top will flatten out as the relative humidity falls and will rise again as it increases. A flat top on the other hand will also rise with humidity, but as it falls with a drop in RH, it can only become concave to a point and then the wood must give way and will split because it has nowhere else to go.

This is not such a problem if you build in a relatively stable environment and then keep the guitar there. But if you were to say build in the tropics when the RH was at 70%, and then you moved south to a cold dry 20% or lower, the guitar top and maybe the back also would probably split.



Cheers

Kim

[Allen]

There is another advantage to doming a sound board. It tensions the board and thereby increases it's stiffness.

This may allow you to make the soundboard thinner if you like. As most of the mass in a guitar top is in the top itself, and just a fraction in the bracing, a small change in the thickness of the top has a dramatic change in the overall weight of the top.

Plenty have been built both ways, and really Kim's explanation is the overriding factor in building with a dome in the top. At least for me. Each method will produce an instrument with a guitar like sound, but that's just where the magic starts, and the endless pursuit of building a better instrument begins.

Tops can be built with a shallow radius, or a steep one. Can be built like a barrel or just a dome in the lower bout. They will all change the dynamics of the instrument, not to mention the way you have to set the neck etc. to get everything to work out so you end up with a playable instrument.

My advice is to not over think your first or even your third instrument. It's going to take quite a few just to get the skills required to put one together without things going pear shaped. After this you will have some idea of where things can get modified, and more importantly you will have the foundation to know what effect those changes have based on past experience.

[matthew]

I like the sound of a pear-shaped soundboard.

[pavliku]

Thanks Kim and Allen.
Very useful information.
Paul.

[J.F. Custom]

For mine, I put an arch into all soundboards for the reasons mentioned above but a colleague of mine, a prominent and well known Brisbane based luthier builds them entirely flat without exception. For him, he feels he is unable to elicit the 'response' he is after on his soundboards if he builds with a radius.

I certainly haven't heard of him having any issues arising as a result of his building methods and he has been building since maybe the 70's. I think much will have to do with how you build - bracing style, type of instrument, sound you are attempting to achieve, environmental conditions etc etc. Soooo many variables...

Anyway, arching works for me so he must be doing something wrong

Jeremy.

[matthew]

when you guys talk about arch in a soundboard, you are talking about putting a radius in an otherwise flat plate, using a dish, curved bracing and go-bars, right? Are you achieving a three dimensional arch, or more a dome as you would get bending a sheet of paper?

This is a different process and result to carving an arch which has its inherent strength.

[Kim]

For mine, I put an arch into all soundboards for the reasons mentioned above but a colleague of mine, a prominent and well known Brisbane based luthier builds them entirely flat without exception. For him, he feels he is unable to elicit the 'response' he is after on his soundboards if he builds with a radius.

I certainly haven't heard of him having any issues arising as a result of his building methods and he has been building since maybe the 70's. I think much will have to do with how you build - bracing style, type of instrument, sound you are attempting to achieve, environmental conditions etc etc. Soooo many variables... Jeremy.

That is why I have become a little skeptical about the alleged tonal advantages of adding a radius. A lot has been said about it, but most seems to be based upon assumptions. Some of the best sounding production guitars I have ever heard have been made by Larrivee..this is a company that has also been build em dead flat since the 1970's.

I did once believe the preloaded tension adding stiffness theory, but these days I am not so sure. I agree that the wood does resist being flexed, so it is easy to assume it stays in tension once forced into a radius. But wood is wood and the spruce so thin. I think that once it is held in radius for more than a day or so, any tension would dissipate throughout the top as some fibres compress, and others conform. I would guess that at this stage there could be no real difference expect for the structural advantage already mentioned.

Cheers

Kim

[Kim]

when you guys talk about arch in a soundboard, you are talking about putting a radius in an otherwise flat plate, using a dish, curved bracing and go-bars, right? Are you achieving a three dimensional arch?

Yes Matthew, the 'radius' is formed in a dish and runs front to back, left to right like a slice from the top of a beach ball. Many, myself included, then flatten the upper bout area with a substantial upper transverse brace that has no radius at all. This makes it easier in setting up the neck angle and avoid the 14th fret hump so common on acoustic guitars. I does mean that most of the radius of the top is then restricted to that area of the lower bout behind the bridge and extending up to the soundhole.

Cheers

Kim

[matthew]

So what tonal difference do you find when you add "dish"?

Do you find your bracing can be lighter?

[Kim]

So what tonal difference do you find when you add "dish"?

Do you find your bracing can be lighter?

I will allow that question to go through to the keepers of the notion that doming the top has a tonal impact. I have already stated my case based upon my limited understanding, experience, and musings, and that is to say that placing the top into a radius allows a structural buffer against the adverse effects of dramatic reductions in relative humidity from that in which the top had originally been glued to its bracing.

Cheers

Kim

[Dave White]

So what tonal difference do you find when you add "dish"?

Do you find your bracing can be lighter?

From my own perspective, "hunting out" bass response is easier than "hunting out" treble response. Keeping the top thicker and arching/doming means that the top's pitch goes up and helps with the trebles. I also believe that a domed top gives better projection and cut through of the instrument's sound - like in archtops but the top isn't carved into the arch. Lastly I believe that you end up with an interactive potential-to-kinetic energy system. You have the top and braces in tension with some stored potential energy. The strings activate this and turn it into kinetic energy - my physics analogy is probably pretty crappy here. I profile my braces to a 13' radius but I don't use radius dishes to glue up - just strategically placed pieces of cork - so the final radius/arch/dome of the top is complex and not necessarily a 13' spherical surface.

Some builders like Howard Klepper in America advocate cylindrical arching as this brings the lateral stiffness of the top into line with the greater longitudinal stiffness.

Looking at pictures of the so called "killer" pre-war Martins their tops are in pretty much the same domed shape that mine start at - even though they may have been flat originally. Why wait 70 years

[Nick]

Having knocked up a few Selmer (overarched, you could say) italian mandolin type of tops, I'm in the camp of a radius providing that little extra tension in the tops to add projection & to my thinking,a more responsive top. Not only can the top be made thinner but in my head it would react quicker also, a tensioned drum head has more attack & quicker response than an untensioned one. I'm not saying the flat top doesn't have tension as the strings are a natural provider or stress but I just see it as having "just a bit less". Having said that I don't use that 'extreme' radius on my ordinary guitars, dropping back to the more traditional 30 foot radius.
In violins the brace is carved 'over radius' so that the already carved top is pulled into tension when gluing the brace on, this provides it with the projection/volume that violins are known for. Over time the brace looses this tension & the volume drops off, that's where it's easy for violins being hide glued together, you just remove the top, carve off the old brace & glue a new overarched back in, bringing it back to it's original state, unfortunately we can't do that with guitars .
I guess at the end of the day it all comes down to what 'seems right' for each individual builder. If we didn't have different thoughts on things like this, every guitar would be identical.

[matthew]

In violins the brace is carved 'over radius' so that the already carved top is pulled into tension when gluing the brace on, this provides it with the projection/volume that violins are known for.

many schools of thought on that one There are some that "spring" the bassbar and many like me who don't. The big difference with the arches I think is the 'recurve" - that area of the plate where the curve transitions from convex to concave and which provides the flexibility to move.

[woodrat]

Interestingly I noticed that the tops of Brook guitars were flat and I asked them about it during my recent visit. Their take on the subject was that they build them extremely dry and naturally expect them to take on a rise as they are used in the normal humidity levels in the UK i.e. quite damp. So maybe that is another way of prestressing a top by using hydraulic pressure to pump it up after it is built!

John.