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Hi everyone, I am new to this forum and hope you can help me with a query about soundboard radius. I have made a few guitars for myself, most of which have been experimental to some degree.

After trawling the internet to research soundboard radius it would appear that most Luthiers build soundboards with a radius in the range of 25 to 30 feet.

This is primarily done to add strength to the top and allow for more control over top thickness and bracing. As I understand it, as the radius decreases, the tone also increases in ‘brightness’.

Of course, the level of ’brightness’ is a matter of personal taste and not everyone wants this.

However, I can't find any information regarding the effects or practicality of a radius less than 25 feet. Does anyone here know if there is a definitive radius at which the severity of that radius reaches a point where the tone would not be considered favourable to anyone?

Although it is common to build a 15 foot radius into the back of the guitar, does anyone know of any studies/experiments or have any experience of pushing the soundboard radius to such an extreme?

Do you think this is something worth experimenting with or would it be a waste of time and money?

Thanks, Fred.

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If you get much smaller than 25’, you will have trouble with neck/fingerboard alignment to the bridge. 25’ to 28’ puts your neck angle with a straight fingerboard right where it needs to be to get a 1/2” string height over the soundboard at the bridge. With more curve than that, you will have to taper the fingerboard or go with much higher strings at the bridge. Of course, you could use a cylinder rather than a dome. As you decrease the radius, the top will get stiffer (all other things being equal) and you will probably lose some bass.

Hans Bentrup, who used to share here on this forum built with 10 or 15 ft radius(I don’t remember which).

Google "Howe Orme guitars" which btw our friend Rick Turner collects and see guitars with a MUCH tighter radius than any you are describing.

I would add that the impact on tone of the radius that you found somewhere on the Internet is BS.... "Brightness" is a function of many things, materials, bracing patterns, top thickness, back materials and even strings and more...

Before doing any experimenting I would also ask if this is the first guitar that you have built? If so I would study convention and get the wood working, neck angles, lots of new stuff down before I attempt to reinvent the wheel. Just a suggestion, welcome to the forum.

If there is, it's more than you can bend a flat plate into. Carved archtops have been going past that limit for a long time, and plenty of people like how they sound (not me though).


Yes, but it is cylindrical radius, which seems to have less effect on tone than longitudinal curvature. Laurent Brondel also uses a similar design.

A lot of guitars have been built with a dead flat top. And what about the fact that the dome helps to control top cracking in changing humidities?

Ed

Fred, I think the advice above relating to reinventing wheels is truth itself. Besides which, without a baseline it's going to be impossible to tell if a modified technique does what is hoped for if it can't be compared to anything. So you're going to need something you made to a known pattern to compare a subsequent modified one against. It's an awful lot of work to build a guitar. I think you're well served building the first one with commonly-accepted parameters and innovating in the future. Is that a lot of work? Yup.

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Peter Havriluk

I was just thinking of him the other day, he's not been around in a while and his posts were always very helpful. Hope he's doing ok.

Hi everyone and thanks for your responses so far. I think this is steering me towards taking the plunge and testing this out.



Thanks Bob, I had considered an 'odd' geometry with the neck joint and I'm sure I can work around that. My main concern is if this would be feasible tone wise. I can live with a little loss on the bass if that's going to be the only downside.



Hi, thanks for the input - I have made a few guitars in the past. These have been either for my own use in which case they have been made in a more traditional style regarding bracing and timbers etc. I have also made a few to test other ideas - some with promising results, others have been stripped and recycled so I'm not averse to a bit of experimenting.



Hi, as regards a baseline, I would keep everything the same as a previous build (materials, bracing, shape etc) with the only variable being the top radius and associated geometry.

However, I think if I do try this I will make it with bolt on neck (as with previous build) and close the box but before finishing the edges of the soundboard and installing the binding etc I will fit the neck and string it up to see if it sounds any good. That way, it won't be too much of a job to replace the top and re-radius the sides to something more conventional.

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I like 26' on the top. To tight and you are likely to add top cracks to the equation. When you look at the modulus of rupture (I know, there is more to wood cracks than just that) you will see spruce rupturing at around 2/3 the pressure it would take on rosewood. So, I do 26' on the top and about 16' on the back.

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Joe Beaver
Maker of Sawdust

First off, experimentation is laudable.

However I agree with Hesh that a guitar is a complex system and you cannot just boil things down to “more top radius equals more brightness”.

If your main goal is more “brightness”, there are many paths you can go down and many easier variables to experiment with in the realm of bracing design, materials etc that don’t involve fundamentally altering the string geometry of the instrument.

Hi, thanks for the response. I agree there are many other variables regarding different aspects of tone. I only mentioned brightness because I had read it from a reputable builder as consequence of a tighter radius.

I am not particularly looking for an overly bright guitar - I had just assumed that as most people build within the 25 to 30 feet radius, some experimentation must have been done to arrive at this as the preferred range. Unfortunately there seems to be little information that I can find but all the responses above have helped me decide if it is worth a test.

I will give it a go and report back (but it may take some time to fit in).

Watch this space.

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Haven't scanned all answers, so maybe duplicate.

Old traditional classical guitars put a decent radius in the lower bout and less or none in the upper bout. In that case, the fingerboard difficulties with radius are not relevant.