Advice on x braced nylon string

[John Cross]

G'day all

I'm partway through a couple of builds and am close to the point of bracing the tops on two guitars. They will both be nylon strings but I'm considering doing one (or both) with a x brace pattern rather than the more typical fan braces. They are not classical copies nor do I have specific expectations of what they "should" sound like. I've heard a couple x braced nylon strings that I thought sounded good and I know a couple folks here have done them. My question is how would you modify a typical steel string x bracing pattern for use with nylon strings? I'd suspect you could go much lighter but how much (i.e target height of braces)? I'm thinking also of only having one tone bar rather than 2. I found a picture of a martin pattern that uses only 1 that I'm using as a starting point. The guitars are both OM sized (1 will be a 12 fret model, the other a 14). Any thoughts, suggestions or advice is appreciated.

[Alan Carruth]

I've made two X-braced Classicals, both on my version of the 12-fret 000 platform. The first, in '01, was BRW/Red spruce, with more or less 'standard' X bracing. Like an idiot I didn't record the top thickness. The main X was 10mm tall at the intersection, 7.5 at the bridge location. The upper tone bar was 5mm at the high point, and the lower on 3mm. I think all the braces were 1/4" wide. I liked the sound, but thought it might be nicer with a cedar top.

The second one is the 'Autumn' guitar you can see on my web site. I don't have the top thickness on that one either, but it came out a lot lighter: 173 grams as against 194 for the spruce one. I used 'double X' bracing on that one, all about 1/4" wide. The main X was 9.5mm tall at the intersection, 9.5 at the bridge. The lower X was only 4.5 tall at it's intersection. Sharon Isbin REALLY LIKED that guitar when I showed it to her.

Hope this helps.

[John Cross]

Thanks very much Alan. That is even more specific than I was hoping to hear.

[Kerry Werry]

Interesting, as a total novice I thought the X brace was not well suited to the Classical guitar.. Alan you have proved me wrong.. (not hard to do)

Kerry

[Mark Swanson]

It isn't that it isn't suited for it, it's just not traditional. Tradition drives much in the instrument world, and its influence is very strong in classic guitars.

[Alan Carruth]

I've noticed in looking at the resonant modes of a lot of assembled guitars that there is one systematic difference between steel and nylon string instruments: on the steel strings the 'cross dipole' mode of the top is generally higher in pitch, usually by 50 Hz or more. The 'monopole' and 'long dipole' modes are usually pretty much the same. Often the cross dipole on a Classical will be close enough in pitch to the monopole that it may cut into the sound the monopole puts out. This would show up as a cutoff of the high side of the monopole peak; instead of extending out to the right it's chopped off more vertically. This can be especially pronounced in Flamencos, which tend to have the cross dipole at a lower pitch than Classicals.

One researcher found in comparing listener response to guitars with the spectra that ones with a narrow monopole/'main top' peak in the output tended to be heard as 'cutting', and even 'harsh' when the peak was particularly narrow. It could well be that Classical makers have taken advantage of this in some degree to help make up for the lack of high frequency in the strings and get a more 'cutting' sound. Fan bracing makes the soundboard more flexible across the top, and lowers the cross dipole pitch. Flamencos often leave the two outer fans out entirely, which drops it even further.

Guitars with a broad 'main top' peak in the output tend to sound 'fuller'. This can be a help with steel strings, which typically need bass to balance out all the treble. X bracing, which adds some crosswise stiffness, bumps the cross dipole up, and allows for a broader 'main top' output peak.

It's possible to make either bracing scheme work on any sort of guitar, of course. But, as is often said: "The race is not always to the swift, nor the battle to the strong, but that's where the smart money goes". If you're X-bracing a Classical top, use a larger/wider pantilla (the 000 is an inch or so wider in the lower bout than most Classicals), and don't make the braces too tall. I've never tried building a fan-braced steel string.