Falcate tertiary brace modification

[camlv]

Hi, my first post so hello anzlf...

I am wanting to make a guitar using all Australian woods and I'm looking at using King Billy Pine for the top with falcate bracing. I know this is not an ideal wood for the top as it is too soft and would need to be thicker and therefore heavier to get the target stiffness. Looking at the bracing that you use I have come up with a slight modification to add more stiffness against bridge rotation whilst adding only a little extra mass to the tertiary braces by curving them (pic below), would this be a workable solution to adding more stiffness? Would this in turn allow a slightly thinner top? And lastly, I could see this reducing the long dipole but maintaining monopole mobility, is that a good outcome if it allows a thinner top? I feel like I am treading in murky waters as I am new to all of this.

Thanks,
Cam

[camlv]

Some what coincidentally I just got around to reading the section of the design book on frequency response. If curving the tertiary braces would reduce the cross dipole I assume this would reduce some of the guitar's mid frequencies and this would be undesirable for a finger style guitar. Would I be right in assuming this? I might save the curved tertiary brace idea for a dreadnaught build.

[kiwigeo]

Trevor is the expert in this but from what I can see the primary and secondary braces should supply sufficient reinforcement to counter bridge rotation and the tertiary brace's function is primarily to provide cross grain reinforcement in the area between the primary and secondary braces.

Over to you Trevor...

[Trevor Gore]

King Billy has about half the stiffness of spruce for around the same density. So you have to increase the thickness of a KB panel by ~25% to get the same stiffness as the spruce, which increases its mass by ~25%, too. So ask yourself "Will a small curvature of the tertiaries add the 25% extra top stiffness required to compensate for the 25% increase in panel mass?" I think you know the answer.

(And whilst asking yourself that, ask what that's going to do to monopole mobility.)

The next question should be " So, how should I increase the top stiffness?"

I think you know the answer to that, too.

[camlv]

I wasn't thinking this would magically negate king billy's lack of stiffness, I was just trying to think of new ways of adding a bit more stiffness.

I have no idea what would happen to monopole mobility if you substitute stiffness provided by the top plate with stiffness from the braces. Sorry if they where stupid questions

[Trevor Gore]

Sorry if they where stupid questions

No, not stupid questions, but the answers are in the book and I'm trying to encourage you think it out yourself, which I'm sure you're capable of.

Let's look at the first part:

" So, how should I increase the top stiffness?"

Well, you don't want to increase top thickness, because it increases mass. You're looking for ~25% increase in stiffness and the most efficient method of increasing top stiffness that much is to increase the stiffness of the primary and secondary braces, either by increasing their height or by adding more CF. All the details are in Section 4.4.

Alternatively, depending on how adventurous you feel, you could thin down the KB until it is the same mass as a typical spruce top and then put a dense CF lattice beneath, rather like a classical guitar, to bring it back up to normal SS stiffness with normal range resonances. I'm doing a koa topped guitar like that at the moment. Use KB, WRC or spruce for the bracing substrate though, rather than balsa (insufficient shear strength). For lattice bracing ideas, check out Fig 4.4-10.

(And whilst asking yourself that, ask what that's going to do to monopole mobility.)

Monopole mobility = 1/SQRT(k*m) where k is equivalent stiffness and m is equivalent mass. So if you increase k and/or m (the stiffness or the mass of the top) monopole mobility gets smaller which decreases your guitar's responsiveness, generally not what you're chasing in an acoustic guitar. The stuff on monopole mobility starts in Section 1.4.9 and it's application is in Section 1.7.2.. Using KB for a top means you start behind the 8 ball, compared to using spruce, because of its low stiffness. So if you want performance comparable to a spruce top, you have to get exotic with your bracing to ensure typical overall stiffness whilst maintaining the commensurate mass.

You can switch the concept around and ask yourself, well, why not use a spruce top and a lattice and go really high mobility? The answer to that is that you could, but you'd likely get a guitar that sounds more like a banjo (because that's where uber high monopole mobility leads), which may or may not be a good thing, depending on how you want to use the guitar. You can choose what level of monopole mobility to aim for by considering Fig. 1.7.8.

[camlv]

Ah thanks for clearing that up,

I had read those sections but I didn't remember the Monopole mobility = 1/SQRT(k*m) equation. I had in my head that an in increase stiffness was one of the goals, as decreasing mass is. I might save experiments with bracing for future builds, that way at least I have a good basis of comparison. Remembering now the trick is to withstand the static forces whilst still providing maximum sensitivity.

[Trevor Gore]

Ah thanks for clearing that up, ...

No worries!