First Falcate underway! Plate Thickness question?

[chrisbaer]

So I'm finally getting around to making a Falcate braced steel string guitar! Very exciting!

I have a couple of questions about plate thickness.

For the Top plate, Trevor lists an f value of 75 to plug into the thickness calculation, but doesn't specify the bracing. For my bear claw sitka top, I get 3mm with an f value of 75, 2.8mm for 70, and 2.6 for 65. Should I use a lower value for falcate bracing?

For the live back plate, I have a nice piece of Madagascar Rosewood - it seems quite dense and stiff. I got some odd values when I tapped it. I tried a few times and got pretty consistent results. It is not a perfect rectangle, but reasonably close.

Flong = 81
Fcross = 80
Fdiag = 67
Mass = 359
Thickness = 4.2

With an f value of 55, I get 2.58mm target thickness. That seems thin to me, but I haven't made one of these kinds of guitars yet.

Do these seem like reasonable numbers?

Thanks!

Chris

[Trevor Gore]

For the Top plate, Trevor lists an f value of 75 to plug into the thickness calculation, but doesn't specify the bracing. For my bear claw sitka top, I get 3mm with an f value of 75, 2.8mm for 70, and 2.6 for 65. Should I use a lower value for falcate bracing?

I use 75 with falcate bracing as per the plan in the back of the Build book. Bearclaw can be less stiff than straight grained timber, thus requiring you to leave it thicker to get the same target vibrational frequencies.

With an f value of 55, I get 2.58mm target thickness. That seems thin to me, but I haven't made one of these kinds of guitars yet.

Do these seem like reasonable numbers?

Yes. That's typical of what I would expect for a live back.

[lamanoditrento]

Congrats Chris on passing the ANLF target thickness rite of passage

[chrisbaer]

Thanks, Trent! I'll feel more of a sense of accomplishment once the top is fully braced! (Fingers crossed)

[chrisbaer]

Thanks, Trevor! Full steam ahead, then!

[chrisbaer]

Well, it isn't pretty (I obviously have to work on my epoxy spreading skills!), but my first falcate top is glued up! Thanks for the help thus far!

[chrisbaer]

Progress! So, my first Falcate guitar is strung up - very exciting! Still have to adjust action and check intonation.

It has a live back, but I've only minimally scooped out the lower brace, so it's pretty stiff.
It is finished. I did thin the top 60mm around the edges.
I have the side weight blocks, but only the bolts in them - 21 grams each.
The bridge is Walnut and pretty light, ~20g (I forgot to weigh it after routing the saddle slot).

Tap testing after being strung up for 24 hrs:

Falcate Back.png (43.87 KiB) Viewed 13980 times

It seems to me that all those numbers are on the high side and the fact that the back resonance only shows up as a tiny peak (which I just noticed! It's not labeled in the first graph) indicates that it's not acting as a live back?

Any suggestions how to proceed?

Thanks so much!

Chris

[Trevor Gore]

The T(1,1)1 and the T(1,1)2 seem a little high, but the T(1,1)1 is likely a consequence of the T(1,1)2. The T(1,1)3 is in about the right place for a live back with a T(1,1)2 at 180Hz. What size top braces did you use and what Young's modulus were they? Also worth checking the T(1,1)2 frequency again. On the chart it looks like it's at a lower frequency than you have it labeled.

[chrisbaer]

Hi, Trevor - Thanks for the response!

I looked at the T(1,1)2 in the first graph data and it's a little lower at 195Hz. The T(1,1)3 is at 221Hz on the first graph. So I can likely lower the T(1,1)2 by adding side mass, correct? Any idea why the T(1,1)3 is so quiet?

The top braces were laminated spruce, perfectly quartered, 5mm thick finished, and 7mm tall tapering down to 5mm or 0mm. I did not check the Young's modulus... That's probably a section of the build I'll have to pay more attention to next time.

I did the compensated nut and saddle and it really seems to have made a huge difference in intonation overall and especially at the first fret, so thanks for that wisdom as well!

[Trevor Gore]

The small peak at 221Hz may not be the T(1,1)3.

You have identified the B(1,1) at 230Hz. Usually the T(1,1)3 is a little higher in frequency than the B(1,1), so I would expect the T(1,1)3 to be around 233Hz. The peak at 221Hz may be either the result of a whole body mode (bending like the fundamental of a marimba bar) or could possibly be a "shorted out" air mode, where the upper bout fails to act as part of the Helmholtz volume due to the narrowing at the waist. Neither of these modes is commonly seen, but nevertheless do occasionally show up.

If the back B(1,1) is at 230Hz (i.e. you are sure your spectrum analyser is giving the right results) and you have very little scalloping of the back brace, it implies that the back is heavy (not unusual for Madagascar rosewood!) That combination of stiffness and mass will likely give a back mobility that precludes an active back. I'd avoid further scalloping of the back braces as that will lower the B(1,1) which is unlikely to be a good idea.

Whilst both the top and back thickness are plausible numbers, they are both on the thick side for wood in my stash. It may be worth re-checking your thickness calcs against Table 4.5-3 just to make sure.

Doing nothing else, just adding side mass to bring the T(1,1)2 to 190Hz should give you a very nice (and loud) non-live back guitar.

[chrisbaer]

Consarn it.

Well, I get closer every time! I'll get there.

Thanks for your help along the way!

Chris