Falcate braced classical

[Trevor Gore]

I was forgetting that you already have the neck on, being a Spanish foot build! That means you may well have some corpus modes showing up which I wouldn't normally be seeing at this stage (and they don't often show up later, either). At least one (or even more) of those "spare" monopoles is likely a corpus mode. If the neck is vibrating a lot, that's the give-away. When these corpus modes show up, there is often one not far from the T(1,1)1, maybe your 136Hz and then one around where the T(1,1)3 often lives, maybe your 220Hz.

Nailing down the back coupled T(1,1)3 peak is a mystery. It should be a monopole but the only peaks close to the 272Hz back peak are cross dipoles at 253 and 279Hz. Can T(1,1)3 possibly ever be a cross dipole mode peak????

As Jeff mentioned, the back may not be coupling in if it's main monopole B(1,1) is at 272Hz. There isn't a coupling mechanism for a top cross dipole to couple with a back monopole, because the top cross dipole will provide no net pressure change inside the guitar to drive the back monopole. Corpus modes ~220Hz can interact with the T(1,1)3 if the T(1,1)3 would normally be in the same zone. When I've seen these two modes close together they are opposite phase and one can cancel the other. It doesn't make things any easier, but fortunately it seems to be pretty rare. Tuners and a fretboard will change the corpus modes significantly and on my builds seems to attenuate them out of significance.

[kiwigeo]

Corpus modes??? Fascinating.

Ok I can test this by clamping a huge weight to the neck and reducing ability of same to vibrate cant I? If I clamp the guitar by its neck to my bench saw table (solid metal) I should see a reduction in the corpus mode peaks. Another job for tomorrow.

Thanks Trev and Jeff for the valuable comments.

[ernie]

Martin , question , Did you use the same sizes for your CL falcate braces as the gore #4 med steel string plan in the book?? The reason I ask is I/ve never done a falcate yet . I just sanded some englemann spruce laminate to 1.7mm to make a falcate brace using a nail board. I made one Cl gtr using the fleta plan except for the soundhole patch, and would like to try to make a falcate brace out of spruce. We get a lot of inexpensive spruce from the west coast at our local home centers .Thanks ernie.

[jeffhigh]

With the back at 270, I doubt you will get a T(1,1)3 showing at all.

For what it's worth, my advice would be to get the bridge on with final sanding polishing etc saddle and strings then retest and decide where your target frequencies will be.

I would suspect that you will find a T(1,1)2 of 190 easiest to achieve by the time the bridge mass, back tuning and maybe a little side mass are taken into account.

[kiwigeo]

With the back at 270, I doubt you will get a T(1,1)3 showing at all.

For what it's worth, my advice would be to get the bridge on with final sanding polishing etc saddle and strings then retest and decide where your target frequencies will be.

I would suspect that you will find a T(1,1)2 of 190 easiest to achieve by the time the bridge mass, back tuning and maybe a little side mass are taken into account.

I was thinking last night that there's not much I can do with the back at 270Hz.

Just to clarify. If I could bring the back resonance down to match the top cross dipole peak at 253Hz that wouldnt work? If I (impossibly) bought it down to the monopole peak at 220Hz that would work? Or is it just that the back isnt mobile enough at 272Hz...ie the monopole at that frequency isnt strong enough to pump the front?

[jeffhigh]

You have not scalloped the main back brace yet have you?
My last falcate Flameco with this back bracing started out around 270 with full height brace, took forever to start coming down as I reduced the center height then suddenly started moving.
Didn't start showing up in the top response till I got down atound 255 or so

[kiwigeo]

You have not scalloped the main back brace yet have you?
My last falcate Flameco with this back bracing started out around 270 with full height brace, took forever to start coming down as I reduced the center height then suddenly started moving.
Didn't start showing up in the top response till I got down around 255 or so

No, lower transverse back brace is at original height. Im concerned that scalloping this brace to bring the back resonance down by circa 15Hz (if its possible) will also lower main top resonance markedly.

If making a new back is going to be my eventual course of action I guess it wont hurt to play around with the back resonances....will be a valuabke learning experience.

[jeffhigh]

You have not scalloped the main back brace yet have you?
My last falcate Flameco with this back bracing started out around 270 with full height brace, took forever to start coming down as I reduced the center height then suddenly started moving.
Didn't start showing up in the top response till I got down around 255 or so

No, lower transverse back brace is at original height. Im concerned that scalloping this brace to bring the back resonance down by circa 15Hz (if its possible) will also lower main top resonance markedly.

If making a new back is going to be my eventual course of action I guess it wont hurt to play around with the back resonances....will be a valuabke learning experience.

Yes it is possible and yes it will lower the main top
That can be useful depending on your target for the top, I'm inclined to think you will not achieve 202 once the bridge, sadlle strings and finish is on, if you are on say 198 then you can push it down to 190 by a combination of scalloping the back and side masses.
Or you can leave the back it as it is and consider it non active, your choice.
Replacing the back won't help, It is the frequency that you achieve which interacts with the top

[kiwigeo]

So target for T(1,1)3 will be in the vicinity of 240Hz as per G and G but exact frequency wont be known until I get there....if I get there?

[jeffhigh]

On my last, I was looking for a 190 top but by the time I lowered the back close to 4 semitones above and did everything else I had to retarget it to 180 which is fine for a flamenco.

See where you are at on assembly, maybe drop the back down to 255 and you will see what is feasible for the top.
Everthing you do affects everything else, approach your targets but leave a little for fine tuning and side mass.

[jeffhigh]

I als retuned a classical which I built in 2010 using Bogdanivic bracing which had always felt a bit constrained.
I dropped the top from 205 to 190 by sanding down the fan braces and adding side mass. The back by chance ended up at a good place.
Made a hell of a difference.

[Trevor Gore]

What Jeff said. (Thank you, Jeff!)

[kiwigeo]

Thanks Trev and Jeff. Ive got an interesting day ahead......I dearly want to spend all day in the workshop tap and Chladni testing but the boss has a day off work so she has other plans. Time to see how devious I can be......

[kiwigeo]

This bldi guitar is going to be the end of me. I redid the tap testing but had guitar sitting on foam pads on my table saw this time around. results are significantly different. Plot shows top and back testing results...average of 5 runs for each. Back resonance has dropped significantly to 257Hz.
T(1,1)1 has dropped to 86.8Hz and T(1,1)2 has dropped to 194Hz. I have yet to re-do Chladni tests on all peaks. Will do this tomorrow and put up results.

[jeffhigh]

No idea why it should have dropped from previous.
Try it in playing position, what are you using to tap?
Does the top feel really soft? it may stiffen up with the bridge especially if you do a CF laminate, and that may bring up the T(1,1)1 too

[kiwigeo]

No idea why it should have dropped from previous.
Try it in playing position, what are you using to tap?
Does the top feel really soft? it may stiffen up with the bridge especially if you do a CF laminate, and that may bring up the T(1,1)1 too

Previously I had the guitar sitting upright on its butt on my bench. Trev mentioned possibility of bench affecting readings. The latest set of data was recorded with guitar sitting on four pieces of foam on the saw bench.

For tap tests I'm using an eraser on a piece of dowel.

Top doesn't feel stiff. Around the bridge area its fairly stiff as theres a dirty great bridge patch underneath.

Out of interest I did some tap tests with a 12gram weight (20 cent coin) stuck on the bridge patch. T(1,1)2 dropped to 172Hz and T(1,1)1 dropped by a couple of Hz. A 36gram weight dropped T(1,1)1 to 78Hz and T(1,1)2 to 153Hz.

The extra monopoles between the T(1,1)1 and T(1,1)2 peaks that Trev thought might be corpus modes are of great interest to me. I did tap tests with 3 ebony fretboard blanks clamped to the neck, 2 G clamps on the headstock and with the guitar clamped by its neck to the saw bench......in all cases the extra peaks still remained and amplitude and frequency were unchanged. I did some tap testing on a bridgeless fan braced classical I have hanging around the shop and this guitar also has similar extra monopoles between T(1,1)1 and T(1,1)2 peaks. Both guitars were built using Spanish Method.

[kiwigeo]

A plot showing spectrum of top with and without 2 G clamps attached to the headstock. Note that with the clamps on the headstock the peak at circa 18Hz disappears but all other peaks remain with some peaks reduced in amplitude. Im no expert (yet) but to me this suggests that the peak at 18Hz is definitely a corpus mode peak but what the other "extra peaks" between the T(1,1)1 and T(1,1)2 peaks are remains uncertain.

[kiwigeo]

It started as a straight forward classical build...until Martin discovered Gore and Gilet's evil books. Things rapidly went downhill and Martin spent the rest of his life chasing "extra monopoles".

mad-scientist-movie.jpg (37.24 KiB) Viewed 17903 times

[Kamusur]

Too much Martin lol

Steve

[Tod Gilding]

It's a lot easier Martin if you have an illudium q-36 explosive space modulator

I think you can get them at Stew Mac, and Chris P will soon have a video out.

[kiwigeo]

LOL...

[kiwigeo]

Redid Chaldni tests on top and no real change in the situation regarding the lack of T(1,1)3 peak on top. Back resonance has dropped to 254Hz but no top response to suggest coupling between the two.

As Jeff suggested Ill go ahead with fretboard and bridge and get the finish on and then decide whether to leave things as they are or play around with back resonance.

[Trevor Gore]

Martin, have you tried just holding the guitar as is you were playing it, and then tapping it?

[kiwigeo]

Martin, have you tried just holding the guitar as is you were playing it, and then tapping it?

Not yet....next on the list of things to do. Should the back of the guitar be against the body?

[Trevor Gore]

When I play (and tap) only the edges are in contact.