1. falcate braced classical

[JurgenV]

After my actually quite nice results with a steel string falcate braced 000 style guitar I thought I can easily adapt a classical guitar to falcate bracing. Little did I know
I should have believed John Parchem build description and stayed with the bracing height for steel strings. What I did was to reduce the brace height to get the flexural rigity to the values I found in the design book. Not the best idea. I closed the box yesterday and cleaned up everything and naturally I did the frequency analysis with VA.
T(1,1)1= 105Hz
T(1,1)2=183Hz
T(1,1)3= 249Hz
Means that would be a very low T(1,1)2 after the binding and gluing the bridge.

Looks like I have to take the top off again. Any suggestions on how to do that the best way without damaging the sides and/or the back?
Never did that before

And I have to find a new top ...

[Jim watts]

Route it off like you're routing for binding. Set the depth to the top thickness and the width off cut equal to the sides = linings. You might need to do a little hand work over the tail and heel block, but otherwise you're pretty much done.

[johnparchem]

I have built all my falcate classical guitars without a lower transverse brace. Without a lower transverse brace, I am between 10 to 10.5 mm on the primary falcates and 8-9 on the secondary falcate braces to get the top strung up to resonate at 190 Hz and 200 Hz respectively. My first guitar had numbers almost like yours, If you built without the lower transverse brace I think that your top can end up at around 180-185 HZ Hz the way it is. My top resonance goes up when I glue on the bridge. With the thinner top and little internal cross bracing the bridge is a very important brace to add cross grain stiffness with no lower transverse brace. Adding the bridge has bumped up the top resonance more than 5 Hz less than 10 Hz. Stringing up the guitar lowers the resonance around 5 Hz.

After I ended up with a lower than expected number on my first classical I really thought through how I went wrong. I had a couple of thoughts, the first is my target resonance for a classical is 10-20 Hz higher than what I want in a steel string. So the top needs to be stiffer relative to its mass. My classical top is about 20% thinner than a steel string using the same plate. My first thought was that the reduced mass of the top would completely mitigate the drop in stiffness from thinning the top with regards to the top's resonance. The flaw in my assumptions was to dismiss the size of tops contribution to the overall stiffness. The top plate while not tall at 2 mm is wide. So the top plate is a significant factor in the stiffness of the top. Mass is a proportional factor in the tops resonance while the height of a brace is a cubed factor in stiffness. So the change in stiffness form thinning the top can have a bigger effect than the change in mass depending on the top plates overall contribution to the top's stiffness.

Also while I pretend to think in engineering like ways a lot of what I do is intuitive. When I was initially coming up with brace heights they were not only taller than my steel string falcates but were much taller than the fan braces I was using. That difference kept pulling me back from my engineering mind set. It was not until I ended up with too low of a top resonance that it occurred to me that my intuitive sense was dismissing the length difference in my primary falcate braces relative to the fan braces that start at the lower transverse brace. The stiffness of a brace has a cubed relationship to its length.

[JurgenV]

Ok, old top off, new top on. Actually easier than I thought. Thanks the advice Jim.

New values after binding and sanding:
T(1,1)1= 90Hz
T(1,1)2=195Hz
T(1,1)3= 244Hz

I had been wondering why the T(1,1)1 is so low until I checked the diameter of the soundhole...
Great! Instead of 85mm I have just 79mm. Does anyone have a good idea to widen the soundhole in a way it´s not obvious?

@John: I guess with the 195Hz it should be ok or what do you think?

[johnparchem]

Ok, old top off, new top on. Actually easier than I thought. Thanks the advice Jim.

New values after binding and sanding:
T(1,1)1= 90Hz
T(1,1)2=195Hz
T(1,1)3= 244Hz

I had been wondering why the T(1,1)1 is so low until I checked the diameter of the soundhole...
Great! Instead of 85mm I have just 79mm. Does anyone have a good idea to widen the soundhole in a way it´s not obvious?

@John: I guess with the 195Hz it should be ok or what do you think?

I think you can hit close to 190 Hz where you are now. That is my normal target. Just need to see after the bridge and strings. Also if you raise the T(1,1)1 the top resonance will go a bit as well. You would get some of that rise back when you tune the back down a touch.

[seeaxe]

After my actually quite nice results with a steel string falcate braced 000 style guitar I thought I can easily adapt a classical guitar to falcate bracing. Little did I know
I should have believed John Parchem build description and stayed with the bracing height for steel strings. What I did was to reduce the brace height to get the flexural rigity to the values I found in the design book. Not the best idea. I closed the box yesterday and cleaned up everything and naturally I did the frequency analysis with VA.
T(1,1)1= 105Hz
T(1,1)2=183Hz
T(1,1)3= 249Hz
Means that would be a very low T(1,1)2 after the binding and gluing the bridge.

Looks like I have to take the top off again. Any suggestions on how to do that the best way without damaging the sides and/or the back?
Never did that before

And I have to find a new top ...

Hi Jurgen, wie gehts??

I am building my first falcate classical and have got to the point where I was planning to use the Flexural rigidity guidelines in the book (section 4.4.5) to determine the height of the primary and secondary braces. If I understand the "books" correctly the size of the brace needs to be determined so that the soundboard and braces have an EI of around 15 to 20 Nm2 50mm in front of the bridge. I had imagined that this needed to be done for each guitar, taking into account the different pieces of wood used.

From your first post in this thread, it sounds like that's what you were doing. My first attempt at the spreadsheet is giving me very high EI for 10mm high braces (5mm thick) which would indicate the braces need to be much smaller. Is that what your calculations showed? Would you mind sharing what brace height you calculated for your first top?

This is the opposite of John Parchems experience above (he calculated higher braces) so I'm more than a little puzzled. Tonight's job is to recheck the spreadsheet.

How did you determine brace height on the second top?

I note that more experienced builders than I simply leave the braces at 10mm plus or minus a bit.

Vielen danke
RIchard

[johnparchem]

After my actually quite nice results with a steel string falcate braced 000 style guitar I thought I can easily adapt a classical guitar to falcate bracing. Little did I know
I should have believed John Parchem build description and stayed with the bracing height for steel strings. What I did was to reduce the brace height to get the flexural rigity to the values I found in the design book. Not the best idea. I closed the box yesterday and cleaned up everything and naturally I did the frequency analysis with VA.
T(1,1)1= 105Hz
T(1,1)2=183Hz
T(1,1)3= 249Hz
Means that would be a very low T(1,1)2 after the binding and gluing the bridge.

Looks like I have to take the top off again. Any suggestions on how to do that the best way without damaging the sides and/or the back?
Never did that before

And I have to find a new top ...

Hi Jurgen, wie gehts??

I am building my first falcate classical and have got to the point where I was planning to use the Flexural rigidity guidelines in the book (section 4.4.5) to determine the height of the primary and secondary braces. If I understand the "books" correctly the size of the brace needs to be determined so that the soundboard and braces have an EI of around 15 to 20 Nm2 50mm in front of the bridge. I had imagined that this needed to be done for each guitar, taking into account the different pieces of wood used.

From your first post in this thread, it sounds like that's what you were doing. My first attempt at the spreadsheet is giving me very high EI for 10mm high braces (5mm thick) which would indicate the braces need to be much smaller. Is that what your calculations showed? Would you mind sharing what brace height you calculated for your first top?

This is the opposite of John Parchems experience above (he calculated higher braces) so I'm more than a little puzzled. Tonight's job is to recheck the spreadsheet.

How did you determine brace height on the second top?

I note that more experienced builders than I simply leave the braces at 10mm plus or minus a bit.

Vielen danke
RIchard

I am not sure of your bracing design. My classical bracing is like the steel strings no lower transverse brace. I'll admit I did not do an Flexural rigidity calculations, but I have built 7 falcate classical guitars and 5 or 6 steel string guitars. The steel string guitars are based on the Gore Medium sized plans. My target resonance T(1,1)2 for a steel string is 180 Hz. My classical guitar tops are about 20% thinner than my steel string guitars, so much less stiff. My classical T(1,1)2 target is 190 or 200 Hz. So to hit that target the braces need to be taller than my steel string. My first falcate classical had braces just like the steel string 8.5 mm. I ended up with a T(1,1)2 of 178 Hz.

It is interesting that using the SS numbers I came very close to my SS target resonance. There are a few other factors involved. On my SS adding the bridge drops the tops resonance a bit more than 5 hz; on my classicals guitars adding the bridge raises the resonance more than 5 Hz. I suspect relative to the SS that with the thinner top and without the hardwood bridge plate the classical has very little cross stiffness. The bridge acts as an important cross brace.

[seeaxe]

Thanks John.
My bracing is going to be more or less the same as the steel string layout. I drafted my own but will amend it slightly. Im intending to put the lower TB in.

I can understand that the bracing needs to be stiffer with a thinner top. Spent last night searching the forum re EI issues and playing with the spreadsheet and can replicate more or less other results so im happy that there arent major errors there.

What im struggling with is that with a 1.75mm top and two primaries at 10 by 5 and two secondaries at 8 by 5, EI is up around 80Nm2. Nothing like the 15 to 20 recommended in section 4.4. And thats before i consider the CF....which will only add to the stiffness.

Seems to run contrary to the books purpose to ignore this and press on. The EI calc is only intended to put you in the right ball park and fine tuning is done later.

I was intending to do some tests on actual laminated bracing made the same time as my falcate braces to check properties. Wondering whether that's redundant.

[JurgenV]

Hi Richard,

actually I tried to use the spreadsheet which workt fine for the steelstring but as you already correctly guessed I changed the EI value to the values given for the classical guitar examples in the design book. The optimization resulted in braces lower than for the steelstring: around 4.5mm
I went with 5mm and as you can see the T(1,1)2 was too low. For the second top I increased the brace height to 8,5mm (more than what I use on the steel string design). And after increasing the diameter of the soundhole to 85mm the T(1,1)2 dropped under 190Hz. I guess I could have gone even a bit higher with the brace height, so perhaps the 10mm you mentioned are a quite good value.

Would be nice to know how Trevor handles the spreadsheet for classical guitars and if there are some adjustments in comparison to the calculation for the steel string guitars.

[johnparchem]

Thanks John.
My bracing is going to be more or less the same as the steel string layout. I drafted my own but will amend it slightly. Im intending to put the lower TB in.
20200403_133723.jpg ...

The lower transverse brace even an open will make a big difference when determining the height of the falcate braces. Brace stiffness falls off by the cube of the length. The lower transverse brace effectively shortens the length of the primary falcate braces and adds in a lot of cross bracing. So your lower calculations may be correct.

[JurgenV]

I finally got the guitar strung up and played it a bit. I like it but it sounds totally different to my handbuilt vietnamese guitar which at the end was the reason for me to start building guitars. Showed it to my standard guitar tester (my guitar teacher) and he likes it as well. His comment: this one I would buy

With bridge, saddle and strings I get the following values:

T(1,1)1= 93Hz
T(1,1)2=189Hz
T(1,1)3= 241Hz

specifications:
Khaya back and sides, Khaya neck, pao ferro fingerboard, bridge walnut
and just to finish this topic here two pictures of the guitar :

[seeaxe]

Hi Jurgen, Hope all is well with you. Your guitar looks very nice.

I'm building my next falcate classical now and am looking forward to the bracing design again.

May I ask, did you do a bridge rotation test for this guitar and if so, what did you measure? On my guitar, I ended up getting the right frequencies (or close enough for a first try) but the top is way stiffer than its supposed to be, with a bridge rotation of less then 1 degree.

Thanks

[johnparchem]

Jurgen, It looks like your classical falcate braces worked out. What attributes of the sound seem different? My falcate guitars all have a characteristic sound that includes long sustain, clarity of each note played and a balance across the strings.

Richard, I personally am happy if I have some bridge rotation but not too much. I focus on the target resonant frequencies. In the Book there are multiple sets of acceptable target resonances, I imagine the bridge rotation will change as one is hitting higher or lower targets. I do not take the 2° as magical. While no rotation could be almost good to way too stiff, some rotation but less than two tells me I am close.

[JurgenV]

Sorry for the late reply. It seemed to me that not much was going on here anymore.

@John: actually your description fits quite well. The vietnamese guitar sounds in comparison more percussive, a bit like a flamenco guitar. Which is surprising because if you compare this one to what is available at the music school it doesn´t

@Richard: I didn´t check the rotation. I was aiming for the target resonances as John is doing.

But in general I would like to know why the spreadsheet didn´t work for the classical guitar. In the meantime I built 4 steelstring acoustics using the spreadsheet and I always get good sounding guitars with resonance frequencies in the target area. Any comments on that from Trevor?

[seeaxe]

Thanks for the reply Jurgen. Yes, pretty quiet here these days. I think the various COVID lockdowns may have meant a few more guitars got built - at least it did for me.

I'm going to go back to the Second Moment of Inertia calc when I do the next top and see what I can see. I think that I will need to make everything substantially lighter to achieve both resonance and rotation. That might mean a fairly short-lived guitar

I had an interesting experience the other day. I took my current conventionally (Torres) braced guitar up to my guitar lesson. My teacher has the new one I made and we played a few pieces together. He reckoned my one sounded better! However, I think that's probably because mine is now 10 years old whereas his is only a few months.

Tchuss!!

[Trevor Gore]

But in general I would like to know why the spreadsheet didn´t work for the classical guitar. In the meantime I built 4 steelstring acoustics using the spreadsheet and I always get good sounding guitars with resonance frequencies in the target area. Any comments on that from Trevor?

I think John (Parchem) has answered your question, Jurgen. The classical guitars assume a "rigid" lower transverse brace (see footnote 18 in Section 4.4.3 in Design). In Section 4.4.5, I mention this again (bottom of p. 4-40) which is the section that includes Fig. 4.4-9 which shows EI for a variety of classical guitars. The LTB makes the effective span is a lot less, as John explained. All the guitars in Fig. 4.4.9 have lower transverse braces, yet there is still quite a wide range of flexural rigidity across that sample of instruments, as discussed in that section.

[JurgenV]

Thanks Trevor.

You´re right it is mentioned in the Design book but I overlooked the footnote and it´s implications on the calculations when not using a lower transverse brace.
If I interpret this correctly this means I need to find a new typical value of the flexural rigidity for classical guitars if I don´t use a lower transverse brace. Ok I mean I already know now that the brace height needs to be higher if use the same bracing pattern as for the steel string but I would like to have the possibility to calculate more or less the right brace heights.
But if I use a ltb the calculations with the typical values for EI would be valid?

[Trevor Gore]

But if I use a ltb the calculations with the typical values for EI would be valid?

Yes. But remember there's a very wide spread of results in Fig. 4-4-9, and the size of the LTB makes quite a difference.