Classical No 13

[kiwigeo]

It's looking good ,
Ive got the books , but I will ask one more cause I'm lazy but interested right now .

The Neck block is not cut at 90 degrees is it ? It has a relief angle of 5 degrees ? or is it 2 degrees ?

The neck block and upper extension is made up square but after sanding the top with the 15' sanding dish you end up with a slight angle..which you then remove so upper bout is flat and it also should be square to front of body. Note that Gore et al get around the issue of positive neck rake by installing a wedge between the neck and fretboard. Its all covered in the books.

While on the subject...if youre following Gore and Gilet's books....read each section throughly before carrying out work on your guitar. You need to understand WHY youre doing things a certain way. Getting the upper bout area of the top flat may seem like a minor thing but its actually critical to avoiding major issues later on.

[Kamusur]

Looking pretty good Martin, won't be long now before you close the box.

Steve

[kiwigeo]

Today's activities. Final preparation of back and then gluing of same to sides.
Trimming neck end of top back to side to allow easier line up during glue up.

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Repair job on some split kerfed lining adjacent to brace slots. Most people wouldnt bother but if it's visible through the soundhole then I fix it. The white square on the back is the outline of the headblock to aid location of clamping caul.

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One can never have enough clamps!

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[kiwigeo]

On a roll......all boxed up.
Top glued up

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Out of the mold!!

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Trimming the back and top overhangs on the router table. Im using a 1/4" shank pattern following cutter. I always draw cutter rotation and cutting directions on the top and back of the guitar so I cant make any mistakes.

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All cleaned up and ready for tap testing.

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[kiwigeo]

Tap testing didn't go as planned. Ive recently switched from testing using VA running on a MacbookPro (Windows 7/Parrallels emulator) to running VA native on an HP laptop. All Im getting is regular peaks on the spectrum at 50Hz intervals.....I suspect laptop fan noise. testing will have to wait until I get a cable to run from my mic preamp to my new MacAir. The MacAir doesnt have a dedicated mic input...you need to either go into headphone socket with a 3 ring mini plug or go in via USB.

I did some rough Chaldni testing and noted strong monopoles at 101Hz (Helmholtz) and 195Hz (T(1,1)2 - main top monopole). There was also a strong cross dipole at 250Hz. The back is giving a strong response (didn't use poppy seed for this one) at 247Hz.

[kiwigeo]

After a bit of juggling around with my tap testing set up I did another run on the top. Helmholtz peak is at 101Hz and main top peak seems to be at 198Hz. Ive presented data recorded with a Shure SM57 mic going via a preamp into the mic input on the HP laptop and also with same mic into USB port via a Shure XLR-USB converter.

[Tod Gilding]

Lookin Good Marty, I have been using a shure SM 58 straight into XLR to USB ( No Preamp) with good results.

[kiwigeo]

Back peak is 275Hz - I dont think Ill be able to bring that down so it effectively couple with the top. I had a similar issue with the last classical I built.

[Trevor Gore]

Martin, I think that split peak at 175/200 Hz -ish is worth looking at more closely. Use Chladnis to figure out the modes and maybe do a tap on the back (sound hole open, mic on the back) to see what the back taps at. Then try both top and back again with the sound hole plugged. You might find another back mode in there somewhere... If you can't find two clear resonant modes, one at ~175 and the other ~200 Hz, you might need to do some work on your tap technique to figure out why a spurious peak is being generated.

[kiwigeo]

Martin, I think that split peak at 175/200 Hz -ish is worth looking at more closely. Use Chladnis to figure out the modes and maybe do a tap on the back (sound hole open, mic on the back) to see what the back taps at. Then try both top and back again with the sound hole plugged. You might find another back mode in there somewhere... If you can't find two clear resonant modes, one at ~175 and the other ~200 Hz, you might need to do some work on your tap technique to figure out why a spurious peak is being generated.

Thanks for the advice Trevor.

The back peak at 275hz was recorded while tapping the back with soundhole open and mic on the back.... it was a clear sharp peak.

Tomorrow I'll investigate the double peak at 186/197Hz further as suggested.

[kiwigeo]

I played around with my tap testing technique and managed to get a slightly cleaner spectrum for top and back. I did some quick Chladni testing and found following. Note I've averaged 5 tap tests for each plot:

Top with soundhole open (solid red line):

1. Strong monopole at 198Hz - looks like this is T(1,1)2
2. Unable to get clear Chladni patterns for peaks at 245, 294, 400 and 498Hz. Peak at 400Hz is most likely long dipole and at 498Hz is Cross tripole?

Top with soundhole blocked (dashed red line):

1. T(1,1)2 peak appears to move from 198 to 181Hz. Gives strong monopole Chladni pattern.
2. peak appears at 542Hz. Couldnt get a Chladni pattern off this peak.

Back

1. Unable to get Chladni patterns off peaks at 220 and 252Hz.
2. Strong monopole from peak at 273Hz. I still think this is main back resonance.
3. with soundhole blocked (dashed blue plot) amplitude of spectrum seems to increase.

[jeffhigh]

You're still full height on the lower bout back brace are you?, so scalloping that will bring the back frequency down, as will sanding the panel.

[kiwigeo]

You're still full height on the lower bout back brace are you?, so scalloping that will bring the back frequency down, as will sanding the panel.

That's correct Jeff....main back cross brace is full height. This is the direction I was thinking of going but will I be able to get main back resonance down far enough to couple with top?

[jeffhigh]

Absolutely, though the amount you take off before you start seeing a difference may surprise you.
And then there is what the bridge will do to the top with adding significant stiffness as well as mass
Take things slowly

[kiwigeo]

Absolutely, though the amount you take off before you start seeing a difference may surprise you.
And then there is what the bridge will do to the top with adding significant stiffness as well as mass
Take things slowly

Ok thanks alot Jeff. Over the next few days I'm going to play around with temporary weights on back and top to get a feel for changes that result in resonant peaks.

[Trevor Gore]

Well, that peak at 175Hz seems to have mostly gone; but I can still see something vestigial on both the plugged and unplugged top plots. Any idea of what was causing it?

[kiwigeo]

Well, that peak at 175Hz seems to have mostly gone; but I can still see something vestigial on both the plugged and unplugged top plots. Any idea of what was causing it?

Yes there's still something there. I couldn't get any sort of Chladni response even with the 50watt Bradley signal generator cranked up full tit and worrying when my neighbour would appear at my door to check out the alien space ship in my workshop.

I can't see any peaks measured with the mic on the back that are close to 175Hz so Im not sure what's going on.

[Trevor Gore]

I still think there's something happening that shouldn't be regarding your tap results.

Maybe try recording in Audacity and then playing back in audio to see if there is extraneous noise happening somewhere in your micing system. Otherwise, I'm out of ideas!

[kiwigeo]

I still think there's something happening that shouldn't be regarding your tap results.

Maybe try recording in Audacity and then playing back in audio to see if there is extraneous noise happening somewhere in your micing system. Otherwise, I'm out of ideas!

I've been in the shop all morning working on my technique but cant get rid of the peaks at 175 and 181Hz. I tried switching from a Shure SM58 to a PG57 but no change. Mikes are pointing midway between bridge and soundhole and are about 2feet away from the instrument.

Will keep playing around and report if anything changes.

[kiwigeo]

Yesterday was spent playing around with poster putty and weights trying to get a feel for the way the instrument responds to same.

1. 16gram Dummy Bridge (CF/Blackwood sandwich) taped to top

-T(1,1)1 dropped from 102 to 99.8Hz
-T(1,1)2 dropped from 198 to 187Hz

2. 474gram metal weights bolted to sides

T(1,1)1 dropped from 102 to 100Hz
T(1,1)2 dropped from 198 to 192.6Hz

3. Response of main back frequency (275Hz) to addition of differing weights of poster putty added to centre of back lower bout.

5gram - 265Hz
10gram - 256hz
15gram - 244Hz
20gram - 235Hz
25gram - 232Hz

Note no change to main top resonances noted.

Comments

1. What I think I should be aiming for

- T(1,1)1 of 100Hz. Currently 102Hz
- T(1,1)2 of 190Hz. Currently 198Hz
- A T(1,1)3 peak no less than 4 semitones above T(1,1)2...ie around 250Hz. Current value is 275Hz.

2. Questions/Ways forward.

- Bearing in mind addition of a bridge (circa 15gm) is going to cause a drop in T(1,1)2 should I be skipping any thinning periphery of lower bout before doing binding?
- Main back resonance needs to be reduced to around 250Hz. This is best achieved by reducing stiffness of lower back brace.

Any thoughts/advice would be much appreciated.

[jeffhigh]

Don't assume that the T1(1,2) is going to drop as per the taped on dummy bridge
On a falcate classical without a bridge plate there is a very flexible area right at the centre so the bridge stiffness offsets the mass, the frequency can even go up.
4 semitones above 190 is 240 not 250 I suspect that there will be better coupling with a more flexible back rather than just creating a lower frequency by adding mass . As you approach this target it will start pushing T1(1,2) down

[kiwigeo]

Don't assume that the T1(1,2) is going to drop as per the taped on dummy bridge
On a falcate classical without a bridge plate there is a very flexible area right at the centre so the bridge stiffness offsets the mass, the frequency can even go up.
4 semitones above 190 is 240 not 250 I suspect that there will be better coupling with a more flexible back rather than just creating a lower frequency by adding mass . As you approach this target it will start pushing T1(1,2) down

Thanks for the input Jeff,

More questions:

1. In light of your comments regarding the back do you think my first step should be to work on thinning the back and trimming back resonance to around 250Hz?

2. Do you think loosening up the top by thinning around periphery of lower bout is a good idea at this stage?

Cheers and thanks Martin

[jeffhigh]

1. Only if you think you have made the back too thick to start with
2. I would probably just install bindings do some minor thinning then apply finish and get the bridge on and strung up before retesting and then doing the back brace scalloping. You will have a better idea of your targets then.
I was a bit slack in recording some intermediate results on my last nylon falcate, they will certainly give you guidance for the next.

[DarwinStrings]

Looks like you are going along fine Martin, I will be onto a nylon string after I finish the two steels I am on at the moment and am wondering what height you used for the King billy main and secondary falcates? Also I seem to remember you used spruce for your falcates Jeff and wonder what height you used for your nylons?

Jim

[kiwigeo]

Looks like you are going along fine Martin, I will be onto a nylon string after I finish the two steels I am on at the moment and am wondering what height you used for the King billy main and secondary falcates? Also I seem to remember you used spruce for your falcates Jeff and wonder what height you used for your nylons?

Jim

Jim,

1. The falcate braces are about 7mm high in the bridge area on my classical. The braces consist of 3 x 2.2mm thick strips of KBP and I usually make the raw brace around 10mm high.

2. I used a bridge plate on my first falcate classical but found it stiffened up the bridge area too much. The current build doesn't have a bridge plate.