Fleta Tap

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soundshooter
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Fleta Tap

Post by soundshooter » Wed Jan 29, 2014 4:05 am

I just closed the box on a Fleta style classical. My first Fleta and guitar #4. I was expecting to see the "close space top and asymmetrical cross dipole" signiture. If I tap near the were the antinode might be I can excite a peek that I believe is the cross dipole pole but the frequency seems quite high and and difficult to excite. Is this something that develops once the bridge is on or is there some way to encourage the asymmetrical dipole through further thinning?
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FranK
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Trevor Gore
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Re: Fleta Tap

Post by Trevor Gore » Wed Jan 29, 2014 10:13 am

It looks like you have all the right things going on, but everything looks a bit high. Being on a log scale it's hard to read exactly what the numbers are.

As you add mass (the bridge) and loose stiffness (edge thinning, taking the tops off the braces) the spectrogram will tend to horizontally compress, so the peaks tend to get closer together. As the back is stiff in this design (four ladder braces) the top has to be pretty compliant to get the main air resonance down. With lots of bracing inside, you can survive with quite a thin top.

I'm assuming you thicknessed according to material properties and left the lower diagonal "flying".

What to do next? Add the bridge mass and see how far the resonances drop. Edge thinning will bring the main air down as well as the main top. Sanding the tops off the braces should also compress the peak spacing.

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Re: Fleta Tap

Post by soundshooter » Wed Jan 29, 2014 12:54 pm

Hi Trevor, Thanks. I already have done some edge thinning. I used some low density Engelmann for bracing, so I added .5mm to all the bracing. Also the back and sides were made from EIR 3mm thick. I experimented with the deep hard maple kirfing by CF re-enforcing. Between light brace material, the heavy sides and stiff kirfing I thought the .5 would be justified but maybe I over did it. I was hoping for a higher voiced instrument so it seems it is going work to out with some brace thinning. I'll experiment with the scale on the spectrograph. Good thing to know thing compress some. Thanks

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Re: Fleta Tap

Post by Dave M » Wed Sep 03, 2014 6:57 am

Frank I have also recently finished the box on a Fleta and my resonant frequencies are also high. I wondered where you went from there?

More thinning of the top? Reduce braces?

Did you get it finished and what resonances did you end up with? More importantly if you did finish did it sound good?

Cheers Dave
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Dave

soundshooter
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Re: Fleta Tap

Post by soundshooter » Wed Sep 03, 2014 11:53 am

I ended up reducing my brace height. I took quite a bit off the braces to get to my target. Maybe to 3mm . The guitar did sound quite nice. I've just boxed one now and instead of using an "f" of 60 I think it was 55 Maybe even 50. (away from the shop right now so I don't know for sure.) About 1.87 mm for a light strong engelmann top. It looks like I won't have to touch the bracing at all on this one.

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Re: Fleta Tap

Post by soundshooter » Fri Sep 05, 2014 12:51 pm

Sorry
Last Fleta (reduced brace hight) 98.5 190 235
1.98mm thickness of revised box. f=55 looks like I'm about right. but i'll let you know what happens

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Re: Fleta Tap

Post by Dave M » Sun Sep 14, 2014 7:05 am

Frank thanks for the detail.


I started my build before I had gone properly through the design book and before I had got a grip on VA. I chose a conservative thickness for my Engelmann top of around 2.2 using the brace designs as per GG's Fleta design. Your figure of 1.87 for the top sounds tremendously thin but I suppose as Trevor points out there is a fair amount of bracing under there.

I am encouraged that the general shape of my frequency spectrum is looking much like yours only with higher frequencies.

I am intending to do a lot of thinning of the top round the edges and then if necessary of the braces as you did to get the frequencies down.
I will also be checking what the addition of the bridge does. I have a traditionally shaped bridge in rosewood with a weight of 21 grams.
And I guess the mobility testing will also tell me if I am getting into the right area for the top properties.

So excuse my ignorance but you refer to figures for f in your post and I am not sure what this refers to...

Cheers Dave
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Trevor Gore
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Re: Fleta Tap

Post by Trevor Gore » Sun Sep 14, 2014 9:14 am

Dave M wrote:So excuse my ignorance but you refer to figures for f in your post and I am not sure what this refers to...
Ahh... the "f" word....

...refers to the "calibration constant" in Eqn. 4.5-7. Various values are give on Page 4-62.

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Re: Fleta Tap

Post by Dave M » Mon Sep 15, 2014 4:52 am

Trevor thanks for that.

Given that it is clearly at the heart of your methodology I was indeed showing my ignorance!

I guess that the mixture of mental work along with the attempts at accurate woodworking is something we don't often attempt. Well that's my excuse.

Never mind I am catching up Dave
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