frequency responce curve interprtation

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benvaneven
Gidgee
Posts: 9
Joined: Sun Apr 30, 2017 6:10 pm

frequency responce curve interprtation

Post by benvaneven » Sun Jan 07, 2018 7:58 am

I am new to the forum. I am building guitar number 6. Still learning.
I have read Trevor Gore's/Gerard Gilet's book last year.
For this guitar I did not yet follow the instructions for thicknessing the top and back of the guitar.
I thickness the fan braces by clamping the top in a mold and then I search the monopole using chladni patterns.
mold.jpg
I took a frc of the guitar I am currently building. No bridge, no bindings, just a closed box and neck. It is a classical guitar.
I see 2 frequencies,213Hz(G#3+44) and 227Hz(A#3-46) for the top and I can actually hear the frequencie changing while I am tapping the top. The frequencie for the back is 244Hz(B3-21) and another little peak at 250Hz(B3+21).
frc.png
Here is the frc of the uncoupled top, 218Hz and 228Hz.
frc uncoupled top.png
Maybe it has something to do with the bracing. I use a asymmetrical fan bracing with 2 little lateral braces.
bracing.jpg
Looking forward to your comments.
Ben

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Trevor Gore
Blackwood
Posts: 1362
Joined: Mon Jun 20, 2011 8:11 pm

Re: frequency responce curve interprtation

Post by Trevor Gore » Sun Jan 07, 2018 10:59 am

The 213Hz peak is probably a fairly strong asymmetric cross dipole. Chladni patterns will confirm that (or not). The frequencies will change when you glue the bridge on, with the monopole peak T(1,1)2 likely reducing in frequency and the cross dipole T(2,1) possibly increasing in frequency. That's what I would expect, but whether or not that happens will depend a lot on the precise details of the bracing you have. The completed classical guitars I have measured with asymmetric bracing usually have the asymmetric cross dipole higher in frequency than the monopole, which means the resonances on yours will need to "cross over" to give a "normal" response.

Keep us posted on how it works out!

benvaneven
Gidgee
Posts: 9
Joined: Sun Apr 30, 2017 6:10 pm

Re: frequency responce curve interprtation

Post by benvaneven » Mon Jan 08, 2018 10:15 am

Thanks for the quick responce.
I will keep posting about the build.
Ben

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