checking resonances before glueing bridge.

[Bruno Piancatelli]

I just closed the box on my current build (wrc and wengue) and I was hearing the tapping on the box some how high in pitch and dry or short in duration of the tone.
at least comparing it to my prior cedar build. Im aware that I built this top with stiffer braces (sitka) instead of wrc on the other one.
I know that after the bridge is glued and with the strings tension the top frec. should drop. (as well as air main?)
the question is how much should I expect it to go down?
I took some samples with Visual analyzer mayb you can help me read them.

the first one is from the top. I get what Im guessing the main air at 110hz what seems high. some shallow ones at around 200hz and then some other peaks at around 234hz and 252hz.


this one is the back and it shows and clear peak at 243hz.


what do you think, should I leave it like that and see what comes around, maybe do some tweaking after the guitar is finished?

[Robert Webster]

Pretty hard to know what to tell you without info like the dimensions of the box and the diameter of the soundhole. BTW, I couldn't get the images to open on my Win7 desktop. Anyone else have that problem? Also, were you taking the recordings from in front of the soundhole or in the middle of the lower bout?

Bob

[senunkan]

What I did recently when I glued on the bridge is to place the bridge on the top (unglued) and tap and then record the response.
It should give a good approximation (within a few hetz difference)
Of course, since it is unglued the guitar top has face up.

[Bruno Piancatelli]

Pretty hard to know what to tell you without info like the dimensions of the box and the diameter of the soundhole. BTW, I couldn't get the images to open on my Win7 desktop. Anyone else have that problem? Also, were you taking the recordings from in front of the soundhole or in the middle of the lower bout?

Bob

thank you bob, sorry I wasnt specific enough.
the box is 370mm LB and 490mm long. depht is 100mm at the end block and 94mm at the heel . soundhole is 87mm wide.
soundboard thickness 2,5/2,6mm around the bridge and 2,3 on the periphery. braces are a made of split sitka very stiff. final weight of the braced soundboard was 107 grams. the dentellones are 9mm on the glueing surface.
here´s a photo before closing it:


the back thicknes is between 2,3mm and 2,2mm. braces are 8mm x 18mm made of cedro.


im thinking since its already closed ill have to take a shot and finish it just like it is and in case of too tight sounding I can try shaving some braces to loosen up the sound a bit.
I dont feel like thining the soundboard anymore. I rather play safe on that matter.
But I´d like to hear some opinions anyway.

thanks a lot!
Bruno

[Alexandru Marian]

Did you glue the binding yet? If you have some tiny gaps through the kerfing the taptone will be quite dead. Put some sticky tape around the corners and tap again.

[Bruno Piancatelli]

Did you glue the binding yet? If you have some tiny gaps through the kerfing the taptone will be quite dead. Put some sticky tape around the corners and tap again.

well, you were right ..
there are gaps showing around the lower bout on the back kerfs, because the sides were a little narrow and I had to glue the kerfed linings a little higher than the profile of the sides to get me the body depth right.
I put the tape around and the sound changed a lot. I´ll re check on VA
thanks!

[Robert Webster]

While you're at it, could you give us the dimensions of the fans and the thickness of the bridge plate? Thanks.

Bob

[Bruno Piancatelli]

While you're at it, could you give us the dimensions of the fans and the thickness of the bridge plate? Thanks.

Bob

yes Bob, the fans are all 5mm wide.
6mm tall for the three on the center. and around 5mm the outter ones. but they ended up a bit lower afer all the shaving. the V braces are 5mm wide and 4mm tall.
the bridge plate is 30mm wide and 1,5mm in thickness.

here´s the VA result with the kerfs gaps sealed.



the air mode seems to be in the normal range now. 97hz around G.
the other 2 noticeable peaks are at 233hz and 247hz. should that be T(1,1)2 and T(1,1)3 respectivey?
and then that double shallower crest between 198 and 207. but I dont have a clue what it can be.