Odd frequency spectrum

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mak009
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Odd frequency spectrum

Post by mak009 » Sun Oct 24, 2021 9:57 am

Hi,

This is a first crack at a falcate braced top (yep-it needs cleaning up)
So I tap tested it out of interest and a strong 57hz spike showed up which isn't present on other acoustics I have. I get that they are fully built and rigid but can anyone explain the low frequency spike?
Top glued.jpg
Top glued.jpg (24.92 KiB) Viewed 5768 times
Spectrum.jpg
Spectrum.jpg (60.63 KiB) Viewed 5768 times
Thanks in advance

seeaxe
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Re: Odd frequency spectrum

Post by seeaxe » Sun Oct 24, 2021 12:18 pm

I'm not the falcate guru but I would not take too much notice of what the spectrum checks say until the end of the process when you have the guitar strung up. I have no idea what the particular peaks means, if it means anything.

Its probably a good thing to do them as you go through, so that you can see differences between the different builds at the same stage.
It will probably change when you add the outer layer of CF on those braces, then again when its on the sides, again when the box is closed etc etc

Cheers
Richard

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kiwigeo
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Re: Odd frequency spectrum

Post by kiwigeo » Sun Oct 24, 2021 6:36 pm

What's the mains frequency where you are?
Martin

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lamanoditrento
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Re: Odd frequency spectrum

Post by lamanoditrento » Sun Oct 24, 2021 7:01 pm

Hi Mak009,
Are you building a falcate ukulele or is this another type of instrument? I'm having trouble understanding the shape, soundhole position etc in your pic. I am also having trouble picturing your secondary falcate braces falling under a bridge footprint and no bridge plate for reference.
Trent

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kiwigeo
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Re: Odd frequency spectrum

Post by kiwigeo » Sun Oct 24, 2021 7:24 pm

Was looking at the layout of those secondary braces.....they should normally pass under the wings of the bridge. Mak009.....do you have The Books?
lamanoditrento wrote:
Sun Oct 24, 2021 7:01 pm
Hi Mak009,
Are you building a falcate ukulele or is this another type of instrument? I'm having trouble understanding the shape, soundhole position etc in your pic. I am also having trouble picturing your secondary falcate braces falling under a bridge footprint and no bridge plate for reference.
Martin

mak009
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Joined: Thu Jul 15, 2021 12:17 pm

Re: Odd frequency spectrum

Post by mak009 » Sun Oct 24, 2021 9:19 pm

Martin,
No the books were knocked back by local library so-I admit-I'm cobbling info together.
Falcate2.jpg.pdf
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mak009
Wandoo
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Re: Odd frequency spectrum

Post by mak009 » Sun Oct 24, 2021 9:23 pm

I'm working on a simple model that says the rolling moment around the bridge is the issue that needs resisting.

I've laminated the primary falcates (tho' they aren't CF reinforced at the moment_ But they are extremely resistant to longituidanal forces.

Also- I probably should have installed the bridge plate and re-bated the falcates-but another argument is leave the primary braces intact and cut the bridge plates as inserts to sit in between. You could argue that the attachment to the top 'plate' then becomes a limiting facor in limiting deflection.

I might test it with a rig like Portland show on their video and see how much deflection I get around 180lbs.

I appreciate any input-the last build was a standard X-brace top-it's OK but overbuilt I suspect.

Mains here in Victoria is standard 50hz???

Maybe the peak is just the initial percussive 'thud' from hitting the top before it begins to resonate???

Thanks for taking the time to look and offer suggestions

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kiwigeo
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Re: Odd frequency spectrum

Post by kiwigeo » Mon Oct 25, 2021 9:10 am

That pdf is a pic of one of my builds.....be careful as its a multiscale instrument..all the bracing and the bridge are assymetrical :)
mak009 wrote:
Sun Oct 24, 2021 9:19 pm
Martin,
No the books were knocked back by local library so-I admit-I'm cobbling info together.Falcate2.jpg.pdf
Martin

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