Odd frequency spectrum

[mak009]

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 (24.92 KiB) Viewed 7821 times

Spectrum.jpg (60.63 KiB) Viewed 7821 times

Thanks in advance

[seeaxe]

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

[kiwigeo]

What's the mains frequency where you are?

[lamanoditrento]

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.

[kiwigeo]

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?

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.

[mak009]

Martin,
No the books were knocked back by local library so-I admit-I'm cobbling info together.

Falcate2.jpg.pdf

(88.54 KiB) Downloaded 360 times

[mak009]

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

[kiwigeo]

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

Martin,
No the books were knocked back by local library so-I admit-I'm cobbling info together.Falcate2.jpg.pdf