Newbie trying to shift frequencies

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KreherGuitars
Beefwood
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Newbie trying to shift frequencies

Post by KreherGuitars » Mon Apr 20, 2020 8:01 am

I am on my second build and I am really trying to get some get the targets I'm aiming for. I have some design flaws I am working through and will tweak on future builds. It's an x braces om style guitar. Air is at 109 hz and top is at 238 hz. The top was at 250, and with edge thinning dropped very easily. My target is somewheres 105 and 225hz or less right now for air and top. I have been cramming my arm I'm the soundhole and doing some additional scalloping but nothing is shifting a single hz. Am I'm going about it wrong?

Alain Lambert
Wandoo
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Re: Newbie trying to shift frequencies

Post by Alain Lambert » Tue Apr 21, 2020 9:05 am

I just did that recently on a 000.
You need to reduce top stiffness. Your best bet is reducing the X brace height.

Here is a spectrum of the original :
000 top.jpg
The next chart show how it changed after 1, 2, 3, carving session of the braces.
Notice that the air mode did not changed much as this is mainly driven by the box volume and sound hole diameter.
The yellow line show where I stop.
I could take the top down from 218 to 202 Hz
000 top carved.jpg
Go slowly and measure often.

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kiwigeo
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Re: Newbie trying to shift frequencies

Post by kiwigeo » Tue Apr 21, 2020 10:31 am

Is the bridge on yet? If it isn't then make sure you're allowing for effect bridge will have on main top frequency.
Martin

blackalex1952
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Re: Newbie trying to shift frequencies

Post by blackalex1952 » Wed Apr 22, 2020 12:44 pm

Here are my immediate thoughts...the top is probably rather stiff, perhaps it wasn't thin enough? Given that frequencies repel, the soundboard and the back along with the air resonance will repel each other frequency wise...so if the soundboard frequency (the top) was lowered to around 185 or 190Hz then I would expect that the lower frequency which is registering on your software most likely will drop to an acceptable frequency of around 90 to 95 Hz. So long as the frequencies are not scale tones, ie notes that the frets and the tuning dictate.The idea is to get the soundboard, the air resonance and the back plate to work together in order to produce an acceptable performance for an instrument of that design, in this case an OM body shape. So, how thin was your soundboard prior to bracing? Did you perform some stiffness tests of any kind on the soundboard prior to bracing? How high are your braces after the last brace carving session? Has the bridge been glued on prior to these resonance tests? Do you know how to adjust the bridge rotation to around 2 degrees? How have you braced the guitar and how is the back performing? Chladni tests using tea leaves, poppy seeds or fine sand can reveal where the nodal lines are forming for the different modes of vibration on both the back and the soundboard as well as confirming the graphic readouts the software is showing. How arched is the soundboard? One other question is: do you have the Gore/Gilet books and do you have a kind gentlemanly and experienced luthier available for a few tips, ie, a mentor with some experience? A friend like that is worth their weight in gold! Good luck and welcome to the world of luthierie!!!!
Cheers! Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

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graham mcdonald
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Re: Newbie trying to shift frequencies

Post by graham mcdonald » Thu Apr 23, 2020 8:33 am

I would keep in mind that this is your second guitar, and while having these resonance targets to aim for might be good, we don't have any idea on what basis you are aiming for these specific frequencies (other than they are in the usual range of such things) or how you have built the guitar - materials, thicknesses, bracing pattern etc. There are a whole bunch of variables involved that we don't know about, and if you are only building your second guitar, lots of factors that you might not fully understand either. I suggest you string it up and see what it sounds like. If it doesn't work pull the soundboard off and make another one.

Remember, this is not brain surgery, no one dies if you get it slightly wrong.

Cheers
Graham McDonald
http://www.mcdonaldstrings.com

KreherGuitars
Beefwood
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Re: Newbie trying to shift frequencies

Post by KreherGuitars » Mon Apr 27, 2020 4:53 am

Thanks Graham. That's the best advice.

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