Simple deflection test for soundboards

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Peter Lynch
Beefwood
Posts: 26
Joined: Mon Sep 17, 2018 10:51 pm

Simple deflection test for soundboards

Post by Peter Lynch » Fri Aug 06, 2021 7:52 pm

I am looking for a simple way to work out how thick I should make a soundboard .
I watched a utube video where the presenter had a simple jig where he rested the soundboard on two 3/4 inch battens 18 inches apart with a 1/2 inch batten halfway between the battens . He then rested a 5ib 4 oz weight centrally on the suspended board and watched to see the deflection of the soundboard narrow the 1/4 inch gap between the soundboard and the 1/2 inch batten .
He then used a sanding machine ( which I have ) to gradually thin the top until the deflection just about touched the 1/2 inch batten .
He took his top down from 3mm at the start to 2.7mm finish .
Has anyone else used this method , I don’t have all the electronic gear that others use and am unlikely to get it while I am loving the hobby I am not intending to eventually sell anything I make , I will either play it myself or I have a brace of grandkids who play acoustic guitar and are eagerly waiting their turn for one of pops guitars .
So my questions are
1. Does anyone on the forum use the above method
2. Or is there another simple way for me to work out the best thickness for a soundboard . I am building small ( 0 , 00 , 000 ) guitars without a heap of expensive electronic equipment .

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kiwigeo
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Posts: 10580
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Simple deflection test for soundboards

Post by kiwigeo » Fri Aug 06, 2021 8:08 pm

Peter Lynch wrote:
Fri Aug 06, 2021 7:52 pm
2. Or is there another simple way for me to work out the best thickness for a soundboard . I am building small ( 0 , 00 , 000 ) guitars without a heap of expensive electronic equipment .
Peter, a tap testing rig wont cost you the earth. You've already got a computer. One of the free/shareware FTT programs won't cost you anything ( I use VA). Your only extra cost is a microphone and maybe a preamp.
Martin

Dave M
Blackwood
Posts: 594
Joined: Tue Jul 15, 2014 6:44 am
Location: Somerset UK

Re: Simple deflection test for soundboards

Post by Dave M » Sat Aug 07, 2021 2:24 am

Deflection testing is a perfectly viable method for measuring the Youngs modulus of a soundboard - I would do it with a series of weights and a dial gauge on a half top board. It would need to be smooth and rectangular. The formula is widely available on the net.

However the problem then is to use that figure to decide on a thickness. This is where G&G's methodology comes in, whereby a mix of empirical measurements and theory is brought together to indicate a proper thickness for any level of stiffnesss you may have.

As an amateur builder myself I don't worry too much about getting this spot on. Particularly with steel strings you tend to get plenty of volume even if you are a bit over with your thickness.

Cheers Dave
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Dave

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