measuring soundboard deflection/stiffness

Construction and repair of Classical Guitar and related instruments
Ryeman
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Joined: Sat Oct 22, 2016 4:31 pm

measuring soundboard deflection/stiffness

Post by Ryeman » Fri Apr 05, 2019 3:38 pm

After I had made my last classical guitar I wished I had thought to measure the stiffness of the Western Red Cedar top. I measured and recorded its thickness, which of course is better than nothing but not the full picture.
At the moment I am just finishing thicknessing the sitka spruce top of a steel-string guitar, working pretty much in the dark as I have never made a streel string before. For future reference in case I make another, what is the best way to measure its stiffness/flexibility along the grain? I have tried clamping the bottom 3 inches to the bench, so 440mm overhangs. Then measuring the deflection when a given weight is placed right at the upper, overhanging end. But there has to be a better way than this, and one which would be relevant to classical guitar construction as well.
Any thoughts?

Alan

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Beowulf
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Re: measuring soundboard deflection/stiffness

Post by Beowulf » Fri Apr 05, 2019 4:37 pm

There is a bit of information and a couple of pictures in the Daniel Friederich section of Roy Courtnall's, "Making Master Guitars". Also, do an internet search for "La guitare FRIEDERICH ET LEIPP bulletin GAM 92.pdf"...in french, but again a few diagrams of testing setups.
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)

Ryeman
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Re: measuring soundboard deflection/stiffness

Post by Ryeman » Fri Apr 05, 2019 5:02 pm

Thanks Beowulf. I did a search and managed to get the introduction but wasn't able to download the article owing to some problem or other...And I don't have Roy Courtnall's book, unfortunately. Apologies for sounding so negative!

Alan

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Beowulf
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Re: measuring soundboard deflection/stiffness

Post by Beowulf » Fri Apr 05, 2019 5:59 pm

Ryeman wrote:
Fri Apr 05, 2019 5:02 pm
Thanks Beowulf. I did a search and managed to get the introduction but wasn't able to download the article owing to some problem or other...And I don't have Roy Courtnall's book, unfortunately. Apologies for sounding so negative!

Alan
PM me your email and I will send you a copy of the article.
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)

jim watts
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Re: measuring soundboard deflection/stiffness

Post by jim watts » Fri Apr 05, 2019 6:11 pm

I currently check sound boards using Trevor Gore's technique of tapping, but prior to that I did deflection testing. Here's the rig I used to accomplish that task to give you some ideas. It can test the board both crosswise and lengthwise. I set a weight on top of the platform shown in the picture. Once I had deflection numbers I could calculate the modulus of elasticity, but this isn't necessary if you always us boards of the same thickness and overall size, or you can thin to a given deflection.
test jig 1.jpg
test jig 3.jpg
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Alan Carruth
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Re: measuring soundboard deflection/stiffness

Post by Alan Carruth » Fri Apr 05, 2019 6:49 pm

Both acoustic testing, via tapping or Chladni patterns, and deflection testing can work well. Both have strengths and weaknesses. I've used Chladni testing for years, in part because I already had the setup. It's not the quickest way, but it give you more information: the Young's modulus along and across the grain, with the associated damping factors, and density (which you need to measure to get the others). One advantage of Chladni patterns over tapping is that it alerts you to when there might be confounding factors, through the mode shapes. Tapping doesn't get you that information, but is quicker.

Deflection testing as Jim shows, using 'free-free supported' plates, can also be accurate. Some folks advocate clamping one end and measuring the deflection of the free end, but there's a risk: if the clamp is too loose you get a low reading, and if it's too tight you crush the wood which can also throw the reading off. From what I understand, with deflection it's best to take two readings, since you can get slightly different ones depending on the grain of the wood.

I'd advise setting things up so that you can calculate the Young's modulus of the wood, rather than simply getting a deflection alone. Young's modulus is a standard engineering measure, and can be used to calculate the 'proper' thickness of the top. It's also handy to be able to compare your results with those of other makers: rather than having to re-invent the wheel you can talk to other makers who've worked it all out. It can shorten the learning curve a lot.

Ryeman
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Re: measuring soundboard deflection/stiffness

Post by Ryeman » Fri Apr 05, 2019 6:54 pm

Many thanks everyone. Much to think about here. Will reply more fully when I have mulled things over. And have more time.

Alan

Ryeman
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Re: measuring soundboard deflection/stiffness

Post by Ryeman » Sat Apr 06, 2019 8:41 am

Beowulf, thank you for your kind offer. PM sent.

Jim, I think I will take a tip from your photos and support the soundboard in two places, measuring deflection in the middle. Clamping the board at one end and measuring deflection at the other (which I described in my opening post) is an inferior method. I already found, as Alan suggested, that it was inconsistent. Also, it tells you nothing about the important area of soundboard clamped to the bench.

Alan, my mind is more comfortable when working along intuitive rather than scientific lines. But I do see the value of your approach, and will investigate further.

Thanks again everyone.

Alan

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