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Here's a pic of a unbraced 3 piece cedar uke top 1.9mm thick or 0.072" I was using my computer speakers and the only pattern I could coax from the top was this at 319 Hz. I'm not sure how to read unbraced patterns but I'm assuming the frequency is too high.



Sorry about the poor pic it's off my mobile phone

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Formerly JJH

I learn more from my mistakes than my successes

Wish I could help but I know nothing about ukelele soundboards. I'll give you a free bump though!

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Formerly known as Adaboy.......

John-
Give David Hurd a shout if he doesn't turn up here....
He's the "Left-Brain Lutherie' guy and a ukelele expert, so could point you in the right direction, no doubt.
ukulele@ukuleles.com

Cheers
John

Here's a pic of an non-book matched 2 piece top at 352 Hz

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Formerly JJH

I learn more from my mistakes than my successes

Though I'd put you guys on more familiar ground here's a top I'd poorly jointed and thought I'd make it a test top for my first chalandi attempt it's a spruce top not yet braced and 2.8mm or 0.110" thick and with my computer speakers I could only get a response at 326Hz this time the top is book matched not like the uke ones.



I have Alans DVD but it doesn't show anything about unbraced top tuning though it is briefly mentioned, any clues or tips

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Formerly JJH

I learn more from my mistakes than my successes

Rotate it clockwise once and you'll have Mr. Potato head.

Here are some more pics of the first top now its braced and a couple of Chalandi patterns all comments welcome.

Here is the uke bracing i used x bracing because I want to use a pinned bridge.


Here is the top at 137 Hz


and at 176 Hz


At the moment having nothing to compare to I'm not 100% sure what the results mean but I guess as I log up a few more builds understanding will grow!

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Formerly JJH

I learn more from my mistakes than my successes

That's because the bracing is VERY important, integral in fact, and so you have almost no information from which to work if you're trying to work with an unbraced top so far as modes go. Stiffness, sure, but you can test that with a weight or a bonk. Tuning an unbraced top is like guessing how to tune the suspension on a car when you've got nothing but four tires in front of you.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

When you look at this stuff, it's helpful to think about where those patterns come from. Basically, they're 'standing wave' patterns along and across the grain of the wood.

Think about something like a round cake pan with a little water in it. If you drip a drop into the center of the pan, the wave runs out in all directions, and hits the rim at the same time all around if you've done it right. Then the wave reflects back into the center and meets in the middle. The same thing happens if you tap a piece of something like glass or expanded styrene bead board just in the center; assuming that it's supported all around the edge a little wave will run out and back. If, either in the cake pan or the bead board, you drive the thing in the center at the right frequency you'll get a standing wave pattern, with a 'node' or null point in from the edge at the place where the 'incident' and 'reflected' waves meet and cancel out.

If you had a round piece of wood it's easy to see that you'd be less likely to see a nice circular pattern. Wood is stiffer along than across the grain, so the lowest frequency standing wave you'll see will involve only, or mostlly, bending across the grain, and then you'll get another similar pattern at a bit higher pitch with bending along the grain. If you make the wood plate elliptical, longer along than across the grain, and get the proportions just right, you could get a nice elliptical ring pattern. The exact proportions that would work best would be a bit different for different peices of wood, depending on the ratio of long-grain to cross-grain stiffness. Unbraced guitar top pieces also show a series of modes, with the shape of the mode depending on the stiffness ratio of the wood and the 'aspect ratio' (length over width) of the plate.

Mark Blanchard found that some tops show a nice 'ring-and-a-half' or 'ring+' pattern, with the ring part closed, when they are unbraced. He also found that these are the easiest tops to 'tune' using Chladni patterns. In fact, he found that tops that did not have a closed ring when they were unbraced were very difficult to get a closed ring on when they were braced. In a sense, the bracing doesn't really do all that much to alter the stiffness ratio of the plate, which leads him to say that the 'sound is in the top'.

As a result, Mark tries each top at the size and shape of his largest pattern, which is wider in relation to it's length than the others. If the cross grain stiffness is not high enough to close the ring on the 'ring+' mode he'll cut it down to the next smaller size and see if that works. He just keeps going until he finds the shape that top 'wants' to be. At present I'm gathering data on the stiffness ratio of tops, and their unbraced mode patterns, to see how the two relate.

The bottom line here is that you should not throw out that floppy top; just make it into a narrower guitar!