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I've been down this road many times before. Just haven't had this result yet.
I'm trying a little different bracing scheme with three different tops. One adi, one redwood and a white spruce top.
All are braced almost identically.

So I tested the frequency peaks using fft and did my glitter test.
The issue is that I don't recognize which one is the ring+ mode in any of them.

The results were interesting in that they were all pretty close with where the peaks lined up even though they were of differing materials. I guess I braced them intuitively well enough so far.

Someone please look at the data and pictures and let me know if I captured the illusive ring+ mode in any of them. Thanks much


Here are the braced tops


Here's some data

Redwood top spectrum analysis


Redwood chladni pics





Adi spectrum plot


Adi top pics











White spruce data



White spruce pictures

Pics 6, 7, and the last one are probably the ring and a half mode. Difficult to read the frequencies but if they are above 300 Hz (which I thought I saw) that is on the stiff side.

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Formerly known as Adaboy.......

The second pic in each group should be it I believe. They all look high to me.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

The first pic in each group look like the ring mode, 2nd pics look like ring +, I agree with Burton, they are high yet.

Chuck

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I don't know the usual Hz intervals for steel string modes but the braces look bulky. Give them the chisel and make the section triangular or at least bullet shape rather than plain square.

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Build log

I agree with Alexandru too much bracing

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Funny thing this thread crops up.... I PM'ed a couple of people yesterday about free plate tuning and Audacity. I am familiar (in theory, anyway) with Chladni patterns and free plate tuning. I haven't seen any posts about using spectrum analysis for tuning free plates (before the box is closed). Tons of info about tuning, modes, etc. after construction is done. Has anyone had any success with using spectrum analysis to tune the plates prior to joining? It seems from the comments that some inference can be made from the spectrum plot about the bracing on the unattached top. I'm looking for a method to get my tops/backs close to final dimensions prior to closing up the box. I know that plates joined to the sides will respond differently, and that small changes (binding, etc.) can make a significant difference. Anybody have any empirical data? I'd love to hear your thoughts.... I think I will try a spectrum plot on the free plates of my current build and see how much they vary when the box is closed/bound.

--Jaybird

Thanks MUCH for the responses.

I am aware that the braces are still on the thick side.

Right now, they are at height and dimension for a typical XXXXXXXXXXXX plan.

Before I dig in and get them down where my builds usually end up, I wanted to get a handle on what it was I was looking at and trying to affect.

Now that I know which peak to concentrate on, I can get the chisels and finger planes out and get to work.

PLUS, I'm using these three tops in a physics demonstration for the school I teach (music) at.
In a time crunch and panic, I had to put this out to others who've been at this point in the build before.

Will keep the data going if folks are interested in the thread.


Dave

As you carve you'll notice all the peaks going down. Only rarely they go up when you manage to get a better stiff to weight ratio than before. In any case i too start doing the Chaldni with the rough bracing and check constantly as i carve them and try to see how things develop. Often when the braces get optimized you'll notice the patterns improve as well. You might also see two adjacent modes merging into a single peak, with a different pattern.

Jaybird, spectrum analysis in itself is not very helpful on a free plate, because often some modes are missing; or you have extra; or you have two-in-one. You can easily decide which peak is the ring or whatever. But finding the peaks easily saves you the treble of sweeping the tone generator to the wits end. You see the peaks, set the generator to that frequency and see how it looks. You want those important modes in the 150-350hz interval to be at the right frequencies (plus minus 10-20Hz depending on what sort of sound you want, bassy, tight/bright, or balanced), and you want the shapes to look nice and clear. Sometimes one mode or another will be missing or sound weak, fortunately this doesn't seem to be the end of the Earth, the guitar will still sound OK.

Spectrum can be used in itself once you complete the guitar. It shows you the air frequency and the soundboard frequency with ease and if you think they are off you can do some things. But really its better to get it right before assembly.

The most important thing is to do it for every guitar and record the data.

Tip and rant: I sometimes see people leaving all sort of random overhangs around the top while claiming they do tap tuning or Chladni or whatever and I really cringe about it. A few mm extra of width can really change the tone or peak positions. You should either use the same exact width of overhang (not always possible if the raw top was just big enough) or better trim it to the exact final shape. Otherwise you really compare apples to oranges.

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Build log

I'm pretty sure that the modes at 357 on the Redwood, 354 on the Adi, and 345 on the White spruce, are the 'ring+' modes. In each case I'd be removing major amounts of wood from the tone bars at the lower ends. The asymmetry was one thing that prompted me to go to 'double X' bracing: trying to symmetric modes with asymmetric bracing wasn't making a lot of sense.

Yes, the ones Al point out are the ones I think are the ring and a half modes. I can see the frequencies now that I looked at them on the computer. I didn't realize they were links when looking at them on my cell phone so I just referred to the order the graphics showed up on the screen.

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Formerly known as Adaboy.......

Thanks again all who helped.

Here's an update.

I started carving braces tonight. First I profiled them and took an audio picture.
Was watching the 345 reading on the fft plot.

on profile, the 345 peak dropped to 332, then 320. But it looked like this and something wasn't quite right



Then I lopped off the bottom of the bass bars and shaved some from the X and tested again. The mode I'd been looking at pretty much disappeared. The one just below looked HUGE on the spectrum plot and turned out looking like this.



Pretty sure I found what I was looking for. Now I'll start working with the braces and trying to get this one to close up.

Dave

OK, that looks like the old 'two modes at the same pitch' trick: it happens all the time. About all you can do i keep trimming and hope that things will sort themselves out before the braces get too short. Fortunately, they can get pretty low down at the lower ends before you get into trouble, and that's where they have the most effect on the modes.

As you work on that mode at 282, you should find that the higher one will look less and less like a 'ring+', and more and more like a star.

Dave - I can't offer anything on the Chladni work but I love that rosette!