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I have a weird one. Maybe it's an epiphany. Maybe an idea. Maybe it's just the regular chaos in my head.
Working on a L00 top. My usual bracing pattern, which is x with 2 fingers, and a perfectly perpendicular to the grain leg between the bridge plate and the point where the ends of the X meet the edge of the top. A-frame up top

Usually this is an easy bracing scheme to close the ring+ mode, which is my usual go-to.
Not this one.



With one leg, the ring will not close. no matter what I do to the rest of the braces.
So I tried an extra leg closer to the end block.
Not much of a difference.

Then I scraped both legs off and just for fun looked at the pattern. Perfect ring. But the top jumped up to 245hz. My target hz of the free top is around 225hz.
Which is where the idea came in..... does a top need legs? If you feel like answering questions, this is it. The question. Does a top need legs. Or, just how much bracing is required for a good sound?(good is of course subjective)
If two tops are made, identical in as many ways as possible, but one needs three braces to close a ring+ at 225hz and another needs zero braces to close a ring+ 225hz, will they act and sound the same? Will the zero brace top implode or explode before the other one?

I digress

My gut says it needs some support. So I moved the leg between the two legs I had previously put on it and the ring closed. There's a nasty old straight line in the mode which tells me, something is a little heavy. I'll play with it until it rounds out. And it'll land somewhere between 235hz and 225hz.

Here is a drawing of the results.




Then I spent another hour or two working on it. What I found was pretty cool.
The first mode I was tuning wasn't the ring+
But by tuning it, and working with it, the actual mode I wanted to work on, came together. It was up in the 260s.
As predicted I put a brace on, tested and found the actual mode, then started working on that.
And it turned out that the leg brace is in fact needed.
My gut was right.
It never did quite close up though. One of the reasons it was so elusive was because it was almost on top of another mode. And the two modes did not want to play well together.
AND there's some bearclaw in this piece of wood, which as we know, figure can mess with the way a top works.
Case in point. This top was a commission. Picked from my stash by the client. But after the rosette was inlaid, some figure showed up that I thought was a sticker shadow. It wasn't. Hence the brown blotch in the upper bout. It is destined to be a sunburst blues guitar.









enjoy,
dave

That's interesting Dave. Obviously you know what you are doing, and looking for in a top. I know Alan Carruth uses Chaldni patterns too. I've never tried it.

I'm working on a viola, and carving it to tap, and scratch tones, inside and out. But I'm not ignoring what the free plate tones are. I don't think that they relate to actual tones they will produce when the viola is finished, but they should give some sort of idea on how they will work.

The belly is pretty flimsy. WRC from HD. It is fairly thick as viola tops go; but it's red cedar, and not Sitka, or Englemann. I noticed that thinning it, and the back, the main high resonance stayed almost the same all the time thinning. The back dropped from F to E thinning it from 184 to The low note on audacity is around low Ab on a guitar, but I hear something way lower that that on the belly.

What I noticed is that thinning it, a couple other lower modes showed up. Then the higher frequencies grew, and when I was done with both; according to the scratching and tapping; there are MANY higher frequencies that are as high as the low ones, up to 1200hz or more. So about an octave and a half above the ring tone of around E; 330 or so. It seems pretty high for a viola, more like what is usual for a violin. It isn't a large viola, only 15 3/8" or so. The back feels a little stiff, but the belly is not stiff at all. I need some stiffness somewhere! They both have about the same ring tones. So the scratching thing works.

I'm thinking that having a LOT of frequencies up high on the chart would be a good thing.

It should work on arch top guitars for sure. Maybe even for regular flat tops. Carve the braces to get as many peaks on the FFT chart as possible, while still keeping stiffness. I haven't tried it yet. I was quite surprised at the viola back and belly.

_________________
Why be normal?

I'm on the computer this morning and I have the screenshots of the viola back and belly. The belly ring tone is about D, 290 or so. I see on the graph before the bass bar was in, it was D too. I don't remember hearing it drop to D. The highest point on that graph was at A 440, so the lower peak at B 250 joined with it, and I heard 345? Ears are funny things.

I also hear a low D, Bass guitar D 73hz like the dah dah dah dah dahdah, in the beginning of Close to the Edge. That's what I hear. The chart says Ab. It's a lot lower than that. Other than that, the pitches are very hard to hear on the belly. You can hear just about anything.

The back has a more open sound.

I think the more peaks that you have the better. Is that what people are talking about when carving braces, when they hear the sound opening up? They also warn that you don't want to go past that.

Wow. Those got huge! They are only 250. I'll take out the left side of the chart.

_________________
Why be normal?

Interesting.

Modes that have overlapping band width can drive you nuts. They push each other's pitches around and shift shapes in odd ways when you remove a little wood.

I've been using a 'double-X' pattern, with the lower X arms being parallel to the main X brace, but behind the bridge and much lower. This adds stiffness both along and across the grain, where your 'leg' just adds cross stiffness.

Mark Blanchard spent a bunch of time looking at 'free' plate modes of unbraced tops. He found that when the stiffness ratio of the wood works well with the shape you get a 'closed' ring+ on the unbraced tops, and that it was hard to get 'closed' modes the tops that didn't once they were braced. Wider outlines call for higher cross grain stiffness. Duh! Mark boils it down by saying 'the sound is in the top'. You seem to have added cross stiffness to a top that didn't need it.

You might be able to make a top with no bracing that would work, but it would need to be thick to withstand the static bridge torque, and the extra weight would cost sound. We use bracing to cut weight, but the bracing makes the top 'lumpy', messing up the higher order resonant modes. Unbraced tops tend to have lots of higher modes, and so do tops where the top and brace stiffness is balanced well, as shown by lots of smooth and 'closed' Chladni modes on the 'free' plate. We're not trying to predict the assembled mode pitches, which are less significant than you might think: we're trying to get something that is stiff enough and works well as a unit.

So the trick IMO is to try to get the bracing out of the way: make the top the 'right' thickness based on the wood, use a pattern that concentrates wood where it's structurally most useful and cuts down on mass/stiffness where it's not needed, and then trim it to get the largest number of clear modes. So far as I can tell, the critical area is between the bridge and the sound hole; once that caves in you've lost it. The two most successful patterns, the Torres style 'fan' and X bracing, both do that. FWIW, Mike Mahar, who has been using 'free' plate tuning for years, feels that top bracing below the point where the X hits the sides in the hips is superfluous, and he could be right.

Ken,
We shoot for perfection and hope for excellence.
I stumbled on this free plate tuning thing about fifty guitars ago, and have found that if nothing else, it helps with the consistency of tone on the instruments I build.
But I still have so much to learn.

Al,
I've said it before, and will say it again, when I first started this obsession every post you made cause my eyes to roll back in my head and I'd lose consciousness.
But the older I get, the more I learn and the more what you say makes sense.

Thanks for chiming in.

You've coached me through some overlapping modes before so I was ready for this one. It came as no surprise.
What floored me was that I was so wrong, before I figured things out.

I totally agree that we're trying to find that delicate balance between structure and tone. The chladni modes are useful guide to help find that balance.



dl

When the guitar is finished, would you mind taking patterns from the finished instrument? I’d love to see how much change occurs….

Most of the modes you see on a 'free' plate (not glued to the rim) will have non-moving node lines that intersect the edge at more or less a right angle (are 'normal' to the edge). In these modes the 'free' plate is bending around the edge, but once you glue it down it can't. I suspect (but cannot prove) that these modes set up conditions of stiffness and mass that may produce more or less similar looking modes on the assembled guitar, but generally at a much higher pitch. I've convinced myself that the more of these sorts of modes you have and the better they look the better the guitar tends to turn out. Since, as Feynman said: "You are the easiest person for you to fool", I don't dismiss the notion that I'm wrong on that.

At any rate, those higher order modes are hard to drive well enough on an assembled instrument to look at the Chladni patterns. You have to get enough amplitude to be able to toss the glitter (or whatever) up into the air to form a pattern, and that takes a lot of power for those higher modes. Since the vibrating areas are small you have to use a small speaker to drive them, and small speakers with high power capability are not common. Holography provides a way of looking at them at more reasonable power levels, but we don't all have an optical bench in the shop.

The ring+ mode is interesting in part because it does seem to more or less 'carry over' once the box is assembled. The node line of the ring runs pretty much parallel to the edge of the lower bout, so that what changes when the top is glued down is the impedance of the edge, but not it's bending stiffness. In other words, for a bending wave traveling out from the middle of the 'ring' the impedance goes from very low (effectively zero) on the free plate to very high once it's glued down. In either case the wave reflects from the edge, but what changes is the phase of the reflected wave. Since the pattern is, in effect, an interference pattern caused by meeting of the incident and reflected waves, what changes is the location of the stationary node line where the two cancel out.

You nearly always (I've only seen one that didn't do that, and it was a very bad guitar...) get a 'ring' in the lower bout of an assembled guitar. On looking at the data for a number of guitars of the same pattern (12-fret 000) I noted that the assembled pattern was at a lower pitch than the 'free' one, but that the pitch dropped less when the 'free' plate pattern was 'better'. In this case, that's defined as a smoother 'free' ring that is more 'closed'. Those also tended to be 'better' guitars by most people's judgement. Note that the pitch of this mode is affected by a bunch of things other than what the top itself 'wants' to do: you can't read too much into that data, but it is consistent enough to be interesting.

Holography gets easier and cheaper all the time, so I keep hoping I can get into it at some point and get a better look at things.

My take what Al said is this:
The Ring+ is a closed ring on a closed box. Tuning the free plate (top) to close that ring reduces the need for the body to exert energy closing it up and thus the top rings freer.

I find that doing this for each guitar lends a VERY consistent tone to all the instruments I build.


Thanks Al for chiming in!!!

dl