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Well, I'm voicing the top on my OM. The 1 1/2 ring mode started out looking like this:



There were certain things I wanted to do no matter what so I decided to scalop around the perimeter, then taper the upper arms of the main X-brace, then scallop the lower legs of the X-brace some, then check the 1 1/2 ring mode and see where I was at.

Well, here is what I have on the first check at 229 Hz:



Wow, better lucky than good. Only problem is, I still haven't scalloped the finger braces where they join the X-brace. I really want to scallop this area as I think it will help the bass "cut" that I like so much. (I like that bass punch of the old bluegrass Martins) Also, I need to thickness the ends of the UTB and the upper X-brace arms to fit the rims then clean up the taper of the upper X-brace arms. Thoughts?

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Formerly known as Adaboy.......

Here is what the bracing looks like:

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Formerly known as Adaboy.......

And a view of the bracing from the neck block:

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Formerly known as Adaboy.......

I received a little advice from Mario and I'm going ahead with scalloping the finger braces where they meet the X-brace.

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Formerly known as Adaboy.......

Hey Darryl,

I think definitely reduce where the fingers meet the X. Maybe a little on the upper arms of the x also. Otherwise it looks great. 229 is a pretty good number I think, but who knows really? When you are about to join it up trim the top plate to as close to final dimensions as possible and then re-check it, the number may go up slightly. It looks like you have a lot of overhang on the upper bout from your pencil line but that may be something else. Overall though it looks really great.

What did Mario tell you?

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

So what do you do, shape braces in a standard way and quit when the chladni pattern becomes what you want, or does each chladni shape tell you something about where to chisel the brace?

Hello Darryl,

I'm by no means an expert on Chladni patterns, but your 1 1/2 looks real good!

When I did my top (sadly my camera got broken, I couldn't make pictures of the last result) I got it also closed, after a lot of "head-scratching".

The frequency of my top is 242 Hz. While shaving off braces, I noticed that the finger braces hadn't that much influence as the X-braching and the lower legs.

Bracing those (X and legs)..whow..really influence the pattern.

Those Chladni patters are fascinating, I'm glad Alan talked/viewed me into this...


Regards,

Edzard

Burton, I was planning to scallop the finger braces down where it meets the X-brace. But when I did my first modal pattern check and ended up closed and at 229, it had me wondering how I should preceed. Mario recommended scalloping the finger braces anyhow.

I don't yet have the UTB and upper arms of the X-brace thicknessed to fit the rim. Once I've done that, I'll clean up the taper on the upper X-brace arms. Those pics are definitely not of a finished product.

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Formerly known as Adaboy.......

Burton, thanks for the suggestion to check the 1 1/2 ring mode again after getting closer to the finished shape. You are correct that the lines on the top represent the outside of the rim so quite a bit of material will be removed. This will reduce the mass around the perimeter so I would guess slightly more scalloping could be done near the center to balance everything out again. That's a good thing since I will be scalloping more on the finger braces etc.

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Formerly known as Adaboy.......

Here is another question for the experts.

This guitar has a 3 3/4" soundhole which is a little smaller than normal. If anything, I would guess this will tend to couple the top and back more than it would with a larger soundhole. I want to try Al's method of tuning the back plate main frequency a half step above the main frequency of the top.

I checked the 1 1/2 ring mode on the unbraced back and it was at 133 Hz. How much will the 1 1/2 ring mode be raised when I add the back braces? Hopefully, the braces will stiffen the back sufficiently to get to (or above) 229 Hz (the current 1 1/2 ring frequency of the top). I'm using the old dimension Martin back braces that are shallower and wider than the newer style.......if it makes a difference.

My thought is to finish tuning the top, brace the back, and (assuming the back main frequency will be higher than the top main frequency) tune the back by weakening the braces till it has the same 1 1/2 ring mode frequency as the top. My understanding is that the frequency of the top will lower about a semi-tone as the top plays in.....if true, then maybe I would have a nice couple between top and back plates. Sound reasonable?

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Formerly known as Adaboy.......

I am neither a Chladni Expert nor am I a guitar maker... But I am working on more video material of Benno Streu. He talks about these things all the time. Stay tuned. I guess there is something interesting coming up in the next days.

Regards,
Holger

Anyone who's interested in the work and findings of Chladny, needs to hear Benno Streu on the subject.
He is the expert on Chladny and Chladny patterns and how the top responds to different frequencies
and also how to maximize the response of the top overall.

I don't know of the folks at Gourmet Guitars ever actually came around and released the DVD featuring
Benno uncovering and sharing a lot of what he knows, but if they have and you're able to get a copy,
you will be fascinated by what he has to offer the guitar world.

Regards,
Kevin Gallagher/Omega Guitars

Darryl,

I do tune the back to match the top before finishing but I do most of it after the box is closed. I haven't put a whole lot of time into tuning the free back plate with the patterns. I mostly do it by feel and then perimeter sand once the guitar is strung up to bring it down a bit to match the top. Sorry that is not much help.

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

Just received American Lutherie #99. Dana Bourgeois has an article on voicing which includes Chladni patterns. I only got to glance at it last night but am looking forward to sitting down and reading it.

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Steve Smith
"Music is what feelings sound like"

Burton,

After the box is assembled, how do you isolate the back frequency where you know it's frequency compared to the top? I would guess the main air frequency of the box influences the 1 1/2 ring frequency of the back after assembly......but for all I know this may be the one to to be concerned with.

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Formerly known as Adaboy.......

Hey Darryl,

Once the box is assembled, put your hand through the soundhole and tap on the bridge position and on the back in roughly the same place. By blocking the soundhole you remove the main body tone and can hear the top and back separately. This is not the most scientific way to do it, but it works. Another more foolproof way is that as the back gets closer to the top in frequency you will get more "sustain" when you tap on the top with the soundhole open. The body resonance seems to get better and better the closer you get. Once they are matched, though, they are in phase (I think) and immediately it goes from great to nothing. It is like a giant wolf note. I go by this after getting as close as I can tapping on the top and back. I should check the frequencies but I have been lazy about that part.

I am going on Al's advice to match them but I have heard other people say that they make it a little higher or lower depending on what they usually see when the bridge goes on and it gets strung up. Matching them has worked very well for me so far. Also, at ASIA I am pretty sure I heard Brian Galloup say that Jeff Traugott also looks to match them at this point. I figure that is not bad company.

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

Here is a clip of Benno Streu from their website.

http://www.customguitarvideo.com/2009/0 ... -streu-01/

Very interesting stuff.

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Mark Burton

Hook 'em Horns

Burton, sounds like you match the frequency of the top and back. Then when the extra mass of the bridge is added and after the top breaks in, the top frequency will drop so the back frequency is a little higher? (doesn't Al recommend a semi-tone higher?) Sounds good.

I assume the dominate frequency you hear on the back will also be the 1 1/2 ring mode?

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Formerly known as Adaboy.......

I tried posting this a day or so ago, and got dumped. Let's see if this works.

Mike _really_ wanted me to talk about back tuning in the video, and I _really_ didn't want to. Compared with backs, tops are all 'much of a muchness', but it's difficult to make any sense of backs. I end up going by feel and experience, as much as anything, and the more I learn the less sure I am. In the end, it's what the back does on the assembled box that counts.

One thing I need to make clear is that there is no reliable relationship between _any_ free plate mode and _any_ assembled mode, so far as I can tell. The edge stiffness is too different. One thing I have seen is that, if you think of the 'ring+' mode as being in some way 'the same' as the 'main top' mode, and track the change in frequency from one to the other when you glue the box up, you'll find that the change seems to relate mostly to the shape of the 'ring+' on the free plate. The more 'rounded' the 'ring+' mode is on the free plate, the less the frequency changes when you glue it up. Getting the ring to 'close' does not seem to make as much difference.

As Burton says, I like to see the main top and back tap tones at the same pitch when I first put the box together. This can be hard to hear, as the 'air' mode masks the higher frequencies. That's why blocking the soundhole is helpful, although, of course, it also alters the tap tone pitches a bit.

In my experinece, routing the binding rabbets will drop the tap tones a little, as well as killing the air mode due to leakage and drag. The tap tones go back up a bit when the binding goes on, but not all the way back up, and, of course, the air mode comes back. Gluing on the bridge generally drops the 'main top' tap tone about 1/2 semitone on my guitars, and the top 'plays down' another half semitone or so in the first month, usually. This leaves the top about a semitone below the back; close enough to couple strongly, but not so close as to cause a 'wolf'. Sometimes you get a bit of 'chuff' at first, before the top plays down, but that generally goes away.

I'd love to see that video of Benno: but on dial-up.....

So Al or anyone else, would it work to try to get the top and back frequencies matched after the binding ledge is cut and before the bindings/purflings are glued in place? At least the main air pitch is mostly gone at this point so I'm wondering if that would be a good time to try to match top and back frequencies.

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Formerly known as Adaboy.......