MIMF

I am making my first archtop with a laminated back ( 5 layers of .020 veneer with a layer of .060 Nomex in the middle layer vacuum formed over a carved mdf plate) The resulting lay-up has a great tap tone and seems quite stiff, but it is way thinner than a carved back. I am wondering if I should brace it like you would a flat top back, or just leave it as is. Any thoughts, tips, or advice?

I don't have experience with your situation, but I would venture to say that if it seems stiff enough to you and has the tap tone you desire I'd think you could leave it unbraced regardless of its thickness.

The whole point of an archtop back is that it's "live" - it vibrates in much the same way as the top, and contributes greatly to the tone and volume. Bracing will only deaden the ability to vibrate. The back carries little load, and a layup with nomex at 160 thou nominal will be fine. A carved back is normally between 125 and 185 thou depending on where you measure. What are your goals with a laminated plate? Will this be primarily an electric or acoustic guitar?

Acoustic. The back (and front) only have nomex in the center - basically the area inside the recurve, thus the edge is ~.100 (.02 * 5) and the center is ~.140. The nomex is sanded after the first lamination to make a gradual transition. My idea is to create a fairly rigid plate with a more flexible perimeter - trying to duplicate the effect of the thicker back and thin recurve.

Sounds like a very cool concept!

I 'tune' the tops and backs of my archtops using Chladni patterns. I work on the thickness of the plates before cutting the holes in the tops, and try to get them to be pretty much the same in terms of pitches and mode shapes in both plates. These are, of course, the resonances that you hear as 'tap tones'. Once I've gotten them matched I cut the holes, and the resonant patterns and shape in the top all change. Then I put in the bracing over size, and trim it down. It's 'right' when the mode shapes and patterns are restored to what they were before cutting the holes. So it looks as though the bracing is there to replace the stiffness you lose when you cut the holes. That's why bracing is not needed on the back, IMO.

Excellent analysis! It certainly makes sense to me. I have only read about Chaldini patterns, but have no experience. How do you excite the unattached plates? Am I correct that you vary the input freq until you get a stable pattern and that is considered the freq of the plate? You mention that you try to get the top (before f holes) and the back the same. I'm sure I have read somewhere (?maybe Benedetto?)that they should be "close, but NOT the same" Sorry for so many questions, you just seem to have a very unique handle on what is going on here.

I use a sine wave generator with a ~30W amp and frequency counter to drive a hand-held 5" speaker. The plate is supported on foam pads at what one hopes will be the stationary nodes along the edges, and glitter (or sawdust) is sprinkled on. When you hit a resonant frequency the powder jumps off the moving areas and gathers along the stationary lines in between to form the patterns. The shapes of the patterns tell you something about the distribution of mass and stiffness within the plate, and the frequencies tell you about the relationship between stiffness and mass.

There are lots of opinions about everything related to these tap tones, and its hard to 'prove' that anybody's is better than anybody else's. I have found that when I get the 'ring' mode to be at the same pitch in the top and back before assembly, the 'main top' and 'main back' modes (which are roughly analogous) will will be pretty close to the same pitch once the box is together. The mass of the bridge (and probably the down bearing force as well) lower the 'top' pitch a little, and the back pitch can rise, probably due to the neck pulling on the back. This puts the two modes roughly a semitone apart on the finished box (if I ate all of my veggies...) which is where I like them. Other people have their own likes and dislikes.

Alan Carruth wrote:I 'tune' the tops and backs of my archtops using Chladni patterns. I work on the thickness of the plates before cutting the holes in the tops, and try to get them to be pretty much the same in terms of pitches and mode shapes in both plates. These are, of course, the resonances that you hear as 'tap tones'. Once I've gotten them matched I cut the holes, and the resonant patterns and shape in the top all change. Then I put in the bracing over size, and trim it down. It's 'right' when the mode shapes and patterns are restored to what they were before cutting the holes. So it looks as though the bracing is there to replace the stiffness you lose when you cut the holes. That's why bracing is not needed on the back, IMO.

I really like this idea of the bracing replacing what the f holes remove. That being the case, I assume that "parallel" braces would do this better than 'X' braces, yes?? (Obviously talking about the front now, not the back)

Maybe not so much 'better' as 'different'. Both work in my experience. X-bracing adds some cross grain stiffness, which might be a benefit is a wider box.