I asked Peter Coombe if he'd seen this before since he does so much Chladni testing but he hasn't. Just for my own curiosity has anyone seen this "batman" pattern before? It's a slab-cut big leaf maple mandolin back graduated to near final size. It is strongest around 580 Hz.
Is it possible that it is touching something in the middle? The dust converges to the places where the plate doesn't vibrate, if your plate is being touched by something in the middle, that would affect the vibrational mode, and look something like this.
Another explanation is, of course, that your mandolin is predestined to be used for the new Batman movie soundtrack.
Well that's a 12" woofer so it's not even close to touching the middle. It's sitting on 3 small pieces of cork that are glued to the metal rim of the woofer. The corks are at 12 o'clock (up near the the button), and 4 and 8 o'clock.
I've seen some patterns that are similar on violin plates and even some guitar plates but not exactly like this. I guess it just shows the wide differences in pieces of wood.
Do you think it'll play the batman theme by itself in the night when no one is around?
I have no idea what it means, but what I do find interesting is the symmetry with which it is responding. The point of interest for the pattern itself is the semi-circle that looks like it falls along the inside of the recurve. I would have (without ever running the test myself) expected that to be a very active area.
I'd suggest making some form of support that allows you to change position of the cork pads. Once you find basic shape of the nodes, you can move the support blocks under the nodes to get cleaner results - if the plate is supported at vibrating parts it will all vibrate and much of the dust will follow with gravity to the deepest spot. The puddle in the center can quite possibly be that. Sometimes position of support pads will enforce one mode (the one that has nodes closest to the support pads) over others in the same frequency range (especially at higher frequencies). Try to use lighter dust (I have no idea what you are using, but finely ground black tea worked best for me, Earl Grey is my preference :-)) and sprinkle it on plate while it is vibrating and less so into the center of plate.
What I see in the pic looks to me like a mode with three vertical nodes (nodes are bent because arching and graduations) like the one at 175Hz on the pic here: http://en.wikipedia.org/wiki/File:Chladni_guitar.svg
Never even thought that the black pepper would fall to the center, you may be right Adrian. Unfortunately I can't test it since it's been glued to the rim. I'll try black tea in the future. I think the mystery is solved.
Now that I had a better look at your pic I see that the mode you have there has the two node lines closer to neck C shaped one on bass side other on treble side, not going across the plate as I thought.
I believe I posted pic of modes I found on an F-5 plates many years ago. Here it is again....
You can see that there are several modes with three node lines. The assymetry and weigth of scroll makes the shapoes more complex and at lest the lower ones seem to come in gorups of same pattern just rotated few times. What you have may be equivalent of my 400Hz mode or 500Hz mode or some sort of combination. Position of your pads and excitation center may affect which modes you'll find and how they will look. BTW, I used very soft foam as pads (1/4' thick 1/2' squares) on old standard speaker box with thin cloth front panel. I laid it with speaker up and placed the pads wherever I wanted them and watched to have the center of the speaker under the spot I wanted to excite most.
I suppose the A style plate will have less modes and will be more widely spaced in frequency as there is less assymetry to it.
Weird chladni pattern around 580 Hz.
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