Official Luthiers Forum!

I recently bought Al Carruth's free plate tuning DVD.

Today I started searching for the ring+1/2 mode on my first top. I've been from 70 hz to almost 400 hz in increments of, first 5hz and then 2 hz. Nothing I encountered look like what I thought was a ring + 1/2. Could someone please define these in terms and elderly hillbilly can understand?

I did find that at 216hz this thing went nuts. I had to back the speaker away from the top almost 6" to keep from blowing everything completely off the top. I'm guessing this is probably the main top resonance. It was many times more active than any other frequency.
All in all a fun day, I just wish I understood it all.

cecil

It can be difficult to find Chladni patterns the first few times. Try to visualize the ring + mode, and place the speaker above where you think the top will be vibrating for that mode. You will also need the foam support blocks in the general area of where the node line comes off the plate.

You may be using too much power, and also remember that the pattern will not be "textbook" shape right off, so they can be difficult to identify. Keep trying, that's what I did.

Hopefully Alan will chime in with more helpful suggestions.

Chuck

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Thanks Chuck, I've got the foam blocks and varied the power. I'll deep looking.

cecil

As Chuck said, the most difficult thing at first is just getting to where you can find modes. It doesn't help that the shapes and pitches will probably not be like the textbook. That's one reason I used a variety of tops in the video: some of them looked 'right', and some certainly did not, so you could see the difference. The important thing to see was the method underlying the search.

I'd say that 5Hz increments are probably too coarse, and even 2 Hz might be.

Even though I work mostly with the 'ring+', I always start looking for modes at the low end, and work upward. Usually the lowest pitched mode on a top is the 'first bar' mode, with node lines running across the plate at the wide part of the lower bout, and a little above the waist. Next usually comes the 'T' mode, with node lines on the center line, and a crosswise line somewhere between the wide part of the lower bout and the waist. As with all modes, the shape and pitch of these depends on the stiffness and mass of the system, and if they're abnormally low or high in pitch, the other modes are likely to be so as well. These lowest modes tend to be pretty consistent in shape, though, and finding them is good practice. The higher modes tend to vary more.

Sometimes a really strong mode will show up almost no matter where you support the plate or drive it. Most of the time, though, you've got to at least be pretty close to get things to work. Note that, as you go up in pitch, there will be more and more modes closer together, and changing the supports or driver position a little can cause a big change in the mode shape that shows up. The rule is that you can have more than one mode at a given pitch, but will never see the same mode at two pitches unless there's something else vibrating.

Hunting modes is usually a matter of intuiting the correct setup, and then sweeping really slowly up through the frequencies, moving the speaker around a bit, and watching like a hawk for any little wiggle of the glitter. Once you see something, you move the speaker to that spot, and try to figure out where the stationary node lines are so you can move the supports. As things get clearer you make small adjustments on the pitch, supports, and speaker location until you get the strongest and clearest pattern you can. Remember the rule of thumb that the next mode will probably be moving where this one is stationary, and stationary where this one is moving, and position your pads and speaker accordingly.

Thanks Alan. I understand the process a little better now. I'm using audacity so one hz is as fine as I can get. i'll try that next while being more vigilant.

cecil

So, you bought a FREE DVD?

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Old growth, shmold growth!

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And the word of the year is.........Thixotropic! "

Heh, heh, and where did you get that word? Kind of sticky, doesn't flow well and be on the level with the rest of the words. Just not a team player that word...

Alan D.

I heard it from some guy at one of Robbie's master classes, the one on double tops.

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Old growth, shmold growth!