Identifying a peak

[Craig Bumgarner]

I wonder if anyone can help me identify the peak at 244hz. Here is a screen shot of the FRC in VA for taps the coupled top, fully strung. It's a good sounding guitar, so worth figuring out.

FRC.jpg (76.94 KiB) Viewed 8576 times

At 203hz is the T(1,1)2 and at 226hz is what I'm pretty sure is the back.

But what is the peak at 244hz? Chaldni image at 244hz with the bridge intact suggested maybe it was a long dipole. The guitar is ladder braced and there is a brace either side of the bridge. These braces are often the node for the long dipole, but the node is usually stronger than this one. With the bridge removed it looks like another monopole. T(1,1)4?

Actually, I'm guessing it is the long dipole. This guitar has a fairly thin top (2.0 - 2.2mm) and my limited observation to date has been that thin tops over ladder bracing makes for low long dipole peak. My guess is the bridge is part of setting up the dipole.

[Craig Bumgarner]

Another thought, if this peak at 244hz is a monopole of some sort, I guess I would have expected the frequency of this peak at 244hz to have risen with the removal of bridge mass (~ 11gr), but this did not happen like it does with T(1,1)2 This lack of change seems to reinforce the idea that 244hz is indeed the long dipole of the top, T(1,2) as the mass of the bridge is on the node and so the frequency is not affected significantly by the mass. The mass might in fact help strengthen the node. Am I thinking straight?

[Trevor Gore]

There can be a back long dipole in that frequency range that also looks a lot like a monopole, as the dividing node line is in much the same place as the top of the ring on the back monopole. So you might be getting some sort of interaction between those two long dipoles (top and back) and possibly an air mode, too. And yes, the mass of the bridge on the dividing node line will have little effect on the frequency of a long dipole, so your thinking seems pretty straight!

[Craig Bumgarner]

Thanks for the reply. Yes, I think a long dipole on the back in this range is a possibility. Before I wrote the OP, I ran a Chladni on the back around 245hz and got a rather weak long dipole. The back has three ladder braces, so a weak dipole is not so surprising I guess. This was before your reply and at the time I didn't think enough of the result to take a picture and unfortunately, I gave the guitar back to the owner last night. With the three brace back, the long tripole on the back is much stronger at 394hz.

In another guitar I've been testing with a molded arched back and a single ladder brace in the middle shows a very strong long dipole in the back at 279hz and this can clearly be seen in the top as well at 279hz. The top Chladni looks suspiciously like the one in my OP.

What would you call what is seen in the top? T(1,1)3 is actually the back's main monopole, B(1,1) showing up in the top as a monopole. But this is the B(1,2) showing up in the top as a monopole (if that is what it really is), would it be T(1,1)4? Just a nomenclature question.

Thanks for your help on all this.

BTW, I notice a consistent 10-15hz discrepancy between taps into VA FRC (higher) vs. Chladnis using a Bradley signal generator (lower). I'm not really worried about this but wondered if you or anyone reading has insight in why this might be.

Oh, and I would consider some luthier workshop tourism sometime maybe, I'm on the east coast of the USA, so it is half way round the world, but the wife and I have always wanted to see the other half. How can I get a heads-up on your workshop schedule?

Craig

[Trevor Gore]

What would you call what is seen in the top? T(1,1)3 is actually the back's main monopole, B(1,1) showing up in the top as a monopole. But this is the B(1,2) showing up in the top as a monopole (if that is what it really is), would it be T(1,1)4? Just a nomenclature question.

Good question! T(1,1)4 is as good as any I suppose. The mode seems to be an interaction between the very mobile top monopole and a back long dipole venting through the sound hole, giving a residual monopole coupling. I've seen traces of it a few times, but never particularly strong. Still unsure whether it's a good thing or not. It will depend on where the peak sits and its phasing (which I haven't measured) relative to others.

BTW, I notice a consistent 10-15hz discrepancy between taps into VA FRC (higher) vs. Chladnis using a Bradley signal generator (lower). I'm not really worried about this but wondered if you or anyone reading has insight in why this might be.

The Bradley is likely spot on (but worth checking, anyway). VA's accuracy depends on your computer. On my last one it read ~ 1% low. This one is spot on. I've found correspondence between tapping and Chadni frequencies to be very good, but you have to drive the same surface in each case, i.e. the T(1,1)3 and the B(1,1) will not be the same frequency due to the way the coupling works.

Oh, and I would consider some luthier workshop tourism sometime maybe, I'm on the east coast of the USA, so it is half way round the world, but the wife and I have always wanted to see the other half. How can I get a heads-up on your workshop schedule?

Probably best to keep an eye on these pages. I will put you on my "interested" list also and let you know when something is planned.

[Craig Bumgarner]

Just did the obvious test of setting the Bradley to 200.00 hz and using VA to read it, exactly the same, so it must be my technique. Thanks,