Chladni of Main Air Mode

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Dave M
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Chladni of Main Air Mode

Post by Dave M » Sat Jan 14, 2017 3:18 am

This has been sparked off by the discussion of Simon Ambridge's pictures of a Hauser. And what good eyes you must have Trevor...

I have been checking a couple of my boxes and my patterns look just like the top monopole patterns though messier. The air mode certainly isn't out near the sides on either of them as Trevor has pointed out one would expect. Below are pics of my old Aranjuez which is lightly built, lightly fan braced guitar. The other is a falcate braced with side masses.

Does anyone have an interpretation of what is happening?

Sorry should have said the discussion was over on the Delcamp Forum
Attachments
Main Top aran small.jpg
Air mode Aran small.jpg
Main top 02 small.jpg
Air 02 small.jpg
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Dave

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Trevor Gore
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Re: Chladni of Main Air Mode

Post by Trevor Gore » Sat Jan 14, 2017 12:30 pm

Dave M wrote:Does anyone have an interpretation of what is happening?
In a word, No!

The smaller diameter of the node line for the T(1,1)2 mode is something I routinely see, though. So here are some possibilities:

Make sure the guitar is not constrained from moving in the expected modes. I use foam supports, which I try to position where node lines are developing, though I'm not particularly pedantic about that. I tend to excite the T(1,1)1 via the sound hole (rather than off the top) and so may be getting the back to move more because of that. Your T(1,1)2s seem to be "cleaner" than your T(1,1)1s and that might be why. Keep the speaker ~ its own diameter away from the sound hole, though, or you effectively partially block the sound hole. (I use a 4" speaker). It's also possible to get interference from the bench itself which can produce odd effects, so I try to keep the guitar supports fairly high, about 100mm (4").

Let us know how you go.

Dave M
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Re: Chladni of Main Air Mode

Post by Dave M » Sun Jan 15, 2017 9:09 am

Just as a holding reply I do have those conditions pretty much nailed down. Foam support, used a more solid bench today, excitation a reasonable distance above the soundhole. I am using VA to provide the sound, through a domestic amplifier into a speaker around 100 mmm diameter in a closed box ...

What I did do today was to excite the top monopole with the frequency indicated by VA, in this case 194 Hz, then changed to the main air at 101 and nothing moved at all so I am exciting that same resonance.

The fact that it happens with two very different guitars suggests a setup problem but I still can't see what it is.

As an aside I can see the Chladni patterns very clearly on both guitars for the cross dipole, though at widely different frequencies, which suggest the setup is working.

I will continue experimenting.

Cheers Dave
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Trevor Gore
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Re: Chladni of Main Air Mode

Post by Trevor Gore » Sun Jan 15, 2017 9:45 am

Dave M wrote:What I did do today was to excite the top monopole with the frequency indicated by VA, in this case 194 Hz, then changed to the main air at 101 and nothing moved at all so I am exciting that same resonance....
I guess that' a another possibility when resonances are close to an octave apart. I would have thought those frequencies are far enough from the octave, though, unless something is giving you a wrong frequency (e.g. reading off an analogue dial)...

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kiwigeo
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Re: Chladni of Main Air Mode

Post by kiwigeo » Sun Jan 15, 2017 11:09 pm

You could try a different blend of tea leaves?? :mrgreen:
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Dave M
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Re: Chladni of Main Air Mode

Post by Dave M » Mon Jan 16, 2017 2:15 am

Yes Lapsang souchong is probably the answer. Less of this builders tea!
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