Curious about a mode at 146-ish Hz

[Jim watts]

I was fortunate enough the other day to have a guitar scanned with a laser vibrometer. I viewed it as an opportunity to check my tap results. They actually line up pretty nicely.
One oddity I struggled with is that most of my taps have a broad peak around 150 Hz or so. I've always thought it as an artifact of my environment as I don't see that in other frequency response curves. This was part of the reason I jumped at a chance to have the guitar scanned.
I have been struggling to try to get rid of this hump at 146 in my response curves as I didn't think it was real, but it looks like it's a real thing. I've seen it in the spectrum of my other guitars too. Everyone else has this nice U shape going on from T1,1 to T1,2, it was driving me crazing trying to figure out what was causing that!
Anyway it appears real as you can see in the vibrometer image.
The guitar was excited with a speaker setting off to the side and the back of the guitar maybe 5 inches away. The volume was low also, just slightly over conversation level I'd say. we Swept the guitar from 50- 500 Hz. These vibrometers are pretty sensitive!
The two images below are the response curve taken back in March (notice large hump around 146) and three of the modes from the vibrometer taken a few weeks ago. T1,1 (98.5 Hz) and T1,2 (189.4 Hz) seem to drifted up about 2 Hz, maybe just different weather condition or having been played in some, I don't know. Possible just different techniques could do it too I suppose.
Any thoughts are appreciated.
Thanks

[lamanoditrento]

Interesting that it is centred just to the right of the bridge with nothing else going on. Is it possible you have a loose brace in that area?

[kiwigeo]

Interesting that it is centred just to the right of the bridge with nothing else going on. Is it possible you have a loose brace in that area?

I think the speaker is off to one side over the back..might have something to do with it?

[Jim watts]

I think having the speaker off to the side is responsible for that. The mode actually sweeps across the top. If I could post a .gif I can show you.

[Jim watts]

Whatdaya know, I can attach a gif.

mode_146.2Hz.gif (834.68 KiB) Viewed 28175 times

[lamanoditrento]

Hmm, time for some poppy seeds then.

[Dave M]

Yes it would seem time to get some Chladni patterns though it may be hard to excite that one since it is broad and not very tall.

Have you tried tapping with open and closed soundhole? That can help pin things down to the top or back resonances.

Let us know what you find.

Cheers Dave

[kiwigeo]

Hmm, time for some poppy seeds then.

Yep..I find if I smoke a few I can see alot of "extra" modes

[lamanoditrento]

Hmm, time for some poppy seeds then.

Yep..I find if I smoke a few I can see alot of "extra" modes

Nothing quite like finding the cosmic fundamental

[Trevor Gore]

That's an interesting mode, Jim!

All I can think of is that it might be a "corpus" mode, and maybe asymmetrical because of the offset excitation.

The first corpus mode is the marimba bar mode of the whole guitar. It has a node usually somewhere between the nut and first fret with the other node being around the bridge location. If you hold the guitar between thumb and forefinger around the first position and tap around the 12th fret, see what you can record. It's a mode that sounds quite clearly but is not normally excited.

[Jim watts]

Thanks for the thoughts.
Dave, I was looking at some taps of another guitar, Also a Hauser size 7 Fan guitar. It has it too. I also has a tap saved of that with the sound hole plugged and it appears to be the first response with the sound hole plugged.

Trevor, I tried to get a spectra plot of tha mode today, but it proved to be difficult with the strings on. I loosened them, damped them and spread them apart so i had a clear shot at the finger board, but it just seemed too rattly to get a clean plot. I try in the near future when I change the strings and report back my findings.
Thanks guys!

[kiwigeo]

A "Gore Mode"........a weird new mode that Trevor throws into the equation just when you thought you've finally learnt about every mode a guitar can generate.