Frequency analysis GS Mini style build: improvement possible ?

[JurgenV]

As at the moment the humidity rose by 10% to 56% I am at the moment blocked. with my two experimental builds I´m at the stage where I could start closing the box but looks like I have to wait a few more days. This led to a situation where I had too much time while being in guitar mode. So I started analyzing a build I was never really satisfied with: a GS Mini inspired small steel string following the plans from Georgia Luthier Supply with a standard back. Body size a lot smaller than everything mentioned in the books: 44cm x 36cm. Usually I give finished guitars away but this one I kept because I didn´t like the sound and I always thought I could somehow improve it.

The frequency response showed the T(1,1)1 at 107Hz, the coupled T(1,1)2 at 195.6Hz and a T(1,1)3 at 237Hz (if correctly identified). Next thing was to check the effect of added mass to back (actually I didn´t expect much to change because there´s no life back) and added mass to the side. For this I used two big toss coins and some poster putty. And that´s what happened:

In all three cases the cavity resonant frequency is more or less at 106 /107 Hz while the shift of the top main resonance was shifted to 191Hz independent if the mass was added to back or sides. But at the same time you can see that something is happening when the mass is added to the back as the frequency resonance in this region changes quite strong.
@ Trevor: can you explain this behaviour?

Ok, back to the main intention of improving this baby: As this was the first guitar of this size I ever built I don´t have a clue on in which direction I should move the frequencies. If I compare it with the values for small steel strings from the book, then my T(1,1)1 is by at least 5Hz too high while the T(1,1)2 can be changed to the recommended top main frequency by adding mass. As usual: Any feedback is appreciated.
And hopefully the humidity is going down again otherwise I have to find an easy way to get back to around 45%. Btw. how long does it take for the wood to be in equilibrium with a changed humidity value? Hours, days or weeks? I would expect something like a few days but a detailed answer would be nice

Jürgen

[JurgenV]

I forgot to include a picture. As you can see nothing fancy but still ...

[Trevor Gore]

Probably worth doing some Chladni testing to set what is actually going on around the 210Hz to 240Hz region, with and without added mass.

[JurgenV]

As soon as I´m back from my business trip I´ll try to put something together. Up to now I don´t have anything to do a chladni analysis. So it may take a while.

[JurgenV]

Ok, I have some pictures to show. First Chladni experiments in my life and most probably not very helpful. I actually checked with all three cases:
without mass
mass at the sides
mass at the back
at and around the resonance frequencies from the spectra

52gr at sides:
106Hz:

192Hz

between 200Hz and 240Hz no clear pattern (around 235Hz some movement of the tea leaves, but after a minute still no pattern. Above and below nothing)

52gr at back
106Hz

192Hz

200-240 nothing visible (see above)

without added mass:

106Hz

195Hz

and again 200-240Hz nothing

Either my setup doesn´t have enough power or there is no real resonance in this region. Btw. I used an old home cinema receiver and 100mm speaker with 15Wrms and jsut to make sure signal frequency was right I checked the output with an app on my phone which at least showed the same value as the signal generator. Oh, btw. I used an online signal generator ... does anyone have a recommendation for a good signal generator on a windows system?

I should stop playing with this new tool for today. I think I had been quite annoying for my neighbours. With getting everything to work it took me quite some time and it was LOUD

Back to glueing. If all goes well I can close the boxes of my parallel builds (x-braced and falcate braced OOO)

Juergen

[Trevor Gore]

It is almost impossible for me to determine what is going on from the pictures, but I suspect that some type of dipole activity was excited when you tapped with the back mass added; either because of the way you tapped on that occasion, or because the added back mass precipitated that type of response. If the back is just plain ladder bracing, some type of long dipole may have formed. Pure speculation, of course. See if you get the same spike in the response by adding the back mass and tapping again.

[JurgenV]

Hi Trevor,
I finally got to check this one again:
I hadn´t been able to reproduce the spectrum. This time I stuck the coins from the outside to the back as I was a bit lazy to loosen the strings so that I could put them inside the guitar. I hope that doesn´t make a big difference otherwise I can repeat the test. I moved the coins to see if there are any changes depending on the position of the added mass but didn´t see anything obvious. And this time I repeated the tapping around ten times. Perhaps I should have done the same the last time but as far as I remember I did it at least twice.
And yes the back has simple ladder bracing and the Scale Length: 596.9mm (23 1/2")

Actually I don´t know what to do with it. I can shift the T(1,1)2 with added mass but otherwise...