Chlandi Patterns on Back

[lamanoditrento]

Hi There,

I have recently tap tested and chladni tested a raw closed box (my first actually) and got the below fcr. I then I did chladni tests and got two strong results on the back; one at 235Hz which I kinda expected, but also got a response at 300Hz. Which while weird because it didn't match the frc, is not as mystifying as the fact the 235Hz looks like a long diapole so a B12? (not sure on the nomenclature) and the 300Hz seems to be the B11/T113? On the top fcr there is no peak at all at 300Hz?

Am I reading this correctly? Any and all comments welcome!

For background, this is a Classical with standard 650 scale, Marty radial brace with CF on top of the bace only. The sound board is engelmann spruce, back/sides are camphor laurel.

Thanks

[kiwigeo]

Trent,

1. The FRC in your post has a strong T(1,1) peak at around 100Hz.....You were tapping the back and the mike was on the back?

2. Did you build with the intention of having a live back?

3. On all my live backs I've seen a main resonance at around low 200Hz mark but the back has also been active at higher frequencies and the Chladni patterns are generally long dipoles, tripoles and on a couple of guitars a quadripole....with complexity of the patterns increasing with frequency.

[lamanoditrento]

Hi Martin,

No I was tapping the top with the mic in front of the top. I was doing the chlandi testing to help me identify the peaks in the frc. My original intention at first was to just build a guitar, but since starting as a student at the Australian Guitar Makers School in Brisbane, I have gone down a rabbit hole of obsession. The AGMS plans have 3 transverse back braces with 4 radial braces. So I think is it the planed to have an active back.

I suppose my confusion is, it seems as if the back monopole is at a higher frequency that the back long dipole which I though was the wrong way around. (as per 1.6.1 in book).

I will do repeat the top and do some back testing again tomorrow and have another look.

[kiwigeo]

Hi Martin,

No I was tapping the top with the mic in front of the top.

If you tap and mike the back you should get frc peaks for all the back modes.

[Steve.Toscano]

Did you purposely match your rosette to the chladni pattern ??

[jeffhigh]

A couple of comments

-I would not expect a back monopole to encompass the whole back, but only the lower bout, that 235 Hz result looks like a monopole to me
-Using a radially braced back does not guarantee that it will be live, that will depend on stiffness and mass

Is this the AGMS modification of the Gore radial back as per Strato's video (with the radial braces arranged around the centre back brace rather than the lower bout brace? I can't see the advantage of this mod.