Frequency response curve, advise please.

[Jorge Bertholdo]

My first tap test just after closing the box is as in image Fleta1408_coup.jpg as the blue line:

What I can see so far is:

1. T(1,1)2 at 215Hz is too hight;
2. The peak at 257Hz I assume is T(1,1)3

After a few sand job on the back (red line):

3. The T(1,1)2 drop a little to 212Hz
4. The assumed T(1,1)3 "disappear"

At first I though the peak at 249Hz was the T(1,1)3. But it have no change after the sanding, so I conclude that it isn't. To make sure that is the case, I make another tap test on the uncoupled top and back:

5. The B(1,1) is at 238Hz
6. The T(1,1) is at 205Hz

So, I ask for some advise on what to do next:

7. Should I sand the borders of the top, to make the frequency drop?
8. Why T(1,1)3 is not so prominent like seen on the book and the others posted here? Is that because it is only 3 semitones higher from the top? B(1,1) at 238Hz doesn't seems it is too stiff.

I don't if that is useful at the moment but, if I did the math right, the "monopole mobility" is 40.37 (s/Kgx10-3) and the "flexural Rigidity" (EI) is 26.32 Nm2

[jeffhigh]

Hi Jorge,
At this stage, I'd be installing the binding, applying the finish and installing the bridge, saddle and strings (get the guitar complete and strung)
All these things will have an effect on the finished resonances.
Tapping the top once you have done that will give you a guide to what you need to do
Making modifications now is too risky
The Fleta has a 4 brace back which may be too rigid to give a T(1,1)3 regardless

[Jorge Bertholdo]

Thanks jeff for your reply,

I was think about doing exactly what you say. By the way, I choose to do the back active, with the radial brace system. I don't know if it work well with fleta, but we'll see.

[woodrat]

Hello Jorge...Welcome to the forum....I couldn't add anything more to what Jeff said as it is good advice....I just wanted to say Welcome to the Forum. I checked out your website....Very nice work...

Also I wanted to ask you what program you used to develop the frequency response curve? It looks different to Visual Analyzer.

Thanks

John

[Jorge Bertholdo]

Hello Jorge...Welcome to the forum....I couldn't add anything more to what Jeff said as it is good advice....I just wanted to say Welcome to the Forum. I checked out your website....Very nice work...

Also I wanted to ask you what program you used to develop the frequency response curve? It looks different to Visual Analyzer.

Thanks

John

Thanks john,

The web site is still a working in progress, I just did so far the violin side, I'm organizing the photos of the guitar works to post on the site. Stay tuned.

The software actually it is Visual Analyzer, exported spectra as text and plotted in a chart in LibreOffice (or Exxel). Thats how Trevor did in the book several times when needed to show 2 curves or more for comparison. What is different is those "check box", that take me a lot of work and headache.

* I discover that when I hide the column where the data is stored the curve disappear. I don't know if this is a bug or it is supposed to happen this way.
* The check box and the chart is 2 independent entities
* a cell can be programed to show the output of the check box (on/off, 0/1, show/hide, etc whatever you want)
* I record a macro to hide the data column, one for each curve
* make each macro to be used as Function
* in another cell I use the "IF" function to compare the check box output cell and call the appropriate macro. "=IF(Data.B2="hide",COUP2HIDE(),COUP2SHOW())". here also one "IF" cell for each curve of the chart.

I was looking for a solution to have several curves in a chart, this could be very confusing if one have 15 curves, for example, but be able to choose only 2 to visualize, like SpectraPlus do, witch is a payed software (very well payed).

cheers

Jorge

[woodrat]

Thanks Jorge...

[Trevor Gore]

Jorge, there will be significant changes to modal resonances as you proceed through the build. The most significant change will be the addition of the bridge so it is worth spending a bit of time attempting to predict what will happen when you glue it on. Simulating the effect of mass is quite easy, but simulating the effect of the added stiffness that the bridge gives is not. Section 4.6.1.3 covers this and has a chart that you may be able to make some predictions from. If you add some mass to the top in the bridge postion (say 15grm or whatever your bridge mass is) and then measure the T(1,1)2 you will be able to see how much the T(1,1)2 drops. If it doesn't drop to your target (say 190Hz) you can be pretty sure you'll need to make the top less stiff, because the bridge itself will add stiffness (as well as mass) and so the T(1,1)2 will not drop as far. At this stage, I wouldn't do any more to the back as that can be easily tuned if necessary when the guitar is finished and it is much easier to make it less stiff than to make it stiffer.

[Jorge Bertholdo]

Thanks jeffhigh, woodrat and Trevor,

Besides the information in the Section 4.6.1.3, the fig. 2.4-5 also give some prediction.

I'm done with the box for the moment, I'll moving with the bindings and I'll be back to tunning at late stage.

cheers
Jorge