opinion about my frequency spectra

[JurgenV]

Hi,
it´s me again. I finally got my old guitar back from it´s new owner to do some frequency analysis and to check the values for the specific mobility. This was my second steel string build where everyone had been totally surprised when they heard I built it myself. And I never came near the sound again

Let´s start with the frequency response.
Equipment: Microphone Samson Meteor (70cm distance from guitar)
Rubber "hammer" made from some big pencil rubber and a wooden stick with around 11mm diameter
Laptop + Audacity and VA
Spectra recorded with the recommended settings from Trevor
Audacity:

VA

In each graph you see the frequency response without a plug(blue) and with a plug (red). With the plug the Helmholtz related peak vanishes and in addition a peak at 214Hz (Audacity) respectively 215Hz vanishes. My interpretation: the easy case from the Book and I can directly get my uncoupled top frequency from these spectra:
VA: 167Hz
Audacity: 163.6Hz

The same done with the back:
Audacity

VA

=> uncoupled back response
Audacity 206.1Hz
VA 204.6Hz

Btw. does anyone have an explanation for the "shaky" graph from Audacity which looks like I have a superposition of a highly oscillating curve?

with these frequencies and the deflections I measured for the top (average over 5 measurements: 0.084mm for 520gr and 0.15mm for 990gr) I calculate a specific mobility of

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Ok, when I see those values and compare them with the books... can that really be right what I interpret into these spectra??? That I accidently managed to get most of the parameters into the ballpark? There are a lot of things I would do different nowadays but as I said, the sound is (not only for me) quite nice. Anyway: If you see some misinterpretation on my side or some comments on the spectra I would appreciate to hear your opinion.

Juergen

Btw. I forgot to say:
it´s a OOO based on the plans from stewmac with scalloped bracing, spruce top and maple sides and back.

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[Trevor Gore]

I think your interpretations are all correct, Jurgen.

Regarding the "shakey" tap on Audacity, I don't know what that specifically means in Audacity. However, if you get a plot like that in VA it means you captured two taps in one buffer (rather than just the one).

Regarding the spectrum plots, if it were a guitar of mine, I would prefer the back to be stiffer. I'd be aiming for a T(1,1)2 of 170 Hz with a T(1,1)3 of 214Hz. That will give a plot with the second peak as the highest (rather than the third) and a very full, detailed response. A 000 size is more normally pitched at 180/226Hz for a more typical 000 sound. But hey, if it sounds good and no wolf notes, it's good!

[JurgenV]

Hey Trevor, thanks a lot for the confirmation of my data.
It does sound nice, but that doesn´t mean there´s nor space for improvement As I wanted to use this guitar as a test object I can try to aim for a back frequency you suggested.
Btw. just to be sure: when you talk about the target frequencies you talk about the values I determined by comparing the spectra with and without soundhole plug and not the peak positions in the "unplugged" spectrum?

Actually I would like to start as soon as possible with the build but looks like I will have to wait bit longer. At the moment my hygrometer shows meager 28% relative humidity in the room where I store my wood. I will check it at work but if the shown values are correct....



Regards
Jurgen

[Trevor Gore]

Btw. just to be sure: when you talk about the target frequencies you talk about the values I determined by comparing the spectra with and without soundhole plug and not the peak positions in the "unplugged" spectrum?

When I reference the T(1,1)1 and T(1,1)2 etc. I am always referring to the fully coupled resonant frequencies, i.e. without the plug, because those are what you hear when playing.

[JurgenV]

Hmm, but that means I aleady have a T(1,1)3 with 214Hz as this is the peak which vanishes from the spectrum when the soundhole is plugged.
So I have to raise the top frequency? Ok, I will check the peak positions of the unplugged spectrum as soon as I´m back form work and post the values. The top and back frequencies I posted up to now had been the uncoupled values.

[kiwigeo]

Some Chladni testing can clear up what's going on...

Hmm, but that means I aleady have a T(1,1)3 with 214Hz as this is the peak which vanishes from the spectrum when the soundhole is plugged.
So I have to raise the top frequency? Ok, I will check the peak positions of the unplugged spectrum as soon as I´m back form work and post the values. The top and back frequencies I posted up to now had been the uncoupled values.

[JurgenV]

Cladni testing could be a possibility but I have already given the guitar back to its owner
In addition I would have to start putting the equipment together as I didn´t start into this direction

[JurgenV]

Ok, finally back form work. If Trevor always refers to the fully coupled frequencies the numbers actually are no longer as good...
the unplugged spectrum with the peak positions:

In this case I´m not only a few Hz too low. 162.5Hz for T(1,1)2 and 215Hz for T(1,1)3 doesn´t sound so good anymore. At least the new owner still likes it. Bu now I´m not sure anymore if it´s a good idea to try to reproduce it as good as possible

[Trevor Gore]

As I interpret those charts I see a T(1,1)1 at ~100Hz, a T(1,1)2 at ~167Hz and a T(1,1)3 at ~215Hz. Stiffening the back will repel the top down less, hopefully bringing it up to ~170Hz. Alternatively, a lower mass bridge would bring the T(1,1)2 up to ~170Hz and by repulsion push up the T(1,1)3 a bit. Either would get you quite close to a set of target figures. The second action would be preferable, but is likely less possible. I'm taking my cue from the VA charts and ignoring the Audacity charts.

[JurgenV]

Hi Trevor,
I fear I might get annoying but I´m a bit confused so I need to ask to get the spectrum interpretation the next time right:
You write that when you talk about characteristic frequencies T(1,1)1, T(1,1)2 and T(1,1)3 you mean the fully coupled frequencies. So I have to look at the unplugged spectrum.
The "air" and back peaks easy to identify because they disappear in the plugged spectrum. In the unplugged spectrum I have three quite close peaks in this area at 162.5Hz, 176Hz and 185.5Hz. How do you identify the T(1,1)3 as 167Hz with just the unplugged spectrum?

Btw. in the audacity data had been an offset which caused the difference in peak position. My fault. With the correct data the difference in peak position is just around 0.5Hz

[Trevor Gore]

You write that when you talk about characteristic frequencies T(1,1)1, T(1,1)2 and T(1,1)3 you mean the fully coupled frequencies. So I have to look at the unplugged spectrum.

Yes.

The "air" and back peaks easy to identify because they disappear in the plugged spectrum. In the unplugged spectrum I have three quite close peaks in this area at 162.5Hz, 176Hz and 185.5Hz. How do you identify the T(1,1)3 as 167Hz with just the unplugged spectrum?

Sometimes you can't, using just the unplugged spectrum. However, if you flip the guitar and tap the back (no need for a plug) you get a peak a little lower in frequency than the T(1,1)3. From that you can usually figure which is the T(1,1)3 in the top tap.