Advice on Resonances

[David Stiles]

Hi
I have finished my first guitar and now looking at tuning the body resonances. It's OM sized steel string, x-braced with live back. Sounds very sweet but I feel it could be improved. There are a few resonance related intonation issues.

So, if anyone is interested in helping me think it through, here are the numbers:I
Bridge rotation is 0.75 deg
The biggest resonance by a long way is at 98Hz. There is are weaker peaks at 184 & 212Hz.

I am thinking that the top and back resonances are too close. Also the top is too stiff and not developing a strong peak.

Should I perhaps be shaving the top braces a little and maybe adding stiffening to the centre of the back brace?

Thanks

[David Stiles]

To illustrate what I am asking about, here is a screen shot of spectrum capture:

In this capture the peaks are 98Hz, 188Hz and 212Hz

[johnparchem]

In general do not look at the amplitude of the peak as your test setup really is not accurate in terms of relative peak amplitude. It is really showing you the locations of the resonances on the frequency spectrum. The resonance frequency values and bridge rotation are better indicators of stiffness. Also when doing the tests where is your microphone? If it is too close to the sound hole. You may be affecting the T(1,1)1 value by closing off the sound hole a bit.

Is this test with strings tuned to pitch? If the guitar is not strung to pitch check again after it is strung up. If it is, the top peak of 184 is not far off, so my guess is the top is not overly stiff. If a check for bridge rotation show some bridge rotation under string tension that is also an indication that the stiffness is not that far off. The back stiffness is a little low at 212 Hz. If this is a test with no strings, the top resonance will probably drop to just under 180 Hz when strung up.

If I was unhappy with how the guitar sounded like a wolf note on F#, I might try to lightly sand the x brace to move the top to 180 Hz. The back will drop a bit as well but less than the top so you will get a bit more separation.

[David Stiles]

Thanks for your reply; it was a great help. I had been concerned about amplitude differences but you put my mind at rest on that point. I was capturing the spectrum with guitar tuned up but dampening the strings. Mic was 25-30cm away from sound hole.

The main issue I had was the G on the 6th string and the open G string notes not sustaining. Today, I shaved a little off the X-braces and that issue has improved. I now have the top resonance down to 182Hz and back is actually up to 214Hz. Bridge rotation is up to just under 1 degree.

I am trying to learn as much as I can from this experience.

[Trevor Gore]

The biggest resonance by a long way is at 98Hz....

The main issue I had was the G on the 6th string and the open G string notes not sustaining....

That was because the the G on the 6th string is at 98Hz, matching the T(1,1)1 resonance.

For a 180Hz T(1,1)2, ideally you'd want the T(1,1)3 at 226Hz and by stiffening the back you may be able to pull the T(1,1)1 up to ~100Hz, which will get you off the G. If it doesn't pull the T(1,1)1 up sufficiently, increase the sound hole diameter slightly until you get there.

[David Stiles]

Hi Trevor, thanks for your advice. I will add a piece to the centre scallop of the back brace and see what happens. I must say, I am pretty happy with the guitar already but quite keen to push ahead and see where it can go. Particularly a mod like this which is reversible.

[David Stiles]

After Trevor's advice I added a piece of spruce into the back brace scallop. This has raised T(1,1)3 to 223Hz and T(1,1)2 is around 180Hz depending on how I tap. T(1,1)1 has moved up from 98 to 99Hz. Not quite enough however 'G' is sounding pretty good. I don't really want to start enlarging the sound hole. So, in the absence of any other easy fixes, I am pretty happy.

[Trevor Gore]

Good to hear that the suggested fix worked for you!