Intonation Problem

[Craig Bumgarner]

Happy New Year, hope you all are doing okay with this virus business. Guitar work has been going along okay here, but I got a problem I can't figure out

My most recent guitar has an intonation problem around A3 (220hz). A3 and the surrounding notes Ab and Bb are all 8-12 cents flat on all three strings where they occur (A string 12th fret, D string 7th fret and G string 2nd fret). Other than these notes, all the rest are within 5 cents, most within 2 cents. All the strings except the A intonate properly at the 12th fret, so don't think it is a bridge location or compensation problem.

Looked at the FRC and saw the main monopole was right on a scale note (B3, 247hz.). Moving this down to 240hz (exactly between B and Bb) with some modeling clay on the bridge did not resolve the problem. Which makes sense as the problem is down around 220, not 240-247. Took a closer look at the FRC:

IMG_8795.PNG (93.38 KiB) Viewed 23057 times

There is a bit of a peak at 218hz and the amplitude does not die off through 220hz before the amplitude climbs the next peak. I'm thinking this is the problem.

So how to get rid of it. Moving it around does not seem to help. With clay stuck to the top, i pushed it down to 213hz (again, half way between A and Ab) but as the peak is broad enough it still seems to be affecting Ab and A. I've tried clay in various spots and antinodes which moves the peaks around but does not change there relative positions or amplitudes.

I've tried to identify the source of this peak by looking at the Chladni pattern. It appears to be a weakly defined monopole. Audibly, it is very noticeable in the 218-220hz range. It is not the main monopole at 240hz which is louder and clearly defined.

IMG_8796.JPG (125.31 KiB) Viewed 23057 times

By closing off the sound hole to see if it is related to the air mode, the air resonance is clearly eliminated but the 218hz resonance remains. So not related to air mode, eh? Also does not seem to related to the back which is pitched high enough that it is not particularly live.

So, I'm stuck. Moving the resonance doesn't seem to working and I don't know how to eliminate it. Thoughts?

[Craig Bumgarner]

Maybe I should add this is a Selmer style guitar, the kind I usually build. An air mode of 96-115hz and a main monopole of 230-260hz is very common in this style guitar.

[Trevor Gore]

Check out that peak at ~440Hz. That's likely your problem...

In the 2nd Edition, Section 22.3.4 shows an example of how to fix this sort of problem on the 2nd partial (not in the 1st Edition). Also likley in the Modal Tuning Course notes.

[Craig Bumgarner]

Thanks Trevor, I’ll check it out in the morning.

[Craig Bumgarner]

I checked that peak near 440. Inspection with guideline indicates it is 452 on that FRC

Having already tried added bridge mass, I took all the clay and the 3.5gr mustaches off. Here’s the FRC:

Peaks are 100hz, 214, 252, 343, 370, 454, 503, 559, 705, 811

This is a nicer solution to the 247hz monopole peak landing smack on B3, and lifted that 452 peak to 454. There is still that little peak at 214 on the slope up to 252. and it seems better defined. But none of these seem to land on scale notes in the problem area, Bb3 (233hz) being the worst, 14 cents flat. A (220) is 12c flat, Ab (208) is 8c flat. Above and below these notes, intonation is normal. Also normal at A4 (440) and A5.

The peak at 370 in the FRC above is right on F# but the intonation of F# is good and no obvious direct problem here.

In the trouble area, the tuner is confused about the frequency it is “hearing”. As is my ear. Tuner shows considerable fluxuation and my ear is confused as to what is the pitch is being heard.

[Craig Bumgarner]

I’ve not resolved this problem yet. Tried tuning the guitar first up then down one half semi-tone with the idea that if the problem was a scale note conflict, tuning half way between scale notes “should” negate this. It doesn’t and the problem zone from Ab to A# doesn’t move.

Tried a different brand of strings, same gauge, no change.

Thoughts?

[Trevor Gore]

I've not applied any more thought to this (a bit busy at the moment), but have you got some sort of tail piece resonance going on?

[Craig Bumgarner]

Thanks, I’ll check that email n the morning. I did dampen the after length of strings between the bridge and tailpiece. That didn’t change things. I'll check the tailpiece itself.

[Craig Bumgarner]

Dampened the tailpiece, no change.

[Dave M]

Did you get anywhere with this issue Craig? It does sound an interesting one

[Craig Bumgarner]

No, Haven’t figured it out yet. Tried Chiladni again and there is definitely a resonance around 220-230hz but poorly defined. I can move it around with clay to n the top, but makes no difference and can’t seem to get rid of it all together