Intonation and Hand position

[jeffhigh]

Ok I have a few instruments now where I have tuned the T(1,1)1 in between scale notes.

Playing the note below this, plucking between soundhole and bridge the note plays a few cents flat, but when I move my hand so I am playing over the soundhole (a more natural playing position except for Flamenco) the note jumps to around 20 cents sharp.

As far as I can work out, having my hand above the soundhole is dropping the T(1,1)1 by about 70 cents, putting it below the fretted note and repelling it upwards.

Any suggestions?

[kiwigeo]

A stab in the dark......perhaps something to do with change in amplitude of higher partials being generated by string when picking position is changed??

[jeffhigh]

I think it is actually the restriction to the soundhole from having my hand in front of it which is dropping the air resonance as compared to it being unrestricted when I am tapping the bridge.
It's typically the G on the low E which is effected where the T(1,1)1 is 100HZ with tap testing

F, F# and G# are ok as is C on the A string (no misplaced fret)

[Trevor Gore]

I just had a close look at this on a classical guitar with the T(1,1) at 100Hz and the T(1,1)2 at 191Hz. The guitar was perfectly tuned on the open strings and played G in tune (according to my Korg CA-1 chromatic tuner) when I plucked the G (3rd fret 6th string) in a fairly normal position. I moved the plucking position around until I got low tuner readings and occasionally high ones and took a spectrogram. A "blow-up" of the first two partials is shown below.

The first peak is split as a consequence of coupling with the T(1,1)1 and the second peak is split as a consequence of coupling with the T(1,1)2. The four peak frequencies (reading off the VA text file) are 96.23Hz, 100.26Hz, 191.11Hz and 196.49Hz. Depending exactly which peak is highest at what moment (which depends on how and where you pluck), that is what your tuner will show. On my CA-1, when trying for it, the note started low then moved in tune. With "normal" plucking, it played in tune. So I would guess that most of the time the CA-1 is firing on the peak of the 2nd partial at 196.49Hz (i.e right on G at 196Hz, with the "error" due to the resolution of the frequency "buckets"). But some of the time it will be firing on the 1st partial at 96.23Hz (which matches with the ~30 cents flat that the CA-1 was reading on those occassions). So I don't think the problem is your hand partially covering the sound hole but more likely one of this ilk.

Basically, you never totally eliminate this pesky frequency shift due to coupling effects in responsive guitars, but pitching body resonances between scale tones minimises the effect. The question is, whilst you can obviously measure it, are you hearing it in a way that bothers you (or your customer)?

[kiwigeo]

Pesky partials