Nodal/harmonic points
 

A result of playing my recent 12 Fret 0 build , I noticed that there is a nodal/harmonic dead on the middle of the sound hole .
This seems to have created the effect of exciting unfretted strings to produce sympathetic harmonic resonances/tones .
I find this to be a pleasing chime during play .
Am I correct in my assessment of this phenomenon ?

What's a nodal/harmonic dead?

I'm assuming it's some sort of typo. But in my mind, anything that excites the strings without actually strumming or plucking is a good thing. If you feel that you don't like this effect, just leave a set of strings on for 6 months and you won't have that problem any more. :)

Yes you are correct, this should be similar to a natural harmonic on the 5th or 7th fret. I'd have to take measurements on your guitar but I'd guess 5th. A lot of guitars do this and fingerstylists capitalize on it

The 5th fret harmonic is repeated at the 24th fret and the 7th fret harmonic is repeated at the 19th fret. So, it might be the 5th fret harmonic or the harmonic at around the 3.2 fret which is the also 31st fret.

A touch harmonic at 1/2, 1/3, 1/4, 1/5, etc. division points along the string's total scale length will get a harmonic. That's in both directions, i.e. from the 12th fret towards the nut or towards the bridge. The same harmonic point at the fourth fret occurs pretty much near the center territory of the sound hole.

It is more obvious and therefore useful on some guitars

There are a bunch of harmonics in the sound hole area. That's the pinch harmonic zone, that electric and especially electric metal guitar players are familiar with.

Hmmm...

Firstly, the node in the middle of the soundhole is probably the 4th harmonic, as rick says, seeing as the centre of the soundhole is (in most acoustics) roughly half-way between bridge and 12th fret. The 4th harmonic node is at 1/4 string length, and produces 2 octaves above the open string. (Compare it with the 5th fret harmonic.)

Secondly, I don't understand what you mean by "created the effect of exciting unfretted strings to produce sympathetic harmonic resonances/tones". Harmonic nodes don't "create" such things (regardless of position). Unless perhaps you actually play the harmonic, maybe...

What makes unfretted strings vibrate in sympathy is fretted notes (or harmonics) that either have the same frequency as the unfretted strings, or share lower harmonics.
E.g., if you were to play that soundhole harmonic on your 6th string (if it is the 4th harmonic), then the first string would vibrate in sympathy, because it's the same pitch. You might also get sympathetic resonance from the A string, because its 3rd harmonic is the same pitch (near enough). You'd get the same effect from the 5th fret harmonic (on 6th string).

"Pleasing chimes" while you play may be due to all kinds of sympathetic resonances (from unfretted strings) depending on what notes you're playing, and is nothing to do with the position of a node relative to the soundhole or any other position. Harmonics are all about string fractions, as rick says. The nodes for fretted strings are of course in different positions from those for open strings.

On my 12 fret the 24th fret harmonic point is found about 1/4" below the center of the sound hole. The second node of the seventh fret harmonic is at the top of the sound hole. Other guitars might differ.