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I did that experiment this morning, removing all the corks and comparing the tone and the spectrum, and looking at some of the modes. It was interesting.
Tone:
It does, indeed, get 'thin' and 'peaky' with all of the side holes open. The bass is just gone, there seems to be a lot more mid-range, particularly irritating mid-range. ;) There's still about the same amount of 'color'.
Modes:
The wood modes seemed to change very little, which was a bit of a surprise. I've seen some of them rise or drop a bit with the opening of a single hole, and expected much more of the same. Instead, the only one that moved a lot was the 'main back' resonance. With the holes closed it's around two semitones higher than the 'main top' mode, but with them all open the top and back are right on top of each other. As near as I can figure out it has to do with the fact that both the top and back are getting into the 'bass reflex couple'. Normally, if it was just the top and the air, they would kick each other apart in pitch more or less, depending on how tightly they're coupled. Which one moves the most depends on the relative masses: usually there's more top mass than air moving, and the air mode drops more than the top rises. In this case, since the back is much heavier than the top you'd think it would move the least. However, what seems to be happening instead is that the top and the air are 'ganging up' on it. Verrry interesting.
Some of the top modes that showed at more than one fequency with the holes closed also did so with them open, indicating that there are still air resonances in the box: it's not really 'open'.
The biggest change, as you'd expect, was with the 'main air' pitch, which rose from 97 to 132 Hz,; about a musical fourth. I did _not_ take the time to measure the pitches of all the other inside air modes: that's kind of time-consuming, and I _do_ have to get some work done now and then! I also figured I'd see the results in the impulse spectrum. I did!
The spectrum:
With the 'main air' mode shifted up so far, and the 'main back' and 'main top' modes right on top of each other, the low end of the spectrum chart is pretty low. The usual broad and tall 'main top' peak is just a 'spike' and it's no taller than was the wider 'top-back' peak of the 'holes closed' condition. There's much less low-end power available.
There is a big gain in power in the range between 250-500Hz: the fundamentals of the notes from the open B on to halfway up the neck on the E. With the holes closed most peaks in that range tend to be shorter than the 'main top' peak, but with them open they are as tall or taller, and some of them are much taller. This is the range of some of the strongest interior air modes, so I'm not surprised that there's power here when they're vented.
Overall the chart reminds me a bit of an archtop F-hole guitar, and not a good one! It is certainly not what I'd expect to see on a classical guitar that has been rated 'not bad!' by some pretty good makers, like Greg Byers, and players too.
So there you are. 20 holes 5/8" in diameter are not a lot compared to the total area of the sides, and it's not surprising to see that they did not 'kill' all of the inside air resonances. Venting those had less efect than I expected on the wood modes, showing that there may not be as much coupling going on as you might expect. Again, the enclosure was not compromised all that much, so.... I think this does indicate the direction you could expect the tone to go if you left the back off entirely: no 'main air' mode and much less contribution from the other air modes, without much gain from 'freeing up' the top. Now we know why we go to all the trouble of making a body....
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