Gena, volume is a factor(for the reverberent field, it makes no difference to the direct sound), but not to the extent that seems to be sometimes imagined(e.g. twice the room volume in no way requires twice the amplifier power or twice the speaker output to get the same level at a listener's position). The Linkwitz and Voelker material on room acoustics that was previously linked includes volume considerations in the analysis. Incidentally, out of curiosity, even before you reported the latest SPL numbers on your room I'd run it through the speech intelligibility calculator at the
MCSquared site which has some pretty good calculators I use when I get lazy and don't want to do the math(nearly all the time). I used a 120 degree horizontal dispersion factor for the M60 and came up with about a 9' "critical distance" which I fooled around with to get some theoretical sound numbers which your measured values were in good agreement with.
