Factor findings :
I am trying to establish the thought that "the tree sounds when the wind blows" - "cause & effect". I recorded the various fan rotation speeds using SF-609 controller and sound level of each setting. If the fan does not have speed sensor I indicated voltage used.
The chart answers a couple of questions in my mind:
1) The higher the fan rotation the higher the sound level.
2) Given the same fan speed
larger fan does not mean lower noise level.
I am making an assumption that most PCs are about 18" from the user; second thought I believe the chart above shows high to low RPM readings corresponding to distance will better serve. Yes, the farther I am away the less the noise. At lowest setting 120 mm fans are not much quieter an 80mm at the same rotation.
I use one each 120, 92 and 80 mm size fans to emulate water cooling setup in a minimum number of fan scenario; 120 mm radiator fan, 92 mm exhaust fan and 80 mm front intake for hard drives. The same setup could be used for 100% air-cooling, say the 120 mm is on the Thermalright XP-120 for instance.
Granted, water cooling may well yield lower CPU temp only to be raised for more overclocking, hence noise is not reduced in some cases.