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.
Base level of my room noise is 37.2 to 37.8 dBA.