Thanks MikeC. It is fun to figure out some things by myself!
So, now some concluding remarks from my side.
Thanks to dukla2000 in this thread
The standard formula for this is
dBA2 = dBA1 - 50 * log (rpm1/rpm2)
so a 4412FGL is 7.0dBA @ 5V.
Yeah, right! The formula ignores among other things the bearing noise that may not decrease at all at lower rpm. Then again I cannot hear my 4412FGL @5V @ 1m: for quality fans the formula is arguably a fair indicator, for other fans I have had the misfortune to own (e.g. Evercool EC12025M12S) it is clear to my ears there are some assumptions behind the formula that will explain cases where it clearly is wrong!
I have been able to calculate the dBA levels for these fans at different voltages (with the friendly support of MS Excel). I stress this is only very indicative, since I am sure that there are numerous variables which from fan to fan need consideration. And I am sure that a dBA figure cannot be a negative number as the 5V of the 4412 2/F2GLL. Also, these calculations are based solely on the manufacturer’s specification, which one must treat with suspicion anyhow.
Nevertheless, the dBA findings are interesting to me, and I hope to others as well. Also confirms for me, I will not exchange my S12 fan, since the 15.4 dBA of the Adda fan at 5V is not perceivable for (most) humans. It’s probably the air resistance in the PSU at 5V that one hears at all, not the fan itself. And at 1m my system is now below the ambient noise level in my room.
V RPM CFM dBA
12,0 1800,0 72,0 34,4
9,5 1425,0 57,0 29,3
7,0 1050,0 42,0 22,7
5,0 750,0 30,0 15,4
Papst 4412 F/2GL
12,0 1600,0 55,0 26,0
9,5 1266,7 43,5 20,9
7,0 933,3 32,1 14,3
5,0 666,7 22,9 7,0
Papst 4412 F/2GLL
12,0 1200,0 41,0 18,0
9,5 950,0 32,5 12,9
7,0 700,0 23,9 6,3
5,0 500,0 17,1 -1,0
12,0 1000,0 37,0 22,8
9,5 791,7 29,3 17,7
7,0 583,3 21,6 11,1
5,0 416,7 15,4 3,8
These findings also confirm to me in a theoretical sense the quality of the Papst 4412 F/2GLL fan which I can definitely sense in practice. Although I must admit, I have not seen/tested a Nexus fan myself; I cannot imagine a discernable difference between the two fans at 5V.
Even though my questions are answered, it would be nice to see this list expanded with other fans which are interesting for silencers, also including the actual dBA figures tested in SPCR labs, and even maybe posted as a Sticky. If anyone is interested in pursuing this, I would be happy to mail the excel table I started – you don’t need to start from scratch.
p.s. if anyone finds an error in my calculations/assumptions, I would be happy for corrections.