It's actually a bit tricky to exactly calculate the resistance needed to reduce your fan to a certain RPM. It's dependent on the amperage of the fan and the resistance of the fan while running. The 5v or 7v mod does work as per silentpcreview, but I prefer to use LNAs myself.
Noctua uses 82 ohm resistors in their blue LNAs and ~51 ohms in their black LNAs. It looks like they are 1/2 watt, but the 82 ohm only drops the NF12s(0.09a) to 9v, so 3v x 0.09a = 0.27w. If you want to build your own LNAs, pick up some 25-250 ohm resistors and if you want to drop to 7v or less, make sure you get 1watt resistors.
XSPC RX series are excellent for low RPM silent operation. The UT60s look great too, but I've never tried them personally and I have no idea why they decided to use m3 screws...
Dustin1706 got a reaction from Brian in Low noise adapters
Dustin1706 got a reaction from airdeano in Best Rad for SILENCE!
