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http://www.silentpcreview.com/article1343-page5.html
We've encountered the NF-F12 PWM previously on the NH-L12. It's one of the more radical designs we've seen in recent years featuring a series of thin, straight struts arranged like a clock around the hub with a few notches cut into each. The blades have very little curvature with the edges almost lining up perpendicular with the struts.
This is the worst possible structure for limiting tonality. Interestingly, Noctua's latest line of fans, the A series, has a design that goes completely against this, with struts and blades curved in opposite directions.
Above 1,000 RPM, the acoustic profile was buzzy with a prominent high pitched tone. At ~900 RPM, it smoothed out considerably, though up close we could hear some bearing chatter. At ~800 RPM and below the fan became very quiet though it was plagued with a dull drone.
Compared to the sample from the NH-L12, this fan had a slightly poorer sound quality, a bit more tonal at high speeds. We far prefer the old trusty NF-P12 to either. Interestingly, the F12 is not equipped on any of Noctua's other coolers, nor has its design been carried over to any of their larger fan models. The F series may simply have been a short-lived experiment that has reached its conclusion.