Noctua NF-B9 Review
 

Well, I was hoping for a bit more performance out of the NF-B9, but at just 1600 RPM, it's asking quite a lot for it to perform better than fans turning at 2000 RPM and higher, and yet it matched or exceeded the performance of the Thermaltake TT-9025A (2000 RPM) fan. So, in this respect, the design of the NF-B9 fan blades can be seen as having a performance advantage over standard fan blades.

http://www.overclockersclub.com/reviews/noctua_nf_b9/