DragonSteelMods.com posted a review on the Noctua NF-P12 120MM Fan
A quote from the article:
Noctua NF-P12 120MM Fan Review
We've reviewed quite a few fans here at DragonSteelMods lately and today we've got yet another one, but this one is a bit different, it's from Noctua. At first glance the NF-P12 looks like other Noctua fans, but upon closer inspection you'll notice it has what appears to be serrated fan blades. The serrations are called Vortex-Control Notches actually and they help to keep the NF-P12 running quietly and efficiently at all of the available speeds.
Noctua NF-P12 120MM Fan Review