Blade Design

Noctua is currently offering fans in two distinct sizes, 120x25mm and 80x25mm. Even so, the differences between these two fans are more than just size. Unlike many other fans where you see very similar blade designs across an entire family, Noctua has carefully designed each of these fans specifically for low speed operation, and as you can see from looking closely at the fan blades themselves they are very different.

Since an 80mm fan has a much larger area devoted to the spindle/rotor assembly relative to a larger design, it required a much different blade design than the 120mm model to attain the flow rate and noise characteristics Noctua desired. To quote from Noctua...

Both the angle of attack and the design of the blade area of the NF-R8 rotor are optimised for maximum airflow and minimum noise emission at speed ranges below 1800RPM. Raising the leading edge above the rotor hub and lowering the trailing edge below the bottom of the rotor hub results in an extended profile chord length, which compensates for the reduction in blade surface caused by the steeper angle of attack and hence increases the overall blade force. The continuous radial increase of the blade curvature towards the blade tips ensures sufficient static pressure despite of the slow rotation speeds. A significant reduction in noise emission is achieved through the rounding of the fan blades towards the rotor hub as well as the rounding of the rotor hub itself. This way, the NF-R8 is significantly more quiet than other high end silent fans at the same performance thanks to its specific optimisations for ultra low noise application.

The 120mm fan is perhaps the most radical departure design-wise compared to other 120mm fans you see, at least on first inspection. Utilizing a scalloped straight blade design, the 120mm fan sports a very different blade profile from what we're used to seeing in 120mm fans, most of which we have seen using varying degrees of swept blade designs. Again, to quote from Noctua...

The Straight-Blade-Design of the NF-S12 series provides the maximum efficiency of a straight fan blade without the drawback of higher noise emission, which arises when straight blades are used at higher rotation speeds. At the low rotation speeds of the NF-S series, the Noctua Straight-Blade-Design achieves a significantly higher flow rate than forward or backward swept blade designs. Furthermore, among other measures to prevent vortices like the rounded rotor hub, the distance between rotor and stator (tip clearance) was increased in order to reduce the problem of tip vortices which plays an important part in the noise emission of axial fans. With its fan blade exactly tailored to ultra low noise application, the NF-S12 series is significantly more quiet than other high end silent fans.

These design characteristics are not only for the overall reduction in sound pressure levels (dB), but are also focused on modifying the nature and quality of the sound made by the fans itself. It's quite clear that Noctua feels they are approaching fan design in a very novel and scientific manner. One final quote from Noctua...

"In the area of PC cooling, the last years saw a growing demand for axial fans with the lowest possible noise emission, which manufacturers mostly met by simply reducing the RPM of existing models without sufficiently adapting the blade geometry and profile to the modified fluid dynamic parameters. The comparison of measured data and simulation outputs showed that the result was an average deficit of efficiency of 10-15%. The complete redevelopment of the blade geometry and profile was carried out with exact fluid mechanical adjustment to minimal noise emission in a speed range of 400-1800RPM at a height of 25mm and a radius of 40 and 60mm. Both fan blades possess 8-12% more flow rate than comparable products at the same total noise emission and static pressure." (Mag. Manfred Stadler MBA, Austrian Institute for Heat-Transmission and Fan Technology (Österreichisches Institut für Wärmeübertragung und Ventilatorentechnik, ÖIWV))

All the technical mumbo-jumbo in the world can't make up for actually testing the real thing, so let's move on, taking a look at the fans themselves and seeing how they perform.