DragonSteelMods posted a review on the Noctua NF-A6x25 FLX 60mm Fan

Featuring advanced aerodynamic design measures such as Flow Acceleration Channels and Noctua’s AAO frame, the NF-A6x25 FLX is a highly optimized, premium quality quiet fan in 60x25mm size. Smooth Commutation Drive technology and Noctua’s reference class SSO2 bearings guarantee superb running smoothness and excellent long-term stability. Topped off with modular cabling, Low-Noise Adaptors and 6 years manufacturer’s warranty, the NF-A6x25 is a premium choice for the highest demands.

Flow Acceleration Channels
The NF-A6x25 impeller features suction side Flow Acceleration Channels. By speeding up the airflow at the crucial outer blade regions, this measure reduces suction side flow separation and thus leads to better efficiency and lower vortex noise.

  Noctua NF-A6x25 FLX 60mm Fan Review