All A-series fans from 40mm through 92mm share some similar features including flow acceleration channels at the tips of the intake side for minimizing vortex noise and improving airflow efficiency, stepped inlet design first seen in the Noctua NF-F12 fans for better airflow attachment to the frame, inner surface microstructures for minimizing sucktion side flow separation, integrated anti-vibration pads, advanced acoustic optimization frames, and Self-stabilizing Oil bearing v2 (SSO2). More information is available on Noctua’s technology page: http://www.noctua.at/main.php?show=technologies&lng=en
The A-series fan blades are very wide with compared to the S or B series noctua fan blades. There are seven in total across all fan sizes and there is a gradual curve on the leading tip and a straighter edge on the trailing tip. There are four standard straight struts on the exhaust side. There are no ULN versions for the 92mm fans but a higher-RPM slim variant is added that is only 14mm thick compared to the standard 25mm fan thickness. The A9 PWM version maxes out at 2000 RPM while the A9 FLX maxes out at 1600 RPM. The A9x14 PWM slim on the other hand maxes out at 1700RPM.
The 80mm A8 fans comes in three variants FLX (standard 3-pin connector), PWM (4-pin PWM controllable), and ULN (lower max RPM). Maximum RPM is much higher with the PWM version going up to 2200 RPM while the FLX version runs up to 2000RPM and the ULN version maxes out at 1400 RPM with a 12V input.
The smallest fan offered in Noctua’s line up is a 40mm variety that is only 10mm thick, ideal for severely height restricted applications and storage cooling. A standard FLX version is available with a 3-pin connector and accepts 12V input but a 5V option is also available, suitable for spaces with limited voltage compatibility such as network storage solutions. Despite the smaller size, Noctua has managed to miniaturize the motor so that there is larger space for the fan blade. Also, the struts are curved unlike the A8 and A9 with a little less curve on the fan blades.