From Noctua.at
NF-S12A: The NF-S12A has been optimised for “low impedance” applications that don’t require high static pressure and thus combines moderate pressure with outstanding airflow and superb quietness of operation. Choose the NF-S12A for case ventilation, applications with little or no obstruction to airflow as well as all other applications where minimum noise emission has first priority.
NF-P12: The NF-P12 has been designed with more pressure demanding “high impedance” applications in mind. It provides an even balance of high static pressure, high airflow and excellent quietness, which has made it a standard choice for low noise CPU cooling, cases with tight fan grills and other low noise cooling applications with mid- to high airflow resistance.
NF-F12: The NF-F12’s unique Focused Flow™ system produces extremely high static pressure and focuses the airflow in order to achieve even better results on air cooling heatsinks and water cooling radiators. With a top speed of 1500rpm, it also offers more performance headroom for less noise-sensitive applications. Choose the NF-F12 if you’re looking for the best possible performance on heatsinks and radiators.