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The higher the FPI, the stronger the fans need to be to push air through and the more impedance air will experience along the way (which translates to noise both from the fan and from forcing air through a restricted space). Lower FPI, then, means less resistance for air passing through and more efficient use of low-speed fans. Putting low speed fans on high FPI radiators makes performance suffer, where putting high speed fans on low FPI radiators is essentially wasted effort. Martin and Skinee have demonstrated the SR 1 consistently beating out higher-density radiators when fan speeds are kept at 1000rpm and lower; above that, their performance drops off pretty quickly.
The HW Lab's SR 1 only has 9 FPI so it's very easy (comparatively) to push air through the rad; I believe most of the high-performance radiators are in the 20+ FPI range. The AX series at 16 FPI is still significantly tighter a space than the SR 1. XSPC's RX series also has very low FPI, and generally these two are the only two radiator series people speak of when it comes to very quiet watercooling.




















