Originally Posted by
Ttaskmaster
The problem is in the combination of whichever fans, and whatever the airflow has to go through.
For example, rads with high FPI combined with close fit fans that have a wide airflow projection, resulting in compression and loads of air bouncing around, with the end product being a flippin' loud air flow!
Unfortunately, without some seriously detailed research and calculation, the best way to figure out these things is just to try some in your individual case.
But basic rule of thumb, the higher the SP rating, the more air will make it through a restriction. The higher the airflow, the more air volume it has available to push through in the first place. High SP and AF values on the one fan are becoming more common, but even today it easily comes down to an SP:AF ratio to find the best.... and that's without factoring in the variables above, or the actual noise ratings.