Could either vary depth, or FPI. FPI would allow for finer grain control, since depth is done in units of an entire core-row. Would speculate that it would take an extremely meaty 12cm fan such that a high-FPI (24-30) single-row core wasn't enough such that moving to a dual-row lower-FPI core was really necessary.
From my limited comprehension of the relationship between fans, performance, and pressure, it would seem to me that double-row cores (deeper cores) very, very rarely make sense for axial fans that aren't in danger of giving their user's tinitis. It would seem to be far better to have an "extreme" model that just uses a higher FPI, rather than a deeper core. Would cost less too I would imagine.
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