[_Nat_]

> why use multiple 1" thick filters to build a box, when you could just use 1x 4" filter.

Probably uncertainty over how effective fans are when immediately obstructed, and maybe concerns about particles possibly getting deflected backwards if a filter messes with a smooth, laminar flow.

This is, if we think of two kinds of airflow:

1. [laminar flow](https://en.wikipedia.org/wiki/Laminar_flow ), which is like a smoothly flowing river;

2. [turbulent flow](https://en.wikipedia.org/wiki/Turbulence ), which is like erratic flow inside a mixer;

then it seems like a box-fan might effect a laminar-flow further away from the blades (under normal circumstances), but could it be more turbulent near the blades?

The boxed-in design seems to try to dodge assumptions about how air flows near the blades, reducing uncertainty about how turbulent-flows might screw with it.