Fan Shrouds
A shroud is a spacer between the fan and whatever it is blowing through. It gives the air a few millimeters to spread before it hits the fin stack, which softens the dead spot behind the motor hub and drops the turbulence noise a fan makes when it sits flat against a radiator or a mesh panel.
Phobya covers both jobs: 120 x 120 x 20mm and 140 x 140 x 20mm plexi shrouds for the gap, and 7mm shroud and decoupling parts in 120mm and 140mm where the point is to stop vibration reaching the chassis.
Alphacool shroud and decoupling parts at 7mm come with 4x M3 x 37mm screws in the box, so you are not hunting for longer hardware after you add the thickness.
ModMyMods flexible fan mounts in 120mm and 140mm, including the V2.0 revision, take the same idea further and hang the fan off the frame entirely.
Adding a 20mm shroud means your existing screws are 20mm too short. Longer 6-32 and M3 hardware is in screws.
