This duct reduces inlet turbulence at the fan blade tips which reduces high frequency fan noise.
The Cetus mk2 with extruder V2 has a 40mm square x 10mm thick 11CFM cooling fan spinning at 9500rpm. Any turbulence causes an unpleasant high frequency whine. Even the wire fan-blade finger guard causes the whine. The fan exhaust side is less important but should still not be ignored.
Reducing noise also tends to increase air flow so a win win.
These are thin walled. I printed using a 0.1mm layer, 0.4mm nozzle, and a track (paving) width of 0.3mm. Takes less than an hour to print and is half the weight of the original wire fan guard.
The thin inlet cone is slightly flexible and I had to put the bolts through the 4 mounting holes by slightly deflecting the cone and mounting the duct with the bolts as an assembly. You can increase bolt clearance by vertical scaling in the slicer before printing.
I designed 3 versions. The center cone style is slightly noisier but provides some finger protection. I suspect the 3 radial cone supports create a small amount of turbulence.
The fan gasket increases the blade clearance on the exhaust side. This further reduces fan noise on my Cetus, on other printers it's a simple quick test. Scale the Z dimension in the slicer for a thicker or thinner gasket.
My conclusion is that this is a good mod. The steppers are now far louder than the fan. The dominant noise from the fan is quiet white noise.
Designed in Fusion 360 and printed on my Cetus.