I agree pretty much 100% with Rick Talbot who did that article for OC. It's not rocket science, pretty simple stuff (as most of all this is, really). But there are issues with implementation. Sure a duct helps things keep cooler -- does it increase noise? I've played with carboard mailing tubes, tunnels built of medite board, add-on plywood and cardboard mufflers for fan in/out vents, and the vinyl ducting above. I don't use them any more because I don't generally find the need to. Undervolting the CPU and judicious use of 5V quiet decouple-mounted fans with completely open vents (no grill) keep my systems quiet enough for me. About the noise level of a single 7V Panaflo a few feet away.
1) tubes are naturally resonant: the air in them will resonate at specific frequencies. Try holding one to your mouth and humming or singing scales or just making noise -- low to high & back. At some freq, you'll actually feel the air resonance of the tube, it will add volume (loudness) at certain frequencies. The stiffer & thinner the material, the more pronounced the effect. A fan is generally a broadband noise maker, therefore, it will excite resonances in tubes. Only way this can be avoided is to keep the fan noise very low.
2) any impedance near / around the fan increases air turbulence. Ducts do, too, in varying degrees. Just try it. Again, this can be avoid with quiet low airflow fans.
3) smooth contours (streamlining) in the tube including in / out ends minimizes turbulence & noise.
4) a thin layer of light smooth foam applied to the inside surface of the duct can also help reduce noise (if the airflow is significant -- maybe 20 cfm & higher?)
Still not sure about one slow big fan vs a few smaller slow fans. I have tried both & think both can work; the former almost requires a duct.
ENuf said. I bore myself going on & on &....