About 18 yrs ago I did a design for AVAYA (nee Lucent) in Denver and figured out how to design cool AND quiet using open frame power supplies and/or motherboard in a large volume box. This kept it quiet and cool for a 19" rack.
1) maximum air velocity by design - use turbulent, eddy current airflow over the hot spots using a Mylar intake spoiler with a plenum or flame-proof thin cover to minimize cross-sectional area and maximize air velocity 1 ~ 5 m/s
2) diffuse and increase exhaust area to reduce air velocity like a muffler then use laminar airflow exhaust without a grill that causes eddy current noise. Egg crating works well.
3) choose fan pressure & velocity ( by CFM, RPM and blade thickness) to drive cabinet airflow impedance
4) Most importantly, keep moving fan blades in a laminar flow plenum, far away from grills that create turbulent noise. Plenums can be like any air duct material, cellulose, metal or plastic.
5)Add damping material to large sheet metal panels to avoid piano board resonance effects at max RPM.
6) use Thermistors to control variable DC-driven fans to enable power and regulate fan speed for hot spots at 50'C (e.g. epoxy thermistor to XFMR)
Summary
- Laminar intake
- Spoiler and plenum to create large eddy currents at max air velocity over hotspots
- diffuse exhaust plenum then laminar output
My inspiration was a DELL tower, circa 2002 with an inline plenum muffin fan to remove CPU heat directly via large hose outside the box, rather than directly CPU heatsink with fan turbulent noise.