Airflow is typically measured for worst-case scenarios or not measured at all but just something pulled off a datasheet and then they install the cheapest fan they can find that meets the specs. My SOP is to open the case, get thermal readings, drop in a Noctua, and keep dropping the voltage while taking thermal readings to make sure nothing is getting too hot. I've got an NVR where I've dropped the speed to the lowest at which it will still start and it's still blowing cold air through the vents, so the 40mm screamer that was there initially was total overkill.

You can also get non-Noctua hydrodynamic bearing fans with polycarbonate blades and silicone mounts which are supposed to be very quiet, but I've never compared one against a Noctua. Anyone done a comparison?