From a quiet PC point of view the semi-passive concept is potentially quite an attractive one. The low power consumption (and heat output) of modern motherboards, CPUs and graphics cards under idle to low system stress conditions does raise the question of whether or not there is any need to run fans at all. Simply keeping them off under all but gaming conditions completely eliminates fan noise. But while getting PSU and graphics cards fans to work semi-passively is easy given the right choice of hardware, it not quite so straightforward to apply to CPU and/or case fans. Some Asus motherboards allow it under BIOS control (typically called fan stop), and Asus Fan Xpert software has a similar facility since version 2. But if you don't have an Asus motherboard the options for semi-passive fan operation may well be non-existent. A potential solution is Arctic's range of semi-passive 140mm fans - the F14 PWM, PWM PST and PWM PST CO models. Incidentally PST here means the traditional Arctic system of daisy-chaining fans in a PWM chain. In its latest form it is a lot neater than the older version that used separate fan rpm sensing cables. The CO (Continuous operation) models have dual ball bearings, the others are fluid dynamic. All of them are rated at 1350 rpm maximum speed.
The semi-passive part works by keeping the fan at 0 rpm under 40% duty cycle as this PWM fan profile published by Artic shows:
As is always the case with semi-passive fan operation there is the issue of the transition from fan off to fan on. In this instance Artic have chosen to start the fan at 600 rpm and from there to move to top speed at quite a steep rate of progression. Motherboard BIOS and/or fan software can still be used to mitigate this behaviour to some extent. But typically once CPU temperatures climb much beyond 75 to 80C then it gets difficult if not impossible to stop PWM fans approaching 100% duty cycle. But this doesn't detract from the fact that except for gaming these fans would simply not be running in normal ambient temperatures. So as an instant plug-in way of eliminating fan noise they may be worth considering. As PWM fans they will need PWM fan headers to work from, or they should work as part of a PWM chain using a PWM splitter cable or the Arctic PST method could be used.
