The 800RPM Scythe Slip Stream (SSS?) starts fine a 5v -- and it is very slightly slower/quieter/less air than the SSS 500RPM at 12v. It blows more air at 12v than a GlobalWin NCB does at 5v; though it is also a bit noisier, too. (The GW NCB has the lousy "parallel" struts, btw...and it makes a much lower pitch noise than the SSS.)
At 5v, the 800RPM SSS and the 1300RPM GW NCB are blowing a very similar amount of air, and the SSS is slightly quieter. The GW NCB sound is dominated by the low pitch blade/strut noise (kind of a constant "purring"), while the only noticeable sounds from the SSS is some very soft bearing "ticks" -- probably when I move it. Each has the whoosh of air flow, of course. Since the GW NCB is too loud at 12v anyway, the SSS is the quieter fan and more useful fan from 5-12v.
The 500RPM SSS won't start at 5v, nor will it start at 7v. But again, it is so quiet at 12v, and frankly is probably only useful at 12v. At 5v (after bump starting it) it barely turns. At 7v the air flow is pretty feeble, though maybe as an intake fan blowing over an HD -- it is so quiet at 12v though, it really only makes sense to use it this way?
Conclusion: no, the 800 model doesn't fall over itself undervolting, so I should get the 800
. The S-flex, FI, had one, I think the 1600 RPM model, which was worse when undervolted, including not running as low, than the standard model of that lower speed.
Starting at low voltage isn't a problem. I only recently threw away my first kickstarter (tube relay, dead-bug RC circuit, rubber band holding it together). I plan to make one with a PICAXE, ASAP, now. I was actually hoping to mess w/ PWM fans (link
), but if I do, it looks like I'll have to do that without any other real goal in mind. Or, you know, Scythe could make a 1200 RPM Slipstream PWM fan...
On the kick-start: you can go find fan controller ICs that will do it all, and they are cheap ($2-3 in single-unit quantities--so negligible when bought by the palette and compared to other costs of a fan, I imagine).