I agree with the above. I technically only needed a 6" fan but opted for the 8" version so I could run it on a very low setting & still get the air movement I wanted. The Hyper fan are EC fans though so a variac will not work on them. The noise they make at low speed is similar to that of a jet engine at low speed. I think the noise is there what ever speed setting you use, its just at higher speeds the air movement noise masks the noise the fan itself makes, unless you buy the stealth version of course.
I have a friend who could not afford the stealth version so has an 8" wrapped in multiple blankets. Obviously not as effective as the stealth model noise wise but from behind a closed door is inaudible.