exactly the large fans can be turned way down slow with the variac controller, but the price goes way up with the big fans, the whoosing sound of the air goes way down when the slower moving air moves thru a 6 or 8 inch tunnel , a 4 inch fan can be ran quiet for sure, and if your tight on money...
I do feel that the sound level with the 4 inch could be very quiet also , if you use the variac speed controller shown in the picture, I can tell you the 6 inch set up that is shown is whisper quiet, the fan is german made, 5 year warranty , and it slows way down and the draw thru the big carbon...
I replaced the fans in mine with super quiet dynamic float type bearing pc fans, you cant even hear it run now, I have photos of this replacement operation if anyone is interested in seeing it
ventalation parts laid out, 6 inch german made can fan, variac speed controller from amazon, way better than speedster controllers, this thing is SICK QUIET, and 14x6 carbon filter
added 1x4 pine board on left door with 2 inch gap on overlap when the door is closed, added 2 inch bolt style latchs top and bottom with small screws into door