Air

Terrabloom are smaller form factor and only minor difference of CFM to AC Infinity. Also, can get a temp control option. EC motors so you know it'll have 0-100 power control and nice and quiet. They also have a new line of "quiet" fans.. i have not tried those and do not know if it is worth the price or not. Looks like they just have a dampener built in. You can buy attachments that do the same thing... easy to see which way is cheaper by checking prices. Biggest key, regardless of brand, is that you go a bit bigger than you need. By running any EC motor at 50 % or so, it'll be quieter than running a smaller fan at 80-100 % in nearly all cases. Running at a lower rpm will extend life of fan, too. I've been running a 6" terrabloom 24/7 for nearly 3 years at this point. They are easy to take apart and clean. I neglect this a bit, but still running smoothly. Compared to those shit fans with a crap 70-100 % power control.. they are a godsend. FFS, you gotta put strip of duct tape on some of those to seal them up better, LOL. all those leather-bound ones suck or anything that looks like them are trash too. You can pick them out.. ipower, vivosun etc.. all trash exhaust fans for a grow tent. Will make ungodly noise within 3 months half the time... Another idea -- 2 4" fans (intake and exhaust) for a 3x3 or 4x4. by running two, should be able to keep both at 50 % or so. Also, could run minimal positive pressure and walls bow out a bit giving a bit more room around canopy. *drawback to a little positive pressure is that the smell could be worse, if you previous piped it outside or elsewhere and any filter will obviously be less useful but would cut down a portion.. even though majority of air would still flow through it, doesn't take much for these aromas to be noticeable, obviously.. Otherwise, i'd go with a 6" fan for that size of a tent. if made of money, get 2, lol.... overkill is great for fans with 0-100 % control. Running lower % is even quieter!