I'm curious if there even is an answer other than AC infinity. I have a Cloudline T6 and it's quiet as hell. I have it set to 7 and have to be within 15 feet or so to hear it and even then it is barely audible. It exhausts directly outside and standing right next to the vent, the only way I can tell it's running is smell or I can see the air moving the grass. Obviously this is all going to be relative. In my case, I work in my basement so I am constantly hearing the HVAC kick on and off as well as my dehumidifier, both of which are waaaayyyy louder than my AC infinity. Then add in my server rack which is quite loud. I will say, though, that it is night and day compared to my previous inline duct fan which was one of the Vivosun behemoths that sound like a fookin rocket engine.