All i can say is my exhaust consists of a carbon filter, 530cfm fan, total of 4 feet of duct but it sounded like a goddamn jetliner in my garage. I grabbed a 8x24 duct with 2 8x6 reducers and 1" egg crate foam (actually foam mattress) spray glued in the duct pipe, taped all the seems and it is long enough to be pretty much the whole exhaust run. I would say is its about 75% quieter and cost about 35. Anybody looking to quite down the air through their ducts cannot go wrong. I had to check my fan to make sure it was on, i have it vented behind a freezer and fridge that are side by side in my garage. I used to be able to hear it clear as day outside with my garage door shut, now it just sounds like my fridge and freezer are running
This is what i got 8x24 round duct 2 8x6 reducers Egg crate foam or the wavy mattress pad or pillow stuff. I picked mine up at Ollies for 7 bucks I simply spray glued my padding inside the duct. I kept mine the full 24" length but you can cut it shorter if needed When i had it all together taped every seem with gorilla tape, i had it lying around. Installed it and was good to go. I can take a pic of it installed later today
Other methods would be to cut your ducting every 2 feet and add screen between the joins, put the fan in a sound insulated box with the holes in the box for air intake similarly baffled (multiple layers of screen break up the ability to leave sound waves at full strength...this is a rough approximation of how a pistol sound suppressor works), and pad (as it sounds like you did) the duct itself so the amount of air running through it, along with the vibration caused by the fan (and the sound waves the fan itself creates) don't get transferred to the outside air as efficiently. Or if you want to dump a bunch of money into it just so you have the "most technically advanced system you know of", I could show you how to build sound baffles INSIDE ductwork...then all you have to do is mount the duct itself solidly so it can't vibrate and create sound, in so doing.