Have my grow in my bedroom and this muffler worked perfect. I would also look at ac infinity fans. Quite possibly the quietest inline fan I've ever heard.
VIVOSUN 6 Inch Noise Reducer Silencer for Inline Duct Fan https://www.amazon.ca/dp/B01CTM0J70/ref=cm_sw_r_cp_api_i_x34QCb07X1P45
To add onto this, you can get a silencer for it. So put it in a box with that sticking out the end and it should be pretty quiet. For even more sound reduction insulated ducting as well and if you're still having problems idk what to say.
I'll start it off.
20 x 3 gal plastic pots
6 inch silencer for your inline fan system - https://www.amazon.ca/VIVOSUN-Noise-Reducer-Silencer-Inline/dp/B01CTM0J70
Up for any offers.
you can also add a duct muffler at the end of your exhaust to reduce noise:
Here is the post for archival purposes:
Author: davedigerati
Content:
>Anyone tried a duct silencer (tons on Amazon from growing operations like this https://www.amazon.com/VIVOSUN-Noise-Reducer-Silencer-Inline/dp/B01CTM0J70 ) and had the stock fans able to handle it?
>Worst case, add a 3rd, quiet fan on the outlet to overcome the increase in backpressure - seems easier than the cooler & ducting ideas...
Here's one VIVOSUN 6 Inch Noise Reducer Silencer for Inline Duct Fan https://www.amazon.com/dp/B01CTM0J70/ref=cm_sw_r_cp_api_MXYyyb87PWV9H