There are some pretty good YouTube videos on how to make an "antminer silencer box."

These are basically either a cool box or thick cardboard box with 5" (125cm) curved AC ducting at either end.

I have tried this on my S9 and it does do a good job of noise reduction.
As a side benefit as my miner is outside in a freezing shed it upped the hash rate slightly as it warmed it up a bit.