HVAC ductwork from an air conditioner hole in the wall

myname

So I'm in a building from about the 70's that doesn't have central air. I wanted to close up all the windows and use a fan blowing outside air in from the hole in the wall the air conditioner usually goes. I'd like to have a couple of turns to absorb some outside noise. What I really want is to absorb midrange noise (about 200hz-600hz). High frequencies are easy to absorb as they don't go around corners well. But those midrange sounds and lower are much harder. Any suggestions? I've heard of mufflers being used to stop noise going from one room to another. I wonder how much airflow these things gobble up? I shouldn't need to bother with exhausting the air as there is about an inch clearance under our front door and other areas where the air can exit the apartment.