My supply line is 8" insulated flex duct in the joists above the ceiling it drops into a MDF box lined with Linacoustic and probably 18' to the T8 fan which is outside the room and you cannot hear it at all until about speed 6 of 8, I usually run it at 2. The return T8 is above the equipment alcove and is also 8" but t's to 6" for the return and 4" to the top of the equipment alcove, both terminate in larger MDFf boxes sealed and lined with Linacoustic. This run is much shorter 10' on the return and about 6' on the 4" and can be heard if run above 3 since I run on 2 not an issue but I would do this one differently next time. I would have used a longer run of flex pipe and gone with 8" to the return and 6" to the alcove. All 3 terminate to the larger ends like Big shows above.