The attached image shows the top of a residential air handling unit, and how a flexible silencer (right) and a spiral duct (left) is attached. The duct is steel, and the silencer aluminium. The owner proposes that fan noise will transfer to the room through the centimeter wide piece of exposed aluminum at the connection of the silencer - supposedly twice as much as it is aluminium, and not steel. I for one have a hard time believing that this can notably add anything to the room's total sound level. The thickness of both is 0,5 mm. What do you think?