A ventilation fan structure has a fan blade-motor portion and a shroud portion independently affixable to the ceiling to reduce noise levels, improve aerodynamic efficiency, and facilitate installation. Mounting members are affixable to ceiling joists spanning the opening for the fan structure. The drive motor and associated fan blades are mounted on the mounting members. The shroud surrounds the ceiling opening and embraces the fan blades.

http://www.freepatentsonline.com/4510851.html