Reduction in cataloged air performance due to a plenum around the fan is called a system effect . System effect is a pressure loss which must be added to the total external static pressure of the duct system . The pressure loss calculation is based on velocity of the air in the discharge ductwork :
System Effect pressure loss =
Co x Air Density x (Duct velocity/1096)2
Co = System effect coefficents
You may experience a system effect from .1 to 1 inch of wg .
