A diffuser for the vacuum fan used with agricultural equipment. The diffuser has a rectangular outlet fixed to the outlet of the fan and a diverging rectangular cross-section outlet for increasing the cross-sectional flow area. A primary flow path between the inlet and the outlet is no less in area than the inlet to the diffuser and a series of rectangular side chambers with perforated walls covered by sound deadening material have predetermined dimensions to decrease the outlet air velocity attenuate selected noise frequencies.