Most noise generated in the air distribution system is caused by fan operation. Noise attenuation devices are usually designed to reduce transmission of fan noise through ductwork. Generally, noise attenuation devices increase the fan power required to move air in the distribution system because they create an added pressure drop. Therefore, the first step in noise control is to select a fan that creates less noise. Low-noise fans, such as "sickle blade" fans, are slightly less efficient than the best axial fans but eliminate the need for noise attenuation devices. The energy engineer has to balance decreased efficiency from noise attenuation devices against less efficient but quieter fans. Designing the ductwork to absorb noise can also influence fan selection. [Kruse, 1991; Micro-Electronics Facility Efficiency Workshop, 1995]
