Keeps noise levels down.72 Using an oversized or undersized fan that fails to operate at or near rated peak efficiency can substantially increase noise levels.73 Locate fan-powered mixing boxes associated with variable-volume air-distribution systems away from noise-sensitive areas.74 Avoids significant noise problems.75 In ducts serving sound-sensitive spaces, keep airflow velocity as low as possible by increasing the duct size.76 Minimises turbulence and flow-generated noise.77 Ducts:78 Do79 Why?80 For consistent and straight airflow, ducts should connect at both the fan inlet and outlet.81 Turbulence at the fan inlet and outlet, and flow separation at the fan blades, can significantly increase fan-generated noise.82 Also, turning vanes near fan outlets can also inflate turbulence and noise, especially if airflow is inconsistent.83 Select air duct noise silencers that do not significantly increase the required fan total static pressure.