The major source of noise in a duct system is the fan. The selected operating point of a fan has a major effect on the acoustic output level or noise generated; the point of maximum efficiency produces the best acoustical effect. However, during system operation when dampering occurs, the operating point moves up into a less efficient region, adding low-frequency rumble. Selecting a fan operating point at a lower total pressure than the maximum for clean filters will also avoid or reduce noise problems. Undersized fans operating at higher shaft speeds produce more noise, and oversized fans operating at lower shaft speeds create more low-frequency noise than fans operating at maximum efficiencies.
Most straight ductwork naturally attenuates noise. Acoustic lining increases noise attenuation. Fittings such as elbows, dampers, branch take offs, grilles, registers, diffusers, air-handling light fixtures, and variable inlet vanes either create or attenuate noise, depending on their geometry and air velocity. Higher air velocity in fittings creates higher noise levels. Duct velocities of 4 m/s (800 fpm) or less generate no audible noise. The following recommendations concerning duct-generated noise are presented in the HVAC Applications chapter, "Sound and Vibration Control" of the ASHRAE HVAC Applications Handbook, 2003 [ASHRAE, 2003]:
If necessary, duct noise can be reduced by using attenuators. The ASHRAE HVAC Applications Handbook, 2003 [ASHRAE, 2003] explains the effect of dissipative, reactive, and active attenuators. The Handbook's system acoustical design procedures should be followed.