High velocities in the duct or diffuser typically generate air noise. The flow turbulence in the duct and the excessive pressure reductions in the duct and diffuser system also generate noise. Such noise is most apparent directly under the diffuser. Room background levels of NC 35 and less provide little masking effect. Any noise source stands out above the background level and is easily detected.
Typically, air noise can be minimized by the following procedures:
? Limiting branch duct velocities to 1200 fpm
? Limiting static pressure in branch ducts adjacent to outlets to 0.15 in. of water
? Sizing diffusers to operate at outlet jet velocities up to 1200 fpm (neck velocities limited to 500 to 900 fpm), and total pressures of 0.10 in. of water
? Using several small diffusers (and return grilles) instead of one or two large outlets or inlets that have a higher sound power
? Providing low-noise dampers in the branch duct where pressure drops of more than 0.20 in. of water must be taken
? Internally lining branch ducts near the fan to quiet this noise source
? Designing background sound levels in the room to be a minimum of NC 35 or NC 40
