Because sound travels in ducts independently of airflow direction, the isolation of noise transmitted by mechanical air ducts (called duct-borne noise) must be carefully evaluated in both the supply and return air systems. Sound- attenuating mufflers and linings, sized to satisfy the requirements of the least critical spaces, can be used in the main ducts, with separate mufflers used in the branch ducts to the more critical spaces (e.g., hearing testing, music practice rooms). In buildings where critical spaces are numerous, large mufflers and linings in the main ducts can be sized to satisfy these critical demands, with the less critical spaces being overdesigned.
Checklist for Duct-Borne Noise Control
1. Provide smooth duct turns and transitions (complicated duct configurations should be avoided).
2. Sound is attenuated by:
- Distance (low-frequency duct-borne sound is reduced by transmission through rectangular duct walls)
- Internal glass-fiber linings ( 1 in thick) and sound-attenuating mufflers (to reduce sound energy by absorption)
- Turns, branches and other duct divisions, lined mixing boxes, and lined plenums (absorptive linings are much more effective at turns than in equal lengths of straight duct)
- End reflection at openings into rooms (small openings don't efficiently radiate low-frequency sound energy)
3. Avoid high air velocities (large duct cross sections permit lower velocities for equal airflow volume).
4. Locate dampers away from air outlets (so noise generated can be attenuated before it reaches rooms).
5. Use round ducts, or rectangular ducts with low aspect ratios (i.e., ratio of duct depth to width), which are less rumble-prone.
6. Use several smaller air outlets rather than a few large ones.
Note: Refer to the latest edition of ASHRAE Handbook (Systems Volume) chapter on “Sound and Vibration Control” for duct-borne noise isolation computational procedures.
Next: Noise Sources in Ducts