CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A technician is designing ductwork serving a conference room with a target noise criterion of NC-30. The current design routes air through a small branch duct with a discharge velocity of about 900 fpm at the diffuser. Which design change would most effectively help achieve the noise target?
- AIncrease blower speed to overcome noise with higher airflow
- BAdd several additional 90-degree elbows immediately before the diffuser
- CDecrease duct size to increase velocity for better throw
- DIncrease duct size to lower air velocity and add internal duct lining near the outlet
Show answer & explanationAnswer & explanation
Correct answer: D. Increase duct size to lower air velocity and add internal duct lining near the outlet
Air velocity is a major contributor to duct-generated noise; increasing duct size for the same CFM lowers velocity and reduces turbulence noise, while sound-absorbing duct liner near the outlet further attenuates noise before it reaches the space, helping meet a low NC (noise criterion) target like NC-30.
Why the other options are wrong
- A. Higher blower speed increases both airflow noise and velocity noise, counterproductive to a low NC target.
- B. Additional close-coupled elbows increase turbulence and noise near the outlet.
- C. Higher velocity increases turbulence and noise, worsening the NC rating.
Noise Criterion (NC) & Duct Design
NC ratings quantify acceptable background noise levels in occupied spaces; duct-generated noise is reduced primarily by lowering air velocity and using sound-attenuating duct linings or silencers.
- Lower velocity reduces turbulence noise
- Duct lining absorbs sound before it reaches the space
- Elbows and fittings close to outlets increase noise generation
Memory trick: Slow the air, line the walls — quiet halls.