Which of the following is the most effective way to reduce noise in mechanical air delivery systems?

Comprehensive and Detailed Explanation From Exact Extract:

Noise in mechanical air delivery systems is often caused by high air velocity and turbulence within

ducts, which generate sound that can be transmitted to occupied spaces.

Increasing the size of the ductwork (A) lowers the air velocity for a given volume of air flow, which

reduces turbulence and noise generation inside the duct. Larger ducts allow air to move more quietly

and efficiently.

Exterior duct insulation (B) can reduce noise transmission through the duct walls but is less effective

at controlling the noise generated by airflow itself inside the duct.

Reducing the free area of the supply air grille (C) increases velocity at the grille, potentially

increasing noise at the outlet and causing discomfort.

Therefore, the most effective strategy is increasing duct size to reduce air velocity and noise.

Reference:

ARE 5.0 PPD – Building Systems and Assemblies, HVAC and Acoustics