Quick Tip: Duct Velocity Limits — Why a Correctly Sized Duct Can Still Be Too Loud

Equal friction sizing gives you the right duct diameter for pressure drop. But it says nothing about noise. A duct that is correct for airflow can be far too loud for the space it serves.

The noise source

Air moving through a duct creates turbulence. Faster air = more turbulence = more noise. The relationship is roughly exponential — doubling velocity increases noise by 16-18 dB.

ASHRAE maximum velocities by space type

Space Type                    Max Velocity (FPM)
──────────────────────────────────────────────────
Residences (main trunk)        600
Residences (branches)          400
Conference rooms / libraries   600-800
Hotel rooms / patient rooms    800
Private offices                800-1,000
General offices / retail      1,000-1,200
Restaurants                   1,200-1,500
Industrial / warehouse        1,500-2,500
Mechanical rooms              2,000-3,000

Real example

Conference room duct: 2,800 CFM

Equal friction method (0.08 in. w.c./100 ft):
  Result: 18" diameter
  Velocity: 2,800 / (π × (18/24)² / 4) = 1,600 FPM
  → 2× the conference room limit

Velocity method (800 FPM max):
  Area = 2,800 / 800 = 3.50 ft²
  Diameter = √(4 × 3.50 / π) × 12 = 25"
  → Meets noise requirement

18" was correct for pressure. 25" is correct for noise. The duct needs to be 39% larger.

Quick sizing by velocity

Area (ft²) = CFM / Velocity (FPM)
Diameter (in) = √(4 × Area / π) × 12

Examples at 800 FPM (office/conference):
   500 CFM → 0.625 ft² → 11"
  1,000 CFM → 1.25 ft² → 15"
  2,000 CFM → 2.50 ft² → 21"
  3,000 CFM → 3.75 ft² → 26"
  5,000 CFM → 6.25 ft² → 34"

When space is too tight for the velocity-sized duct

Option 1: Lined duct (internal fiberglass)
  → absorbs 5-10 dB, allows ~20% higher velocity

Option 2: Sound attenuator (silencer)
  → 15-25 dB reduction, adds 3-5 ft length

Option 3: VAV terminal unit
  → noise break between high-velocity trunk and low-velocity branch

Option 4: Route trunk through non-sensitive space
  → long lined branch attenuates before reaching diffuser

The cost lesson

Correct sizing at design:     +$800 in larger sheet metal
Post-construction silencer:   $4,200 installed

Size for pressure AND velocity. The chart gives you airflow. The velocity table tells you if the space can live with the noise.