Duct Noise Is Set at the Sizing Stage, Not the Complaint Stage

Velocity = CFM / Duct Area

18" round:  Area = 1.77 ft²   V = 679 FPM    → quiet
14" round:  Area = 1.07 ft²   V = 1,122 FPM  → noisy (+8 dB)

Space type           Max FPM    NC rating
──────────────────────────────────────────
Residential          400–700    NC-25
Libraries, studios   500–700    NC-25–30
Private offices      600–800    NC-30–35
Open offices         700–1,000  NC-35–40
Retail, restaurants  800–1,200  NC-40–45
Industrial           1,000–2,500 NC-50+

Fitting                    Added noise
──────────────────────────────────────
Sharp 90° elbow (no vanes)  +12–18 dB
Vaned 90° elbow             +4–8 dB
Tee branch                  +8–14 dB
Abrupt transition           +6–12 dB
Takeoff (no scoop)          +8–15 dB

Round duct:        curved surface resists vibration
                   minimal panel noise

Rectangular duct:  flat panels vibrate (drumming)
                   wider panel = lower frequency = louder
                   36" wide panel >> 12" wide panel

Fix at design stage:
  Select 18" instead of 14" duct
  Cost difference: ~$2–5/linear foot
  For 100 ft run: $200–500

Fix after construction:
  Duct silencer: $2,000–10,000 installed
  Duct replacement: $5,000–25,000
  Ceiling repair: additional cost
  Tenant disruption: priceless

1. Calculate velocity: V = CFM / Area
2. Look up NC rating for the space type
3. Compare velocity to ASHRAE max for that NC
4. If over → select next larger duct size