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Evgenii Konkin
Evgenii Konkin

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

#hvac #engineering #acoustics #ductwork

Duct noise is proportional to velocity. Doubling velocity adds roughly 16 dB. The duct size you choose locks in the noise level permanently.

Velocity drives noise 

Velocity = CFM / Duct Area
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1,200 CFM through two different ducts:

18" round:  Area = 1.77 ft²   V = 679 FPM    → quiet
14" round:  Area = 1.07 ft²   V = 1,122 FPM  → noisy (+8 dB)
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Same airflow. 65% more velocity. 8 dB more noise.

ASHRAE recommended max velocities 

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+
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NC = Noise Criteria. Lower NC = quieter space = lower max velocity.

Where noise gets worse: fittings

Straight duct noise is predictable. Fittings generate turbulence noise 10–20 dB louder:

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
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A vaned elbow is 8–12 dB quieter than unvaned at the same velocity. That is the difference between hearing the duct and not hearing it.

Rectangular duct adds breakout noise 

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
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This is another reason extreme aspect ratios fail — a 36×6 duct is not just higher friction, it is also louder than a 12×8 duct of the same area.

The cost comparison 

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
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The check

Before committing to a duct size:

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
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This takes 30 seconds. The noise complaint lasts the life of the building.

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