🔊 Estimated NC Rating
Result Breakdown
How to Use the Velocity Noise Calculator
Choose supply plenum, return plenum, or branch duct, since each generates noise differently at the same velocity.
Enter the calculated or measured air velocity from your duct or plenum sizing.
Choose the target noise criterion rating appropriate for the space being served.
Click Calculate NC Rating to see the estimated NC rating and whether it meets target.
Understanding Velocity-Generated Noise
Air moving through ductwork, plenums, and fittings generates regenerated noise that scales with velocity, which is why velocity limits are one of the primary noise control strategies in HVAC design. This calculator estimates NC rating from velocity for common application types; see the duct self-noise calculator for a more detailed fitting-specific analysis.
Why Application Type Matters at the Same Velocity
Return plenums generate more noise than supply plenums at identical velocity because return air passes through open ceiling grilles and picks up turbulence from every opening and obstruction along its path, while supply air stays enclosed in ductwork for most of its travel. Branch ducts sit between these two in typical noise generation per unit velocity.
Target NC Ratings by Space Type
Different space types have different acceptable noise floors: bedrooms and libraries typically target NC-25 or below, private offices and conference rooms target NC-30 to NC-35, and open offices or retail spaces often accept NC-35 to NC-40. Confirm the specific project's target against the applicable building standard or client specification rather than assuming a default.
Reducing Noise When Velocity Can't Come Down
When space constraints prevent lowering velocity enough to hit target NC rating, acoustic duct lining or sound attenuators can reduce transmitted noise for the same air velocity. See the sound attenuation calculator in this category for lining and attenuator sizing as an alternative to further duct or plenum enlargement.
Frequently Asked Questions
Most open-plan commercial offices target NC-35 to NC-40, while private offices and conference rooms often target a quieter NC-30 to NC-35. Bedrooms and quiet spaces like libraries or recording studios need significantly lower targets, often NC-25 or below, which requires much lower duct and plenum velocities to achieve.
Return air passes through open ceiling grilles and picks up turbulence noise from every opening and obstruction along the way, which supply air traveling through enclosed ductwork does not experience to the same degree. This is why return plenums typically target 500 feet per minute or below, noticeably lower than the 700 feet per minute often acceptable for enclosed supply plenums.
Generally yes, since regenerated noise from air movement through fittings, transitions, and terminal devices increases with velocity, though the exact relationship depends on the specific geometry, not velocity alone. Sharp transitions, dampers, and diffuser necks generate more noise at a given velocity than smooth, gradual transitions, so fitting design matters alongside raw velocity.
Yes, acoustic duct lining and sound attenuators can reduce transmitted noise for a given velocity, offering an alternative to reducing velocity when duct or plenum space is too constrained to hit target velocity directly. See the sound attenuation calculator for lining and attenuator sizing when space limits velocity reduction as the primary noise control strategy.