🔊 Estimated Sound Power
📏 Velocity Sensitivity
Dominant Frequency Bands
Full Octave Band Table
| Frequency Band | Estimated Lw |
|---|
How to Calculate Duct Self-Noise from Air Velocity
Choose the fitting or terminal generating self-noise, such as an elbow, damper, transition, or diffuser neck.
Enter the air velocity through or at that fitting, in metres per second or feet per minute.
Click Calculate Self-Noise to estimate the sound power level generated at that fitting and the octave band spectrum shape.
Compare the estimated self-noise against the room noise criterion, adjusting velocity if the result is too high.
Typical Maximum Velocity Guidelines by Space Sensitivity
| Space Sensitivity | Main Duct Velocity | Branch Duct Velocity | Diffuser Neck Velocity |
|---|---|---|---|
| Very quiet (studio, concert hall) | 3–4 m/s | 2–3 m/s | 1.5–2 m/s |
| Quiet (private office, patient room) | 5–6 m/s | 3–4 m/s | 2–2.5 m/s |
| Moderate (open office, classroom) | 6–8 m/s | 4–5 m/s | 2.5–3 m/s |
| Less sensitive (retail, corridor) | 8–10 m/s | 5–7 m/s | 3–4 m/s |
| Mechanical / utility space | 10–15+ m/s | 7–10 m/s | N/A |
General guideline ranges. Use this calculator to verify the specific velocity against your project's actual noise criterion target.
Understanding Duct Self-Noise and Velocity Control
Duct self-noise, also called velocity-regenerated noise, is sound generated locally by turbulence as air flows through fittings, transitions, dampers, and diffuser necks, distinct from fan noise that originates at the fan and travels through the duct system. Self-noise is generated wherever airflow direction, velocity, or cross-sectional area changes abruptly, converting some of the kinetic energy in the turbulent flow into acoustic energy, a mechanism covered in the Sound and Vibration and Duct Design chapters of the ASHRAE Handbook, Fundamentals volume.
The Steep Relationship Between Velocity and Self-Noise
Self-noise generation follows a steep power-law relationship with velocity, meaning sound power increases dramatically as velocity increases. As a general pattern, doubling velocity through a given fitting increases self-noise by roughly 15 to 18 decibels, a very large jump for what might seem like a modest change in flow speed. This relationship is the fundamental reason velocity control is one of the most effective noise control strategies available to duct designers, since reducing velocity through a fitting by even a small amount can produce a noise reduction that would otherwise require substantial and costly acoustic treatment.
Which Fittings Generate the Most Self-Noise
Different fitting types generate self-noise with different characteristic severity for the same velocity, and how those fittings are fabricated and installed is itself governed by the HVAC Duct Construction Standards published by SMACNA. Diffuser and grille necks, where air accelerates through a small opening directly adjacent to the occupied space, are particularly effective noise generators and typically require the lowest velocity limits of any duct element. Throttled dampers, especially when significantly closed and forcing air through a small effective opening, generate substantial broadband noise proportional to the pressure drop across them, which is a common issue at a VAV box outlet sized with a VAV box sizing calculator when the box throttles down at part load. Elbows, transitions, and branch take-offs generate self-noise proportional to velocity and the abruptness of the geometry change, with sharp, unlined elbows generally noisier than smooth radius elbows or those with turning vanes.
Frequency Content of Self-Noise
Self-noise tends to be broadband, spread across a range of frequencies, but the spectral shape differs by fitting type and can shift toward higher frequencies at very high velocities or very small fitting dimensions, since turbulent eddy size relative to a smaller opening tends to shift energy generation to shorter wavelengths. This calculator provides a representative spectrum shape for each fitting type to support downstream octave-band-based noise criteria verification using the NC curve calculator or RC rating calculator.
Using Velocity Limits in Duct Design
Rather than designing a duct system and then trying to acoustically treat excessive self-noise after the fact, the more effective approach is setting appropriate velocity limits at the outset of duct sizing, checked with an air velocity calculator against the target noise criterion for each served space, and sizing ducts and terminal devices accordingly. This calculator helps verify a specific velocity against an estimated self-noise level; for a full duct path analysis combining self-noise with fan noise and duct attenuation, use the sound attenuation calculator and fan sound calculator together with this tool.
Frequently Asked Questions
Duct self-noise, also called regenerated noise, is sound generated by turbulence as air flows through duct fittings, transitions, dampers, and diffuser necks, distinct from noise produced by the fan itself and transmitted down the duct. Self-noise increases sharply with air velocity, roughly following a fifth to sixth power relationship, meaning even a modest velocity increase can produce a substantial rise in self-generated sound. Fan noise originates at the fan and travels through the duct system, being attenuated along the way, while self-noise originates locally at each fitting. Use the fan sound calculator to estimate the separate fan noise contribution.
Self-noise generation at a duct fitting is driven by turbulent kinetic energy, which itself scales with a high power of velocity. Because sound power radiated by turbulence-generating mechanisms scales even more steeply, doubling air velocity through a fitting can increase self-noise by roughly 15 to 18 dB, a very large increase for a modest flow speed change. This steep relationship is why velocity limits are one of the most effective and commonly used noise control strategies in duct system design, often more effective than downstream acoustic treatment.
Recommended velocity limits vary by application and target noise criterion, with lower NC or RC targets requiring correspondingly lower velocities. As a general pattern, main ducts serving noise-sensitive spaces such as private offices or patient rooms are often kept below roughly 5 to 6 m/s, while ducts serving less noise-sensitive spaces can run considerably faster. Terminal devices such as diffusers typically need even lower neck velocities than the duct feeding them. Use this calculator to check a specific velocity, and the noise criteria selector to confirm the target NC for your space type.
A duct silencer can reduce self-noise generated upstream of it to some extent, but silencers are generally more effective at attenuating fan noise travelling through the duct than at controlling self-noise generated very close to where it enters the airstream, particularly for terminal devices like diffusers where there is often no practical location to insert a silencer between the source and the room. Reducing air velocity at the source, by increasing duct or terminal size, is usually the most reliable way to control self-noise. Use the attenuator sizing calculator for cases where a silencer is still warranted for upstream fan noise.