🔊 Radiated Sound Power
🔌 Transmission Loss
Dominant Frequency Bands
Full Calculation Table
| Frequency Band | Transmission Loss | Radiated Lw |
|---|
How to Calculate Duct Breakout Noise
Enter the sound power level travelling inside the duct at the section passing through the sensitive space.
Enter the duct width, height, and the length of duct passing through the space, since breakout sound power scales with duct wall surface area.
Choose the duct gauge and whether external acoustic lagging is applied, since transmission loss through the duct wall depends on wall mass and any added treatment.
Click Calculate to estimate the sound power radiated into the adjacent space and compare it against the room's noise criterion.
Typical Duct Wall Transmission Loss by Construction
| Wall Construction | Relative TL at Low Frequency (63–125 Hz) | Relative TL at High Frequency (2000–4000 Hz) |
|---|---|---|
| 26 gauge standard sheet metal | Lowest | Moderate |
| 22 gauge heavy sheet metal | Slightly improved | Improved |
| 26 gauge with acoustic lagging | Substantially improved | Substantially improved |
| Double-wall insulated duct | Improved | Substantially improved |
General relative comparison. Actual transmission loss varies by specific product, insulation thickness, and installation quality.
Understanding Duct Breakout Noise Transmission
Duct breakout noise, sometimes called breakout transmission, occurs when sound travelling inside a duct radiates outward through the duct walls into any space the duct physically passes through, before the airstream itself reaches its intended terminal or diffuser. This differs from the noise that eventually exits at the diffuser: breakout sound escapes through the sheet metal walls of the duct section itself, meaning even a duct that does not terminate in a sensitive room can still be a significant noise source in that room if it merely runs through or above it. Acoustic performance requirements for the spaces most often affected by this problem, such as private offices, patient rooms, and multi-unit dwellings, are set out in the National Building Code of Canada, published by the National Research Council of Canada, which is the reference most Canadian jurisdictions adopt for minimum sound insulation.
Why Breakout Happens
Sound pressure fluctuations inside the duct cause the thin sheet metal duct wall to vibrate slightly, and that vibrating wall in turn radiates sound into the surrounding air on the outside of the duct, just as a speaker cone radiates sound by vibrating. The transmission loss values used in this type of calculation are typically derived from laboratory testing methods similar in principle to ASTM E90, the standard test method for laboratory measurement of airborne sound transmission loss of building partitions and elements, adapted to the duct wall geometry. The amount of sound that breaks out also depends on the total surface area of duct wall exposed to the sensitive space, which is directly proportional to duct perimeter and the length of duct passing through that space; checking this surface area independently with the duct surface area calculator can help confirm the figure used here, particularly for irregular routing where the exposed length is not obvious from the plans alone.
The Low-Frequency Problem
Duct wall transmission loss for a mass-limited sheet metal panel generally increases with frequency, meaning low-frequency sound passes through the duct wall more readily than high-frequency sound. This is the opposite pattern from duct lining, which is most effective at absorbing higher frequencies and comparatively weak at low frequencies. The practical consequence is that breakout noise is frequently dominated by low-frequency content, even for fans and airstreams whose original in-duct spectrum was not particularly low-frequency-heavy, because the duct wall itself acts as a low-pass filter that preferentially transmits the lower bands.
The Effect of Duct Size and Wall Construction
Larger duct cross-sections generally exhibit lower transmission loss than smaller ducts built from the same gauge material, because larger unsupported panel spans are more easily excited into vibration by the internal sound pressure and radiate more efficiently as a result. Simply increasing sheet metal gauge (using thicker material), a decision usually checked against structural and pressure class requirements with the duct gauge calculator, provides some improvement in transmission loss, particularly at higher frequencies, but often only modest benefit at the low frequencies where breakout noise is typically most problematic. Acoustic lagging, a mass-loaded barrier material wrapped around the duct exterior, is usually far more effective across the full frequency range and is the standard solution when breakout noise into a sensitive space exceeds target.
Addressing Excessive Breakout Noise
When calculated breakout noise exceeds the target for the space, the most direct options are rerouting the duct to avoid the sensitive space entirely, adding acoustic lagging to increase transmission loss, reducing the in-duct sound level upstream using a silencer sized with the attenuator sizing calculator before the sound reaches the sensitive section, or in more demanding cases building a dedicated acoustic enclosure around the duct run using gypsum board or other mass-loaded barrier construction. Combine the predicted breakout level with any other noise sources in the room using the room sound level calculator or equipment sound level calculator, then compare the total against the room's target using the NC curve calculator or RC rating calculator.
Frequently Asked Questions
Duct breakout noise, also called breakout transmission, is sound that radiates outward through the duct walls into any space the duct passes through, rather than continuing to travel to the intended terminal or diffuser. It becomes a problem whenever a duct carrying high internal sound levels, typically from a nearby fan, passes through or above a noise-sensitive space such as a private office, conference room, or patient room before reaching a less sensitive final destination, since the radiated sound can exceed the room's noise criterion even though the duct itself is not terminating there. Use the fan sound calculator to estimate the in-duct source level.
Duct wall transmission loss generally increases with frequency for a mass-limited panel like sheet metal duct wall, meaning low-frequency sound is attenuated less by the duct wall than high-frequency sound. This is the opposite of the general pattern for duct lining attenuation, where high frequencies are absorbed more effectively than low frequencies. As a result, breakout noise is often dominated by low-frequency content even when the in-duct spectrum itself is not particularly low-frequency-heavy, because the duct wall passes that low-frequency energy through more readily. Compare against the RC rating calculator, which specifically flags low-frequency rumble issues.
Larger duct cross-sections generally have lower transmission loss than smaller ducts of the same gauge, because larger, thinner panel spans are more easily set into vibration by internal sound pressure, radiating more efficiently. Heavier gauge duct wall provides more mass and generally improves transmission loss, though the benefit is most pronounced at higher frequencies; heavier gauge alone often provides only modest improvement at the low frequencies where breakout is typically most problematic. This is why acoustic lagging, rather than simply upgrading duct gauge, is usually the more effective solution when breakout noise exceeds a target.
The most direct options are rerouting the duct to avoid passing through or above the sensitive space, adding acoustic lagging to increase transmission loss, reducing the in-duct sound level upstream with a silencer sized using the attenuator sizing calculator before it reaches the sensitive section, or building an enclosure around the duct section using gypsum board or other mass-loaded materials for a more substantial noise barrier than lagging alone provides. The most cost-effective solution depends on the specific project constraints, including ceiling space, budget, and how much noise reduction is actually required.
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