Add each element between the source and the diffuser, in order along the duct path.
🔈 Level at Diffuser
📊 Total Attenuation
Attenuation by Path Element
Full Calculation Table
| Path Element | Quantity | Attenuation | Running Total |
|---|
How to Calculate Cumulative Duct Path Attenuation
Enter the sound power level at the source, such as a fan outlet, before any duct path attenuation.
Add each element in the duct path — lined straight sections, elbows, area changes — with its length or quantity.
This calculator automatically includes end reflection loss at the diffuser based on the terminal duct dimension entered, most significant at low frequencies.
Click Calculate Path Attenuation to see the cumulative attenuation and the resulting sound power level reaching the diffuser.
Typical Attenuation Values by Path Element
| Element | Typical Attenuation | Notes |
|---|---|---|
| Lined straight duct | 1–2 dB per metre | Higher for smaller ducts and mid frequencies |
| Unlined elbow | 1–3 dB per elbow | Modest, frequency-dependent attenuation of upstream noise |
| Lined elbow | 3–6 dB per elbow | Combines directional and absorptive attenuation |
| Branch take-off | 3–10 dB per branch | Depends on area ratio between main and branch |
| Area change / transition | 2–5 dB per transition | Larger area ratio produces greater attenuation |
| Duct end reflection (small terminal) | 5–20 dB at 63 Hz, declining at higher frequency | Most significant for small diffuser necks |
General estimating values. Manufacturer-published attenuation data for specific products and duct sizes should be used for final design.
Tracing Sound Power Through the Duct Path
Sound generated at a fan or other source does not travel unchanged through a duct system to the diffuser; it is progressively attenuated by every duct element along the way. Predicting the noise level actually reaching a room requires summing these attenuation contributions from every straight section, fitting, and termination between the source and the diffuser, then subtracting the total from the starting sound power level.
Lined Duct Attenuation
Acoustic duct liner, typically fiberglass insulation faced with a protective coating on the airstream side, absorbs sound energy as it travels along a lined duct section. Attenuation per unit length depends on lining thickness, duct cross-sectional dimensions, and frequency, with smaller ducts generally providing more attenuation per unit length than larger ducts at the same lining thickness, since the ratio of lined perimeter to cross-sectional area is higher in smaller ducts.
Fitting Attenuation: Elbows, Branches, and Transitions
Elbows, branch take-offs, and area changes all provide some attenuation of sound already travelling through the duct, distinct from any self-noise they generate through velocity effects (calculated separately with the duct self-noise calculator). Lined elbows generally provide more attenuation than unlined elbows, combining the directional reflection effect of the turn with absorptive lining. Branch take-offs and area changes attenuate sound in proportion to the change in cross-sectional area, since energy splitting into a branch or expanding into a larger duct reduces the sound power continuing in the original direction.
Duct End Reflection Loss
Where a duct terminates into a much larger space, such as a room through a diffuser, some sound energy reflects back into the duct rather than radiating outward, similar to how a wave partially reflects at any sudden change in an acoustic medium. This end reflection loss is most significant at low frequencies and for small duct terminations, since low-frequency wavelength is large relative to the opening, creating a greater impedance mismatch. At high frequencies, where wavelength is small relative to the duct opening, end reflection loss becomes negligible. This effect is automatically included in this calculator based on the entered terminal dimension.
Building a Complete Noise Prediction
Duct path attenuation as calculated here reduces the fan-generated noise reaching the diffuser, but the diffuser and other fittings along the path also generate their own self-noise from air velocity, calculated separately with the duct self-noise calculator. The attenuated fan noise contribution and the locally generated self-noise contribution must be combined using logarithmic addition, since decibels do not add arithmetically; use the decibel calculator for that combination. Once the total sound power reaching the room from the diffuser is known, convert it to a predicted sound pressure level using the sound power calculator, combine with other room sources using the room sound level calculator, and compare against the target using the NC curve calculator.
When a Silencer Is Needed
If the cumulative path attenuation from standard duct elements alone is insufficient to reach the target noise criterion, a duct silencer can be added as an additional path element. Use the attenuator sizing calculator to determine the specific insertion loss required, then include that value as a custom silencer element in this calculator to verify the complete path meets target.
Frequently Asked Questions
Duct end reflection loss is the reduction in transmitted sound power that occurs where a duct terminates into a much larger space, such as a room, because some sound energy reflects back into the duct rather than radiating outward. This effect is most pronounced at low frequencies and for small duct terminations, since the wavelength of low-frequency sound is large relative to the duct opening, making the impedance mismatch at the termination more significant. At high frequencies, where wavelength is small relative to the opening, end reflection loss becomes negligible. Use the octave band calculator to analyze the full frequency-dependent effect on a specific spectrum.
Lined duct attenuation varies significantly by frequency, duct size, and lining thickness, but as a general pattern, standard fiberglass duct liner provides more attenuation per metre at mid frequencies, roughly 500 to 2000 Hz, than at low or high frequencies. Attenuation per unit length also increases as duct cross-sectional area decreases, since the ratio of lined perimeter to cross-sectional area increases in smaller ducts. Manufacturers publish specific attenuation values by duct size and lining thickness; this calculator uses representative values for general estimating.
An unlined elbow can provide some attenuation of sound already travelling through the duct, particularly at higher frequencies where the change in direction reflects some energy back toward the source, but this attenuation is generally modest and frequency-dependent. At the same time, an elbow with sufficient air velocity generates its own self-noise through turbulence, a separate phenomenon. For overall duct path noise prediction, both effects should be considered: attenuation of upstream noise passing through, calculated here, and self-noise the elbow generates, calculated with the duct self-noise calculator.
Start with the fan sound power level from the fan sound calculator as the initial source level entering the duct path. Apply cumulative attenuation from this calculator to find the level reaching the diffuser from that source. Separately calculate self-noise generated at fittings and the diffuser using the duct self-noise calculator, since this noise originates partway along the path. Combine both contributions logarithmically using the decibel calculator to get the total sound power reaching the room.