Estimate HVAC duct noise at the outlet from duct length, attenuation rate and elbow losses, or find the attenuation rate needed to meet a target level.

Use a level already referred to the same receiver as background; do not mix sound power with sound pressure.

Elbows and background
Blank means no elbows.

Required when elbow count is above zero. Use the measured or manufacturer loss for your elbow type.

Blank omits background; 0 dB is a real level, not silence.

Need air speed first? Use the calculate duct air velocity from airflow and duct size tool for round or rectangular ducts, with results in FPM or m/s. Velocity alone does not determine sound level; this noise calculator uses the attenuation data you supply.

Duct Noise Screening Formula

This tool subtracts supplied straight-duct and elbow attenuation from a source contribution, then optionally combines background sound logarithmically. Use comparable, receiver-referred levels with the same acoustic reference, weighting and frequency scope. Do not directly mix a fan sound-power rating with a room sound-pressure level. Independent, uncorrelated contributions are assumed.

Lduct = Lin - r × L - N × E
Lcombined = 10 × log10(10⁽Lduct / 10) + 10⁽Lbackground / 10))
  • Lin: source contribution referred to the comparison receiver, in dB.
  • r: attenuation rate, in dB/m or dB/ft. L: compatible straight length.
  • N: whole number of elbows. E: supplied attenuation per elbow, in dB.
  • Blank background omits it; zero dB is a real sound level.

Required Attenuation Rate

Select the inverse mode and enter a combined target and positive straight-duct length. When background is included, the target must exceed it. Finite attenuation cannot produce a combined level at or below that background floor.

Lduct,target = 10 × log10(10⁽Ltarget / 10) - 10⁽Lbackground / 10))
r = max(0, Lin - Lduct,target - N × E) / L

Without background, the duct-only target equals the entered target. A zero required rate means the model already meets the target; it does not require negative attenuation.

Worked Screening Example

For an illustrative source contribution of 82 dB, length 12 m, rate 0.7 dB/m, two elbows at 2 dB each and background 35 dB: straight loss is 8.4 dB and elbow loss is 4 dB. The duct contribution is 69.6 dB; the combined level is approximately 69.601506 dB. For a 50 dB combined target with the same source, length, elbows and background, the required rate is approximately 2.344963 dB/m. These are demonstration inputs, not measured product data.

Sensitivity and Limits

Forward scenarios change the straight-duct rate by minus/plus 20% while keeping elbows fixed. Inverse scenarios vary the target by minus/plus 3 dB and flag a background-floor conflict. Blank elbow count means zero; when elbows are included, enter applicable attenuation per elbow. Negative dB levels are valid relative to a reference.

This simplified path model excludes regenerated flow noise, breakout, leakage, directivity, room corrections and a full octave-band source-path-receiver analysis. It does not calculate NC/RC ratings, hearing risk or compliance. Duct attenuation depends on frequency, geometry, lining and installation; enter suitable documented values.

Sources: NIOSH Industrial Noise Control Manual for independent sound-level addition; ASHRAE, Noise and Vibration Control for frequency-specific HVAC path analysis and regenerated noise.