Noise Attenuation with Distance Calculator
Sound level drop-off from a fan or equipment as distance increases.
Enter your values
Enter the sound level at reference distance in dBA.
Enter the reference distance in ft.
Enter the new distance in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Sound level at new distance
- 65 dBA
- Reduction
- 20 dB
- Distance ratio
- 10
Quick answer
For a point source in free field, sound pressure level drops about 6 dB for each doubling of distance.
L2 = L1 - 20 x log10(d2/d1)
At a glance
| What it does | Sound level drop-off from a fan or equipment as distance increases. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Sound level at reference distance, Reference distance, New distance |
| Main output | Sound level at new distance |
| Formula | L2 = L1 - 20 x log10(d2/d1) |
| Cost | Free — no sign-up, no download |
How to use the Noise Attenuation with Distance Calculator
- 1Measure or take the sound level at a known reference distance from the fan.
- 2Enter the new distance you want to estimate.
- 3Read the estimated sound level, based on the 6 dB per doubling of distance rule for a point source.
Inputs explained
- Sound level at reference distance(dBA)
- Enter the sound level at reference distance in dBA.
- Reference distance(ft)
- Enter the reference distance in ft.
- New distance(ft)
- Enter the new distance in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Sound level at reference distance85 dBA
- Reference distance3 ft
- New distance30 ft
Results
- Sound level at new distance65 dBA
- Reduction20 dB
- Distance ratio10
Frequently asked questions
Does this apply indoors?
Free-field spreading loss is reduced indoors by reflections; expect less attenuation than calculated in reverberant spaces.
What about line sources like long ducts?
Line sources attenuate roughly 3 dB per doubling of distance instead of 6 dB.
What do I need to enter into the Noise Attenuation with Distance Calculator?
Just 3 values: sound level at reference distance, reference distance, new distance. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Noise Attenuation with Distance Calculator work out the answer?
For a point source in free field, sound pressure level drops about 6 dB for each doubling of distance. It applies the formula L2 = L1 - 20 x log10(d2/d1) and shows the working so you can check each step by hand.
Is the Noise Attenuation with Distance Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.