Noise Reduction Calculator
Calculate how much a noise barrier reduces sound levels.
Enter source decibels, barrier NRC rating, and distance for the reduced dB level.
Noise reduction in decibels (dB) quantifies how much sound energy is blocked or absorbed by a barrier, ear protection, or acoustic material.
Core formulas: Sound Level After = Sound Level Before − NRR (Noise Reduction Rating) Effective NRR (OSHA formula) = (Rated NRR − 7) ÷ 2 Transmission Loss (TL) = 10 × log₁₀(1 ÷ τ)
Where:
- NRR = Noise Reduction Rating in dB (printed on hearing protection products)
- 7 = correction factor for real-world fit vs. laboratory conditions
- τ (tau) = sound transmission coefficient (fraction of energy transmitted through a barrier)
- TL = transmission loss of a wall, door, or panel
The decibel scale is logarithmic:
- Reducing by 3 dB = cutting sound energy in half
- Reducing by 10 dB = perceived as half as loud to human hearing
- Reducing by 20 dB = 100× reduction in sound energy
Hearing protection NRR values:
| Protection Type | Rated NRR | OSHA Effective |
|---|---|---|
| Foam earplugs (good fit) | 29–33 dB | 11–13 dB |
| Earmuffs (standard) | 24–28 dB | 8.5–10.5 dB |
| Earmuffs + earplugs | 30–36 dB | ~15–17 dB |
| Construction earmuffs | 30–34 dB | 11.5–13.5 dB |
Worked example: A construction site produces 98 dB of noise. A worker wears foam earplugs rated NRR 30. OSHA Effective NRR = (30 − 7) ÷ 2 = 11.5 dB Sound level at ear = 98 − 11.5 = 86.5 dB OSHA safe exposure limit: 90 dB for 8 hours. At 86.5 dB, this worker is within safe limits.
OSHA permissible exposure limits: 90 dB for 8 hours; 95 dB for 4 hours; 100 dB for 2 hours; 115 dB for 15 minutes. Hearing damage is permanent — proper protection is critical.