Noise Reduction Calculator

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

Three ratings that get mixed up constantly

This trips up almost everyone shopping for noise control, and getting it wrong wastes real money.

• NRR (Noise Reduction Rating) is printed on earplugs and earmuffs, in dB. It is what this calculator uses. It tells you how much quieter the sound is at your eardrum.
• NRC (Noise Reduction Coefficient) is a number from 0 to 1 printed on acoustic foam and ceiling tiles. It measures absorption, meaning how little sound bounces back off the surface. It says nothing about how much sound gets through. An NRC 0.9 foam panel will tame the echo in a room and block almost none of your neighbour’s television.
• STC (Sound Transmission Class) is the one that measures blocking, and it is a property of the whole wall assembly, not of a panel you staple to it. If you want less sound coming through, mass and airtightness are what buy it, not absorption.

The short version: NRC fixes how a room sounds, STC fixes what you can hear from the next room, NRR fixes what reaches your ears.

Hearing protection NRR values:

Doubling up plugs under muffs does not add the two ratings together. OSHA and NIOSH both say to take the higher of the two NRR figures, derate that one, then add 5 dB for the second layer. Muffs rated 30 worn over plugs rated 32 give (32 − 7) ÷ 2 + 5, which is about 17.5 dB, not the 31 dB the two labels might suggest.

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.

Distance helps too, and it is free. The calculator applies the inverse square law between the distance the reading was taken at and where you actually stand. Doubling your distance from a point source drops the level about 6 dB, which is worth more than most people expect: stepping from 3 ft to 12 ft is a 12 dB cut, roughly the same as good earplugs, at no cost. It only works outdoors or in a large space. In a small hard room the reflected sound dominates past a few metres and moving further away stops helping.

OSHA permissible exposure limits: 90 dB for 8 hours; 95 dB for 4 hours; 100 dB for 2 hours; 105 dB for 1 hour; 110 dB for 30 minutes; 115 dB for 15 minutes. That is a 5 dB exchange rate. NIOSH recommends a stricter 3 dB rate starting at 85 dB, which is why the two agencies quote different numbers for the same noise. Hearing damage is permanent and it accumulates, so treat the stricter figure as the target.