Noise Exposure TWA and Dose Calculator
Noise dose tells you how much of the daily allowable exposure a worker has used. This calculator combines up to three tasks at different sound levels into one dose and an 8-hour time-weighted average.
Your numbers
Average A-weighted level for this task
Levels below 80 dBA are ignored in the dose, as in OSHA's calculation
Your results
Screening tool using the OSHA 29 CFR 1910.95 equations. Use dosimetry or sound level measurements by a qualified person for compliance decisions, and consider impulse and peak sound limits separately.
Map exposure groups to your assets
Optional: discuss tying sound survey results to machines and areas in your ERP or CMMS.
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How to use the calculator
Enter the average A-weighted sound level and the hours spent at that level for up to three segments of the shift. Levels below 80 dBA are ignored when computing dose. The calculator finds the allowed time at each level, adds the fractions and shows the total dose as a percentage and the equivalent 8-hour TWA. Use measured values from a sound level meter or personal dosimeter, and make sure total hours match the real exposure period. If workers move through many levels, a dosimeter gives a more reliable result than manual segments.
The formulas behind it
Under 29 CFR 1910.95, the permissible exposure limit is 90 dBA for eight hours with a 5 dB exchange rate: each 5 dB increase halves the allowed time. The reference duration is T = 8 / 2^((L - 90) / 5). Dose D = 100 x the sum of Ci / Ti, where Ci is time at level Li. TWA = 16.61 x log10(D / 100) + 90. The hearing conservation program action level is an 8-hour TWA of 85 dBA, equal to a 50% dose. Some other countries and NIOSH use 85 dBA with a 3 dB exchange rate, so results differ.
- 100% dose equals 90 dBA TWA (OSHA PEL)
- 50% dose equals 85 dBA TWA (action level)
- 5 dB exchange rate in OSHA, 3 dB in NIOSH and many countries
- Peak impulse noise of 140 dB is limited separately
Worked example
With the defaults, 3 hours at 92 dBA allows T = 8 / 2^0.4 = 6.06 hours, giving 0.495. Three hours at 87 dBA gives T = 8 / 2^(-0.6) = 12.13 hours and 0.247. Two hours at 78 dBA is ignored. The dose is about 74% and the TWA is about 87.8 dBA. That is above the 85 dBA action level, so the worker needs audiometric testing, training and hearing protection available, but below the PEL. Engineering controls and quieter processes remain the preferred long-term fix.
Frequently Asked Questions
What is the OSHA action level for noise?
An 8-hour time-weighted average of 85 dBA, equivalent to a 50% dose. Above it, employers must run a hearing conservation program with monitoring, audiometric testing, training and hearing protection.
Why is the exchange rate 5 dB?
OSHA's standard uses a 5 dB exchange rate, so allowed time halves for each 5 dB rise. NIOSH recommends a 3 dB rate, which is more protective. Use the rate that your regulator requires.
Does hearing protection reduce the dose here?
This calculator uses sound level at the ear without protection. Hearing protector attenuation is applied separately using NRR derating or a field attenuation method. Do not subtract the label NRR directly.
Can I use it for non-US limits?
The formulas follow the OSHA method. Many regions use 85 dBA with a 3 dB exchange rate, which gives a different dose. Use it as a rough screen only and apply your local method for compliance.
If you want noise survey results from your ERP maintenance or asset data tied to exposure groups, Netray can discuss the approach.
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