Noise Exposure Dose & TWA Calculator

How it works

Dose % = 100 × Σ (Cᵢ ÷ Tᵢ), Tᵢ = 8 ÷ 2^((Lᵢ − criterion) ÷ exchange rate); TWA = criterion + exchange × log₂(dose ÷ 100); OSHA: 90 dBA / 5 dB; NIOSH: 85 dBA / 3 dB

Hearing damage depends on how loud and for how long, so exposure limits are expressed as a daily dose: eight hours at the criterion level is 100%, and every step of the exchange rate up the scale halves the allowed time. The two dominant standards disagree on both numbers. OSHA’s enforceable limit is 90 dBA with a 5 dB exchange rate — 95 dBA for four hours — and it requires a hearing-conservation programme from an 85 dBA action level; NIOSH, the EU and most of the world use 85 dBA with a physically better-founded 3 dB rate, under which 100 dBA is allowed for just fifteen minutes. Combining intermittent exposures gives a single time-weighted average. Manufacturers’ noise-reduction ratings are optimistic, so OSHA derates them heavily when estimating protected exposure.

Worked example

Three hours at 92 dBA, three at 85 and two at 75 under the NIOSH rule: 92 dBA allows 1.59 h (189% of the dose), 85 allows 8 h (38%), 75 adds only 2.5% — total 229%, a TWA of 88.6 dBA; well over the limit, and under OSHA the same day is 84.5 dBA TWA, just under the action level.

Frequently asked questions