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The time-weighted average (TWA) is the single number that OSHA uses to evaluate whether a worker's noise exposure meets or exceeds the action level or PEL. Understanding how the TWA is calculated—and why it matters more than peak noise levels alone—is essential for any safety manager running a hearing conservation program under 29 CFR 1910.95.
Soundtrace automatically computes TWA and dose from monitoring data and links results directly to audiometric enrollment decisions—no manual math required.
A time-weighted average noise exposure accounts for both the level of noise and the duration of exposure. It normalizes any combination of noise levels and durations into a single equivalent number representing what it would mean as a constant 8-hour exposure. A worker who spends 2 hours at 95 dBA and 6 hours at 80 dBA does not have a TWA equal to the simple average of those numbers—the math accounts for the fact that louder exposures accumulate damage much faster.
OSHA uses the formula specified in Appendix A to 1910.95. The noise dose percentage is calculated first by dividing actual time exposed at each level by the maximum permissible time at that level, then summing the fractions. The TWA is then derived from the dose percentage.
| Level (dBA) | Max Allowable Time (OSHA) | If exposed for 4 hrs: dose contribution |
|---|---|---|
| 85 dBA | 16 hours | 4/16 = 25% |
| 90 dBA | 8 hours | 4/8 = 50% |
| 95 dBA | 4 hours | 4/4 = 100% |
| 100 dBA | 2 hours | 4/2 = 200% (exceeded PEL) |
| 105 dBA | 1 hour | 4/1 = 400% (severely exceeded) |
If the sum of dose fractions equals or exceeds 1.0 (100%), the worker has reached the PEL. If the sum equals or exceeds 0.5 (50%), the worker has reached the action level.
OSHA's standard uses a 5 dB exchange rate, meaning that for every 5 dB increase in exposure level, the maximum allowable exposure time is cut in half. Safety managers using NIOSH-calibrated dosimeters will get different TWA readings than those using OSHA settings.
PEL: 90 dBA for 8 hours
At 95 dBA: 4 hours maximum
At 100 dBA: 2 hours maximum
Dosimeter setting: Criterion 90 dBA, exchange rate 5 dB — use for OSHA compliance
REL: 85 dBA for 8 hours
At 88 dBA: 4 hours maximum
Dosimeter setting: Criterion 85 dBA, exchange rate 3 dB — produces results incompatible with OSHA compliance
Noise dose (expressed as a percentage) and TWA (expressed in dBA) are two representations of the same measurement. A dose of 100% equals a TWA of exactly 90 dBA. A dose of 50% equals the action level TWA of 85 dBA. Personal dosimeters typically display both.
| Work Scenario | Exposure Pattern | Approx TWA | Status |
|---|---|---|---|
| Press operator, stamping line | 8 hrs at 94 dBA | 94 dBA | Exceeds PEL — controls required |
| Forklift operator | 6 hrs at 83 dBA, 2 hrs at 91 dBA | ~86 dBA | Above action level — enroll in HCP |
| Office admin visiting floor | 1 hr at 90 dBA, 7 hrs at 55 dBA | ~72 dBA | Below action level — no HCP required |
| Maintenance tech | 2 hrs at 98 dBA, 6 hrs at 78 dBA | ~88 dBA | Above action level — enroll in HCP |
If workers regularly work shifts longer than 8 hours, the same noise levels produce higher dose accumulation. A 12-hour shift at 90 dBA produces 150% dose—significantly exceeding the PEL—even though the noise level itself is exactly at the nominal PEL.
Soundtrace computes noise dose and TWA automatically, flags action-level and PEL exceedances, and triggers the right HCP responses—all in real time.
Book a DemoGet a quote for your facility →A time-weighted average noise exposure normalizes any combination of noise levels and exposure durations into a single number representing the equivalent constant exposure over an 8-hour reference period. OSHA uses TWA to evaluate whether workers meet or exceed the 85 dBA action level or 90 dBA PEL.
Noise dose is a percentage representing how much of the maximum permissible daily exposure has been accumulated. A dose of 100% equals a TWA of exactly 90 dBA (the PEL). A dose of 50% equals a TWA of 85 dBA (the action level). Both are valid representations of the same measurement.
OSHA uses a 5 dB exchange rate, meaning that for every 5 dB increase in noise level, the maximum allowable exposure time is cut in half. At 90 dBA, the maximum is 8 hours; at 95 dBA, 4 hours; at 100 dBA, 2 hours. NIOSH settings cannot be used for OSHA compliance determinations.