The time-weighted average (TWA) is how OSHA measures whether a worker’s noise exposure exceeds the 85 dBA action level or 90 dBA permissible exposure limit over an 8-hour shift. If your TWA calculation comes back above either threshold, specific legal obligations kick in. Here’s the formula, a worked example, and what to do with the result.
Soundtrace computes TWA and dose automatically from monitoring data and links results directly to HCP enrollment decisions — no manual calculation required.
Peak noise levels alone don’t determine compliance. A 95 dBA reading on a sound level meter tells you nothing about whether a worker needs HCP enrollment — you need to know how long they were at that level. A 15-minute exposure at 95 dBA is a small dose fraction. Eight hours at 95 dBA exceeds the PEL. Level without duration is incomplete.
What Is a Time-Weighted Average?
A time-weighted average noise exposure normalizes any combination of noise levels and exposure durations into a single number representing the equivalent constant 8-hour exposure. Because noise damage to the cochlea accumulates exponentially with level — not linearly — a higher level for a shorter time can be equivalent to a lower level for a longer time.
The TWA is the number OSHA compares against the 85 dBA action level and the 90 dBA PEL. It is always expressed as an 8-hour equivalent regardless of the actual shift length. A worker on a 10-hour shift who accumulates the same dose as someone at 90 dBA for 8 hours has a TWA of 90 dBA even though their shift was longer.
A sound level meter reading of 95 dBA tells you the noise level at that moment — not the worker’s TWA. A supervisor walking through a stamping press area for 5 minutes at 100 dBA contributes a small fraction to their daily dose. The press operator working 8 hours at 100 dBA far exceeds the PEL. Same noise level, radically different compliance status. TWA requires both level and duration.
Noise Dose vs. TWA: Two Ways to Express the Same Thing
Noise dose (expressed as a percentage) and TWA (expressed in dBA) are two representations of the same measurement. They are mathematically linked: 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). Personal noise dosimeters typically display both values.
How Noise Dose Is Calculated
OSHA’s dose calculation method is specified in Appendix A to 1910.95. The dose percentage is calculated by summing fractional contributions from each noise level encountered during the shift: for each level, divide actual time at that level by the maximum permissible time (from Table G-16), then sum all fractions. If the total ≥ 1.0 (100%), the worker has reached the PEL.
The 5 dB Exchange Rate and OSHA Table G-16
The exchange rate is the rate at which permissible exposure time changes with noise level. OSHA uses a 5 dB exchange rate: every 5 dB increase halves the maximum permissible time. This is specified in OSHA Table G-16 under 29 CFR 1910.95.
Worked Examples: TWA for Common Industrial Jobs
The following examples show how the same noise level produces very different compliance outcomes depending on duration — and how mixed-level shifts produce TWA values that aren’t obvious from the individual noise readings alone.
Shift Length and Overtime: Why Longer Shifts Raise the Stakes
The OSHA TWA calculation uses an 8-hour reference period regardless of actual shift length. When workers work longer shifts, the same noise level produces a higher dose per shift. A 12-hour shift at 90 dBA produces 150% dose — significantly exceeding the PEL — even though the noise level is exactly at the nominal PEL.
Workers on 10- or 12-hour shifts accumulate more dose per day at the same noise level than workers on standard 8-hour shifts. Employers running extended shifts in high-noise environments should evaluate whether their noise controls and monitoring adequately address the increased daily dose accumulation. The OSHA PEL is based on an 8-hour shift — it does not automatically scale for longer shifts.
OSHA vs. NIOSH Dosimeter Settings: Use the Right One
OSHA and NIOSH use different exchange rates and criterion levels. Using the wrong dosimeter setting for OSHA compliance produces incompatible and misleading results.
✅ OSHA Settings (for 1910.95 Compliance)
Exchange rate: 5 dB
Criterion level: 90 dBA (100% dose)
Action level: 85 dBA (50% dose)
Threshold: 80 dBA (sounds below excluded)
Use for: HCP enrollment decisions, 300 Log, OSHA inspections, all 1910.95 compliance determinations
⚠ NIOSH Settings (Not for OSHA Compliance)
Exchange rate: 3 dB (stricter — halves time per 3 dB)
Criterion level: 85 dBA (100% dose)
REL: 85 dBA as 8-hr TWA
Result: Produces higher TWA from same exposure — values incompatible with 1910.95
Use for: Occupational health best-practice goals beyond legal minimums; not for OSHA compliance
Most commercial noise dosimeters can be configured for either OSHA or NIOSH settings. Confirm your instrument is set to 90 dBA criterion level and 5 dB exchange rate before any monitoring that will be used for OSHA compliance purposes. Monitoring performed with NIOSH settings cannot be used directly for 1910.95 HCP enrollment decisions without recalculation.
A worker is exposed to:
- 95 dBA for 2 hours
- 90 dBA for 3 hours
- 85 dBA for 3 hours
Using OSHA’s formula: TWA = 16.61 × log(D/100) + 90
Where D = noise dose percentage calculated from each exposure segment against OSHA’s permissible duration table.
At 95 dBA: OSHA allows 4 hours → 2 hours ÷ 4 hours = 50%
At 90 dBA: OSHA allows 8 hours → 3 ÷ 8 = 37.5%
At 85 dBA: OSHA allows 16 hours → 3 ÷ 16 = 18.75%
Total D = 50 + 37.5 + 18.75 = 106.25%
This exceeds 100% — mandatory hearing protection and audiometric testing required.
Frequently asked questions
Stop Calculating TWA in Spreadsheets
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See How It Works →- Noise Monitoring: The Complete Employer Guide
- OSHA Action Level vs. PEL: What the 5 dB Difference Means
- Noise Dosimetry: How to Actually Measure Worker Exposure
- Sound Level Meter vs. Noise Dosimeter: Which Does OSHA Require?
- OSHA vs. NIOSH Exchange Rate: The 5 dB vs. 3 dB Difference