Dosimetry That Disagrees With the Sound Level Meter
A 60-person injection molding plant sampled noise with a Type 2 logging dosimeter worn by six machine operators. The instrument was configured with A-weighting, slow response, a 3 dB exchange rate, and an 80 dB threshold; it reported 8-hour TWAs of 89–93 dBA. Hand-held sound level meter readings at the same machines, taken with a 5 dB exchange rate, slow response, and A-weighting, ranged from 86 to 90 dBA. Two dosimeters were removed for the 30-minute unpaid lunch break and the removal was not logged. Calibrator checks read 114.0 dB before the shift and 113.2 dB after. One operator works a 12-hour shift four days a week; his dosimeter reported a TWA of 90 dBA.
1. Explain the disagreement between the dosimeter and the sound level meter, and state which configuration governs the compliance decision.
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The exchange rate explains most of the gap. With a 3 dB exchange rate, allowable exposure time halves for every 3 dB increase, so the computed TWA runs higher than it should for the same sound field. OSHA's criterion uses a 5 dB exchange rate. Reprocess the logged data at 5 dB, A-weighting, slow response, and compare again.
The threshold setting also matters. A threshold of 80 dB captures the quieter minutes needed to characterize the 85 dBA action level. A 90 dB threshold discards that time and biases the result low relative to the action level, while the exchange-rate error biases it high relative to the PEL.
Which numbers to trust. The corrected dosimetry should land near the 86–90 dBA meter readings. Use the corrected values to classify each operator against 85 dBA (program enrollment) and 90 dBA (mandatory protectors).
Judgment point. The 3 dB result must not be reported as the OSHA TWA, but it should not simply be discarded either - it is the raw record from which the corrected value is derived.
2. How should the unlogged lunch removals and the calibration readings be handled?
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Calibration. The pre- and post-shift checks differed by 0.8 dB, within the customary ±2 dB acceptance window, so the samples remain usable. A drift beyond that window would invalidate the sample and require resampling. File the calibration readings with the survey record.
The lunch removals. Taking the dosimeter off for 30 minutes without documenting it makes the reported TWA an underestimate unless the break area is demonstrably quiet or the instrument logged the gap. Do not silently accept those two results. Either resample those operators with the instrument left on through the break (noting the quiet period), or reconstruct the missing time and correct the dose.
Program fix. Train everyone who deploys instruments to log start and stop times, breaks, unusual events (confined spaces, equipment shutdowns), and calibration checks, and attach that log to the exposure record, which is retained at least 2 years.
3. The 12-hour operator's dosimeter reported a 90 dBA TWA. How should that result be interpreted and used?
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TWA is normalized to 8 hours, so a 12-hour shift accumulates more dose than the same sound level over 8 hours. At a constant 90 dBA for 12 hours, dose = 100 × (12/8) = 150%, and:
TWA = 16.61 log10(D/100) + 90 = 16.61 log10(1.5) + 90 ≈ 92.9 dBA.
Practical implications.
- Report both the measured 8-hour TWA and a shift-adjusted value, and base program decisions on the adjusted one.
- Select hearing protectors for the protected exposure across the full 12 hours, not merely against the 8-hour criterion.
- Consider engineering controls, rotation, or administrative limits for extended shifts.
- Confirm how the instrument was worn: a dosimeter left on for 12 hours must be documented as such, or the reported TWA may be misread as an 8-hour equivalent.
Judgment point. The operator is above the mandatory-protector threshold on a shift-adjusted basis even if the raw number reads exactly 90.