Noise Measurement, Exposure Assessment, and Dosimetry
Instruments and the arithmetic behind exposure
Personal noise dosimetry is the reference method: a meter worn on the shoulder accumulates dose over a shift against a criterion level (90 dBA for 8 hours) using OSHA's 5 dB exchange rate. Sound level meters serve area surveys and source identification. Weighting matters: A-weighting approximates the ear's low-frequency insensitivity for damage-risk assessment; C-weighting is flatter, and the C-minus-A difference helps select protectors for high-level, low-frequency noise. Use slow response for steady noise, and verify the instrument with an acoustical calibrator before and after measurement.
Dose converts to TWA by TWA = 16.61 log10(D/100) + 90.
Traps
Sampling a "typical" employee without documenting that others are similar; forgetting that 95 dBA for 4 hours equals the same dose as 90 dBA for 8 hours; and confusing dosimeter settings - a NIOSH-configured unit (3 dB exchange, 80 dBA threshold) reports a very different dose from an OSHA-configured unit on the same worker.
Worked example: 2 hours at 95 dBA plus 6 hours at 85 dBA. Allowed time at 95 dBA is 4 h, so 2/4 = 50%; allowed time at 85 dBA is 16 h, so 6/16 = 37.5%. Total dose = 87.5%. TWA = 16.61 log10(0.875) + 90 ≈ 89 dBA - above the action level, below the PEL.