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Audiometric test results are only useful if they are correctly interpreted. Missing a Standard Threshold Shift means a worker continues accumulating damage without intervention. Misidentifying a false positive STS triggers unnecessary referrals and paperwork. This guide explains how to read audiogram data, how OSHA requires STS to be calculated, and what patterns indicate about the effectiveness of your hearing conservation program.
Soundtrace automates audiogram interpretation — STS determination, age correction, baseline revision, and result notification are handled automatically, with remote audiologist review for every flagged case.
The critical frequencies for OSHA STS are 2,000, 3,000, and 4,000 Hz. An average shift of 10 dB or more across those three frequencies in either ear triggers required employer action within 21 days.
A pure-tone audiogram measures the softest sound (in decibels) that an individual can detect at specific frequencies. Results are expressed as hearing threshold levels (HTLs) in dB HL, where 0 dB HL represents the average threshold for a young person with normal hearing. For OSHA 1910.95, the tested frequencies are 500, 1,000, 2,000, 3,000, 4,000, and 6,000 Hz. The frequencies most relevant to OSHA’s STS calculation are 2,000, 3,000, and 4,000 Hz.
▶ Bottom line: Higher dB HL numbers mean worse hearing. Thresholds below 25 dB HL across all frequencies are generally considered within normal limits for occupational purposes.
| Frequency (Hz) | 500 | 1,000 | 2,000 | 3,000 | 4,000 | 6,000 |
|---|---|---|---|---|---|---|
| Baseline (Right ear) | 10 | 10 | 15 | 15 | 20 | 20 |
| Annual Year 3 (Right ear) | 10 | 15 | 20 | 25 | 35 | 40 |
| Change | 0 | +5 | +5 | +10 | +15 | +20 |
This shows progressive shift at 3,000, 4,000, and 6,000 Hz — the high-frequency pattern characteristic of NIHL. The STS calculation: (5+10+15)/3 = 10 dB — exactly the OSHA STS threshold.
OSHA 1910.95(g)(10)(i) defines an STS as a change of an average of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear. Calculation: (1) subtract baseline threshold from current test threshold at each frequency; (2) if any value is negative, use 0; (3) average the three values; (4) if ≥10 dB, STS is present. When an STS is identified, OSHA requires: notify employee within 21 days; refit or provide hearing protection; require HPD use; refer for evaluation if warranted; evaluate OSHA 300 log recordability using uncorrected thresholds.
OSHA Appendix F provides age correction values that can be subtracted from measured threshold shift to account for normal age-related hearing decline. Age correction is optional. If applied: look up correction value from Appendix F tables based on employee age at baseline and current age, separately for men and women; subtract from observed shift before applying the 10 dB threshold; treat negative corrected values as 0. Important: age correction cannot be applied when determining OSHA 300 log recordability — for recordkeeping, use uncorrected shift.
| Pattern | Likely Cause | Action |
|---|---|---|
| Notch at 4,000 Hz (bilateral) | Classic NIHL pattern — early noise-induced loss | Review noise controls; confirm HPD use; monitor at next annual |
| Shift at 3,000–6,000 Hz (progressive) | Ongoing NIHL | STS evaluation; engineering control review; HPD upgrade |
| Asymmetric shift (one ear worse) | Asymmetric noise exposure; otologic disease; prior injury | Refer for otologic evaluation |
| Low-frequency loss (500–1,000 Hz) | Generally not NIHL; suggests otologic disease | Refer for medical evaluation |
| Sudden significant shift (>25 dB) | Acoustic trauma; equipment failure; TTS from prior shift | Urgent evaluation; review testing conditions; re-test after quiet period |
OSHA 1910.95(g)(7) allows baseline revision when: (1) an STS is confirmed on re-testing as persistent, or (2) a medical evaluation attributes a threshold change to a non-occupational cause. Baseline revision is optional — not required. Most audiologists recommend revising baselines only when the STS is clearly persistent and medically reviewed. Aggressive baseline revision can hide the progressive deterioration the audiometric program is designed to detect.
Program-level metrics to monitor: STS rate by department or area (disproportionate STS rate in one area points to a noise control problem); annual completion rate (percentage with audiogram within 12-month cycle); bilateral vs. unilateral STS ratio (high bilateral rates suggest systemic noise overexposure); year-over-year threshold trends at 4,000 Hz across the enrolled population (flat or improving = program working; progressive negative = it is not).
OSHA 1910.95(g)(10)(i) defines STS as an average shift of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear. Other tested frequencies (500, 1,000, 6,000 Hz) are not part of the STS determination.
No. OSHA Appendix F provides age correction values, but their application is optional. If applied, correction must be documented consistently, and it cannot be used for OSHA 300 log recordability determinations.
Classic NIHL produces a bilateral notch at 4,000 Hz — threshold elevation at that frequency with relatively better thresholds at lower frequencies. Over time, the notch deepens and widens toward speech frequencies (2,000 Hz and below).
Soundtrace calculates STS automatically, applies age correction consistently, flags patterns for audiologist review, and notifies employees within OSHA’s 21-day requirement.
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