An audiogram is the core document at the center of every OSHA-required hearing conservation program — but most safety managers who receive them have never been taught how to read one. This guide explains every element of a standard industrial audiogram: what the axes mean, how to interpret the symbols, what normal and abnormal results look like, how to determine whether a Standard Threshold Shift has occurred, and what to do next.
An audiogram plots hearing thresholds on the y-axis against frequency in Hz on the x-axis. The lower the marks on the chart, the worse the hearing. The critical frequencies for OSHA STS calculations are 2000, 3000, and 4000 Hz. An average shift of 10 dB or more across those three frequencies in either ear triggers required employer action.
- What is an audiogram?
- Reading the axes
- Understanding the symbols
- What does a normal audiogram look like?
- Recognizing noise-induced hearing loss
- How to calculate a Standard Threshold Shift
- Age correction
- Comparing baseline and annual audiograms
- What to do when you see a shift
- Frequently asked questions
What is an audiogram?
An audiogram is a graph that displays the results of a pure-tone air conduction hearing test. The worker listens through headphones and responds each time they hear a tone; the softest level at which they can reliably detect each tone is their threshold at that frequency. Under OSHA 1910.95, industrial audiograms must test at minimum 500, 1000, 2000, 3000, 4000, and 6000 Hz for each ear separately.
Reading the axes
X-axis (frequency): Runs left to right from 250 Hz to 8000 Hz. Low numbers = low-pitched sounds; high numbers = high-pitched sounds. Human speech occurs mostly between 500 and 4000 Hz. The 4000 Hz range is where NIHL typically appears first. Y-axis (hearing level in dB HL): Runs top to bottom from 0 dB (top) to 110+ dB (bottom). Counterintuitively: marks near the top = good hearing; marks near the bottom = worse hearing.
Many non-audiologists instinctively think “higher on the chart = worse hearing.” It is the opposite. Threshold marks plotted near the top (0–20 dB HL) indicate excellent hearing sensitivity. Marks further down (50, 60, 70+ dB HL) indicate significant hearing loss.
Understanding the symbols
| Symbol | Ear | Test Type |
|---|---|---|
| O | Right ear | Air conduction (unmasked) — plotted in red or black |
| X | Left ear | Air conduction (unmasked) — plotted in blue or black |
For industrial OSHA audiograms, O (right ear) and X (left ear) are the critical symbols. Each symbol is plotted at the threshold level for that ear at each frequency, and connected by lines to create the audiogram curve.
What does a normal audiogram look like?
A normal audiogram shows all thresholds between 0 and 25 dB HL across all tested frequencies. The curve is relatively flat. Both ears typically show similar thresholds. Classification: 0–25 dB HL = normal; 26–40 dB = mild loss; 41–55 dB = moderate; 56–70 dB = moderately severe; 71–90 dB = severe; 91+ dB = profound.
Recognizing noise-induced hearing loss on an audiogram
NIHL has a distinctive pattern: a notch centered around 4000 Hz, with better thresholds at lower and higher frequencies. Features: good thresholds at 500 and 1000 Hz; sharp drop at 2000, 3000, and 4000 Hz; some recovery at 6000 or 8000 Hz (the “4 kHz notch”); often bilateral and symmetric; gradual worsening with continued noise exposure.
Not all high-frequency hearing loss is NIHL. Presbycusis also causes high-frequency loss but slopes gradually without the distinctive 4 kHz notch. A clinical audiologist is needed to formally differentiate the causes — safety managers should not make clinical diagnoses from audiogram data alone.
How to calculate a Standard Threshold Shift
OSHA 1910.95(g)(10) defines an STS as an average shift of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. Calculation: (1) pull baseline thresholds for affected ear at 2000/3000/4000 Hz; (2) pull current annual thresholds for same ear and frequencies; (3) calculate difference at each frequency; (4) average the three differences; (5) if ≥10 dB, STS confirmed in that ear.
| Frequency | Baseline (dB HL) | Current Annual (dB HL) | Shift (dB) |
|---|---|---|---|
| 2000 Hz | 15 | 20 | +5 |
| 3000 Hz | 20 | 35 | +15 |
| 4000 Hz | 25 | 40 | +15 |
| Average shift | +11.7 dB → STS confirmed | ||
Age correction
OSHA permits employers to apply age correction factors (OSHA Appendix F) to account for expected age-related hearing decline before determining whether a program-level STS has occurred. Age correction is optional, not required, and cannot be applied when determining OSHA 300 log recordability — for recordkeeping, the raw uncorrected shift is used.
Comparing baseline and annual audiograms
The baseline audiogram is established within 6 months of first noise exposure. Every annual audiogram is compared to the baseline — not to the prior year’s test. This means gradual drift over five years is calculated against the original baseline, making it harder to miss accumulating damage. When an STS is confirmed and determined persistent, OSHA permits the employer to revise the baseline — but only after clinical review.
What to do when you see a shift
When an STS is identified: notify the affected employee in writing within 21 days; if not already using hearing protection, fit and require immediately; if already using HPD, evaluate and upgrade; consider audiological evaluation referral; determine OSHA 300 log recordability using uncorrected thresholds; consider re-testing within 30 days to confirm (a temporary shift can falsely indicate STS); consider baseline revision after professional audiological review.
Frequently Asked Questions
OSHA requires audiometric test records to be reviewed by a licensed audiologist, otolaryngologist, or physician, or by an audiometric technician working under such supervision. Safety managers can learn to read audiograms and identify potential STS patterns, but the clinical determination requires qualified professional review.
A TTS is a short-term worsening of hearing thresholds caused by recent loud noise exposure, typically resolving within 12–16 hours. If tested shortly after noise exposure, an employee’s audiogram may show elevated thresholds that do not reflect true permanent change. OSHA recommends a quiet period of at least 14 hours before baseline audiograms.
Yes. Modern audiometric software including Soundtrace automatically flags potential STS cases by comparing each annual result to the stored baseline and calculating the average shift at 2000, 3000, and 4000 Hz. This eliminates manual calculation errors and ensures no STS goes undetected. The software flags the case for professional review rather than making the final determination autonomously.
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