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Every employer running an OSHA hearing conservation program receives audiogram results for each enrolled worker. But the printout — a grid of numbers showing hearing thresholds at each tested frequency — is opaque to most safety managers and HR professionals who aren’t clinically trained. Understanding what those numbers mean, why the 4000 Hz column matters most, how to recognize the classic noise-induced hearing loss pattern, and when a result requires action is foundational to running a defensible HCP. You don’t need to be an audiologist to read an audiogram — but you do need to understand what you’re looking at.
Soundtrace’s cloud portal presents each worker’s audiometric history in a visual format with STS flags, trend indicators, and Professional Supervisor review notes — so safety managers see what matters without needing to interpret raw threshold tables.
OSHA’s hearing conservation program requires the employer to respond to STS findings — notify workers, refit hearing protection, arrange referrals. A safety manager who cannot look at an audiogram result and understand whether it warrants action is dependent on a vendor notification that may arrive weeks late. Understanding the basics of audiogram interpretation gives employers the ability to act promptly, track trends, and have meaningful conversations with their audiometric testing provider.
An audiogram is a graph with two axes. The horizontal x-axis represents test frequency in hertz (Hz), running from low frequencies on the left (250 Hz, approximately the frequency of a bass guitar) to high frequencies on the right (8000 Hz, the upper range of speech consonants). The OSHA-required frequencies for occupational audiometry are 500, 1000, 2000, 3000, 4000, and 6000 Hz.
The vertical y-axis represents hearing level in decibels HL (dB HL). Critically, 0 dB HL is at the top of the graph, not the bottom. Higher numbers — shown further down the page — indicate worse hearing. A threshold of 0–20 dB HL at a given frequency indicates normal hearing at that frequency. Thresholds of 25–40 dB HL indicate mild hearing loss; 40–55 dB HL moderate loss; above 70 dB HL severe to profound loss.
Audiometric zero (0 dB HL) represents the average hearing threshold of normal-hearing young adults — it is not silence. A worker with a threshold of 20 dB HL at 1000 Hz needs a tone 20 dB louder than the average normal-hearing young adult to hear it at that frequency. A threshold of 0 dB HL means the worker’s hearing at that frequency matches the average young adult reference.
The 25 dB above audiometric zero threshold in the OSHA 300 Log recordability test (1904.10) refers to this scale: a worker whose average threshold at 2000, 3000, and 4000 Hz is 25 dB HL or higher — not just 25 dB above their own prior baseline, but 25 dB above the audiometric zero reference — meets the hearing threshold level criterion for 300 Log recordability.
Noise-induced hearing loss has a characteristic audiometric signature: a notch at 4000 Hz with relative preservation of lower frequencies. The cochlea’s outer hair cells in the 4000 Hz region are the most metabolically active and the most vulnerable to noise-induced oxidative stress. A worker whose audiogram shows thresholds of 10–15 dB HL at 500, 1000, and 2000 Hz, but 35–40 dB HL at 4000 Hz, has a classic 4 kHz notch suggesting noise-induced damage.
The notch typically deepens over years of exposure and eventually spreads to adjacent frequencies (3000 Hz and 6000 Hz) as damage progresses. Early NIHL is often invisible to the worker — they can still understand speech, which is concentrated at 500–2000 Hz — but the 4 kHz notch is already showing on their audiogram. This is precisely why annual audiometric monitoring catches NIHL before the worker notices it.
An STS is calculated by comparing the worker’s current annual audiogram to their baseline audiogram. For each ear, subtract the baseline threshold from the current threshold at 2000, 3000, and 4000 Hz. Average those three differences. If the average is 10 dB or more in either ear, an STS has occurred.
| Frequency | Baseline (dB HL) | Annual (dB HL) | Shift |
|---|---|---|---|
| 2000 Hz | 10 | 15 | +5 dB |
| 3000 Hz | 10 | 20 | +10 dB |
| 4000 Hz | 15 | 30 | +15 dB |
| Average shift | — | — | +10 dB = STS |
An STS — average shift of 10 dB at 2000/3000/4000 Hz — triggers the full response chain: optional retest within 30 days, written notification within 21 days of determination, HPD refitting, retraining, and possible 300 Log entry. A threshold elevation that does not meet the STS definition does not trigger mandatory follow-up, but should still be tracked as a trend. A worker showing 8 dB average shift this year and 6 dB last year is progressing toward an STS and warrants program attention before the threshold is crossed.
Soundtrace’s portal presents each worker’s audiometric history visually — with STS flags, trend indicators, and Professional Supervisor review notes — so safety managers see what matters without interpreting raw threshold tables.
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