Comparing Baseline and Annual Audiograms: OSHA Compliance Guide

Baseline and annual audiograms serve different regulatory functions under OSHA 1910.95 — confusing them is one of the most common hearing conservation compliance errors. The baseline establishes the reference point. The annual audiogram is the measurement. A Standard Threshold Shift only exists in relation to a valid baseline — which means a defective baseline invalidates years of subsequent comparisons.

Soundtrace audiometric testing automates baseline and annual scheduling, STS calculations, and required follow-up notifications as part of a complete OSHA 1910.95 hearing conservation program.

How to Read an Audiogram

An audiogram plots hearing thresholds in decibels hearing level (dBHL) against frequency in Hz. Two conventions are critical for reading it correctly:

• Y-axis is inverted — lower numbers (better hearing) appear at the top. A threshold of 0 dBHL at the top means excellent hearing; 80 dBHL near the bottom means severe loss. This is counterintuitive but standard audiometric convention.
• Frequencies run left to right from low to high — 250 Hz (low pitch) on the left, 8000 Hz (high pitch) on the right. Noise-induced hearing loss typically appears first at the high frequencies, particularly 4000 Hz.

For occupational compliance under 1910.95, the key frequencies for STS calculation are 2000, 3000, and 4000 Hz. A 10 dBHL average shift at those three frequencies in either ear — compared to the baseline audiogram — constitutes a Standard Threshold Shift.

NIHL Patterns on an Audiogram

Noise-induced hearing loss has a characteristic audiogram signature. Unlike age-related hearing loss, which affects high and low frequencies more evenly, NIHL first appears as a notch at 4000 Hz — thresholds worsen specifically at that frequency before affecting adjacent frequencies. As exposure continues, the notch deepens and spreads toward 3000 Hz and 6000 Hz. The interactive audiogram below shows what each stage looks like:

What a Baseline Audiogram Is

The baseline audiogram under OSHA 1910.95(g)(5) establishes the starting hearing threshold for an enrolled employee. A valid baseline requires: administration within 6 months of first exposure at or above 85 dBA TWA; 14 hours of noise-free time before the test; administration by qualified personnel under 1910.95(g)(3); and audiometer calibration within ANSI S3.6 specifications.

▶ Bottom line: An invalid baseline orphans every subsequent annual audiogram. Every STS determination, workers’ compensation apportionment, and causation defense depends on a valid baseline taken before significant occupational exposure has occurred.

What an Annual Audiogram Is

Annual audiograms under 1910.95(g)(6) must be provided to every enrolled employee within 12 months of the last audiogram. Unlike the baseline, annual audiograms do not require a preceding quiet period — though it is recommended, particularly for workers with high noise exposure. Results must be compared to the baseline by a qualified audiologist or physician within 30 days (21 days if an STS is found).

▶ Bottom line: The annual audiogram has no compliance value unless compared to a valid baseline. Annual audiograms without that comparison satisfy the scheduling requirement but not the monitoring purpose of 1910.95.

STS: Watching a Shift Develop Year by Year

The interactive chart below shows a right-ear audiogram for a worker exposed at 93 dBA TWA. Step through years 1 through 3 to see how the 4 kHz notch deepens and the STS develops. The dashed green line is the baseline; the solid orange line is the annual audiogram being compared to it.

Side-by-Side Comparison

The 14-Hour Quiet Period Rule

Under 1910.95(g)(5)(ii), the employer shall not permit a baseline audiogram to be used as a valid reference unless the employee was not exposed to workplace noise at or above 85 dBA TWA for at least 14 hours preceding the test. This protects against temporary threshold shift (TTS) contaminating the baseline. If TTS is present during baseline testing, the resulting reference threshold is artificially elevated — future STS detection becomes harder and genuine deterioration is masked.

This requirement is one of the strongest arguments for in-house audiometric testing over annual mobile van visits, which have no practical mechanism for verifying quiet period compliance on a per-worker basis.

When a Revised Baseline Is Required

Under 1910.95(g)(9), a revised baseline must be substituted when: an STS is confirmed as a permanent threshold shift; an audiologist or physician determines the original baseline is not an accurate representation of the worker’s non-occupational baseline; or improved thresholds occur in subsequent testing. The original baseline must be retained in the employee’s record even after revision. See: Audiometric Recordkeeping: Digital vs. Paper.

Age Correction in STS Determination

OSHA permits employers to age-correct threshold data before determining whether an STS has occurred, using the correction tables in Appendix F. When applied, some apparent STSs are reduced because the shift is consistent with expected age-related decline rather than noise-induced hearing loss. Age correction is optional, not required, and must use the OSHA Appendix F tables with documented calculations retained in the employee file.

Documentation Requirements

Under OSHA 1910.95(m)(3): all audiograms must be retained for the duration of employment; calibration records for each test day must be retained for two years; STS determinations, age-correction calculations, and revised baseline justifications must be in writing and retained; employee notification letters for STS must be retained. Most occupational health attorneys recommend retaining all audiometric records for 30 years beyond employment termination given the long latency of occupational hearing loss claims.

Frequently Asked Questions