Signs Your Audiometer Is Drifting Out of Calibration

An audiometer that is drifting out of calibration rarely announces itself with a dramatic failure. It produces thresholds that are a few decibels off, consistently, in one direction. A technician running the daily functional check catches most drift before it ever reaches a stored test result. A safety manager who skips that habit finds out the hard way, usually when a standard threshold shift gets disputed and the calibration paper trail does not hold up.

Foster Special Instruments runs audiometer calibration for hearing conservation programs across the country, and drift is the single most common reason a unit fails its annual acoustic check. Here is what the drift actually looks like, what OSHA's rule says about it, and when a few decibels of deviation becomes a unit that has to go back to the bench.

What does the daily functional check actually catch?

Under 29 CFR 1910.95(h)(5)(i), the functional operation of an audiometer must be checked before each day's use, by testing a person with known, stable hearing thresholds and by listening to confirm the output is free of distortion or unwanted sound. A deviation of 10 decibels or greater on that check requires an acoustic calibration before the unit is used again. This is the cheapest and fastest drift catch available, and it is also the one most likely to get skipped on a busy testing day. A tester who runs it consistently will notice a slow upward or downward creep in the known person's thresholds long before an employee's own results are affected.

What does a 10 dB deviation actually mean for a tested employee?

A 10 dB shift on the daily check is a flag to stop and recalibrate, not a diagnosis of a specific employee's hearing. But it tells you the instrument cannot be trusted to produce a valid baseline or annual audiogram that day. OSHA treats a standard threshold shift (STS) as an average 10 dB or greater change at 2000, 3000, and 4000 Hz from the baseline, calculated under 1910.95(g)(10)(i). If the audiometer itself is off by 10 dB when that comparison happens, the STS determination is built on bad data, and that is exactly the kind of gap an auditor or an employee's attorney will look for first.

What happens at the annual acoustic check?

At least once a year, the audiometer's calibration is checked acoustically under Appendix E of 1910.95, measuring actual sound pressure output against the tolerances in ANSI S3.6-1969. Appendix E allows deviations up to plus or minus 3 dB between 500 and 3000 Hz, 4 dB at 4000 Hz, and 5 dB at 6000 Hz before anything is flagged. Once a measured level deviates by more than that, an exhaustive calibration is advised, and once any frequency deviates by 15 dB or more, an exhaustive calibration is required before the unit returns to service. Appendix E's linearity check also verifies the output drops in matching 10 dB steps from 70 dB down to 10 dB at 1000 Hz, which is a separate check from the frequency tolerance table.

How often does the full exhaustive calibration have to happen?

Independent of what the daily or annual checks find, 1910.95(h)(5)(iii) requires an exhaustive calibration at least every two years. That interval runs regardless of whether the unit has been showing drift, because mechanical and electronic components age even on a unit that has never failed a functional or annual check. A unit that has already needed an unscheduled exhaustive calibration from drift still owes its two-year exhaustive calibration on the original clock, not a reset one.

Check	Frequency	Deviation that triggers action	Citation
Functional/listening check	Before each day's use	10 dB or greater, or audible distortion	1910.95(h)(5)(i)
Acoustic calibration check	At least annually	15 dB or greater at any frequency requires exhaustive calibration	1910.95(h)(5)(ii), Appendix E
Exhaustive calibration	At least every 2 years	Required regardless of prior drift findings	1910.95(h)(5)(iii)

What are the practical warning signs between scheduled checks?

Besides the daily check itself, a few patterns point to drift before the annual acoustic calibration catches it on paper. A known-threshold tester whose results creep in one direction over several weeks. Multiple employees showing an STS in the same direction and frequency pattern in the same testing cycle, with no obvious noise-exposure explanation. Unusual static, hum, or dropout audible through the headset during the listening portion of the daily check. Any of these is a reason to pull the unit and run the acoustic calibration check early rather than waiting for the annual date.

How Foster handles this

Foster's calibration service checks audiometers against the Appendix E tolerance table and ANSI S3.6-1969 on every annual visit, and performs exhaustive calibration on the two-year clock regardless of what the annual check shows. When a customer's own daily functional checks flag a 10 dB deviation between scheduled visits, we can get a loaner or expedited calibration slot booked rather than leaving a hearing conservation program running on an instrument nobody trusts. For employers tracking audiometer calibration dates alongside baseline windows, STS determinations, and BAT or respirator renewal dates on separate clocks, Foster's compliance workspace product keeps each requirement and due date in one view instead of a spreadsheet per program.

Related reading: see our note on what an exhaustive calibration under ANSI S3.6 actually tests. Foster also offers on-site hearing testing and BAT training for employers managing both programs; request a quote for calibration or testing service.

FAQ

Can a technician keep using an audiometer that just failed its daily functional check?

No. Under 1910.95(h)(5)(i), a deviation of 10 dB or greater on the daily functional check requires an acoustic calibration before the unit is used again that day.

Does a small deviation on the annual acoustic check always mean the unit needs service?

Not automatically. Appendix E allows deviations up to plus or minus 3 dB between 500 and 3000 Hz, 4 dB at 4000 Hz, and 5 dB at 6000 Hz. Beyond that, exhaustive calibration is advised, and at 15 dB or greater at any frequency it is required.

Does a unit that just had an exhaustive calibration still need its two-year exhaustive calibration on schedule?

Yes. 1910.95(h)(5)(iii) sets a minimum two-year interval for exhaustive calibration independent of any unscheduled calibration triggered by drift findings on the daily or annual checks.

What ANSI standard does OSHA reference for audiometer tolerances?

OSHA's rule references American Standard Specification for Audiometers, S3.6-1969, for the tolerances checked in the annual acoustic calibration under Appendix E.

Sources

• 29 CFR 1910.95 - Occupational noise exposure (OSHA)
• 29 CFR 1910.95 Appendix E - Acoustic calibration of audiometers (OSHA)
• Biological check - Acoustical Society of America standards glossary