What Is a Standard Threshold Shift? STS Math, Explained

What is a standard threshold shift?

A standard threshold shift (STS) is a change in hearing sensitivity, relative to an employee's baseline audiogram, averaging 10 decibels or more at 2000, 3000, and 4000 Hz in either ear. OSHA defines it in 29 CFR 1910.95 as the trigger for a required retest and, if confirmed, a set of follow-up actions — employee notification, hearing protector refitting or upgrading, and possible referral for further evaluation. The STS calculation compares only three frequencies, not the full audiogram, because 2000–4000 Hz is the range where occupational noise exposure does the most damage and where early loss is most detectable before it spreads to speech frequencies.

How is the STS average calculated?

Take the employee's current hearing threshold at 2000, 3000, and 4000 Hz in one ear, subtract the baseline threshold at those same three frequencies, and average the three differences. If that average is 10 dB or more, the ear has an STS. The calculation runs separately for each ear — a shift in one ear alone is enough to trigger the requirement, the two ears aren't averaged together. Age correction, when the employer elects to apply it under Appendix F, is subtracted from the current threshold at each frequency before the comparison, which can bring an apparent shift below the 10 dB line for older workers whose baseline was set years earlier.

Worked example: a shift that trips the threshold

Baseline thresholds for the right ear: 10 dB at 2000 Hz, 15 dB at 3000 Hz, 20 dB at 4000 Hz. Current-year thresholds: 15 dB, 25 dB, 35 dB. The differences are 5, 10, and 15 dB — averaging 10 dB exactly. That's a standard threshold shift, and it triggers a required retest, ideally the same day with the employee removed from noise for at least 14 hours beforehand, to rule out a temporary threshold shift from same-shift noise exposure rather than a permanent change.

Worked example: a shift that doesn't trip the threshold

Same baseline. Current-year thresholds: 10 dB, 20 dB, 25 dB. Differences: 0, 5, and 5 dB, averaging 3.3 dB. No STS — even though 4000 Hz alone moved 5 dB and might look concerning on a single-frequency review, the three-frequency average is what OSHA's rule tests, not any one point. This is why a technician reviewing an audiogram checks the average, not the worst frequency in isolation.

Frequency	Baseline	Current	Difference
2000 Hz	10 dB	15 dB	5 dB
3000 Hz	15 dB	25 dB	10 dB
4000 Hz	20 dB	35 dB	15 dB
Average			10 dB — STS

Why does age correction change the outcome?

Hearing sensitivity declines gradually with age even without noise exposure, and Appendix F to 1910.95 provides age-correction tables by sex and age at both baseline and current test to separate presbycusis (age-related loss) from occupational loss. Applying age correction subtracts the expected age-related decline at each frequency from the current threshold before running the STS math, which can move a borderline case below 10 dB. Age correction is optional for the employer to apply, but if it's used it has to be applied consistently across the workforce, not selectively to individual cases.

How does Foster confirm an STS before it becomes a retest headache?

Foster's audiometric software flags the three-frequency average automatically on every annual test against the stored baseline, so a shift doesn't depend on a technician doing the arithmetic by hand under time pressure during a busy on-site testing day. When the software flags an STS, the protocol on Foster's hearing testing visits is to retest that ear before the employee leaves, after a quiet period where practical, so a same-shift noise artifact doesn't get carried forward as a false permanent shift into the employee's record. Confirmed shifts get flagged in the report Foster returns to the employer with the specific frequencies and dB differences, not just a pass/fail flag, so whoever reviews the case — often the audiologist or physician serving as the program's professional reviewer — has the numbers needed to decide follow-up.

Frequently Asked Questions

Does one STS mean permanent hearing loss?

Not necessarily. An STS confirmed on retest triggers required follow-up steps, but repeat testing after a noise-free period can show the shift was temporary. A revised baseline is only warranted per OSHA's rules when the shift persists on a confirmation retest.

Can an STS occur in only one ear?

Yes. The 10 dB average is calculated separately per ear, and a shift in either ear alone meets the definition — the two ears are never averaged together.

Is a standard threshold shift the same as a recordable OSHA case?

Not automatically. An STS becomes a recordable case on the OSHA 300 Log only under specific conditions in 29 CFR 1904.10, covered in Foster's guide to STS recordkeeping.

What happens after a confirmed STS?

The employee has to be notified in writing within 21 days, fitted or refitted with adequate hearing protectors, trained on their use, and referred for further evaluation if the professional reviewer determines the hearing loss isn't work-related or a medical pathology is suspected.

Does the STS calculation use the same three frequencies for every employee?

Yes. 2000, 3000, and 4000 Hz are fixed by the OSHA rule for every STS calculation, regardless of which frequencies an audiometer tests beyond that range.

Foster runs the audiogram math and flags shifts on every visit — see on-site testing for how the annual program works, or request a quote to get a hearing conservation program started.