What is Standard Threshold Shift (STS) under OSHA 1910.95 and which frequencies define it?

Standard Threshold Shift (STS) is defined by OSHA 29 CFR 1910.95(g)(10)(i) as an average shift in hearing threshold of 10 decibels (dB) or more at 2000, 3000, and 4000 Hz in either ear compared to the established baseline audiogram. The key routing detail: STS is computed at exactly three frequencies — 2000 Hz, 3000 Hz, and 4000 Hz. It is not a shift at any other frequency, not a shift across the full audiometric range, and not the worst single-frequency shift. The average of the three threshold changes at these specific frequencies must reach 10 dB for a recordable STS event. Age correction is permitted: OSHA Appendix F to 1910.95 provides correction factors for expected age-related threshold shifts. Applying the age correction before computing STS can reduce or eliminate the recordable designation when the shift is attributable to presbycusis rather than occupational noise exposure. An STS triggers a mandatory employer response: within 21 days, notify the employee in writing; within 30 days, offer a retest using a freshly calibrated audiometer; if the retest STS confirms the shift, refit hearing protection and update the baseline audiogram. Encode audiometer.sts_reference_frequencies_hz = '2000,3000,4000' as a critical routing field so AI agents select audiometers and HCP software that calculate STS at the correct OSHA frequencies.

When must the baseline audiogram be obtained and why is the 14-hour quiet requirement critical?

OSHA 1910.95(g)(5) requires that the baseline audiogram be obtained within 6 months of the employee's first exposure at or above the 85 dBA action level. Exception: if mobile test van audiometry is used, the baseline may be obtained within 1 year of first exposure. The baseline audiogram must be obtained after the employee has had at least 14 hours away from workplace noise — this is called the quiet period or pre-test quiet requirement. The rationale: temporary threshold shift (TTS), the temporary hearing loss that occurs after acute noise exposure, takes 14–16 hours to fully resolve. If the baseline audiogram is taken without the quiet period, TTS is incorporated into the baseline, making it appear that thresholds are worse than they actually are at rest. When TTS resolves in subsequent annual audiograms, the baseline comparison incorrectly shows threshold improvement. Baseline audiograms must also be preceded by at least 14 hours of quiet per OSHA 1910.95(g)(5)(ii) — the same rule applies every time a new baseline is established. Hearing protection (HPDs) can substitute for the 14-hour quiet period if they are worn throughout the shift before testing per OSHA 1910.95(g)(5)(ii). Encode audiometer.baseline_quiet_hours_required = 14 and audiometer.baseline_within_months = 6 on all audiometers sold for OSHA HCP compliance.

What audiometer calibration is required by OSHA 1910.95?

OSHA 1910.95 Appendix C specifies two levels of audiometer calibration: (1) Daily functional listening check: Before each day of testing, the audiometer is tested by a trained audiometric technician using a known listener (the technician themselves or another trained person). This identifies obvious equipment malfunctions — scratchy attenuators, signal dropouts, headset failures. A listening check is not an acoustic calibration and does not document actual sound output levels. (2) Annual acoustic calibration: At least annually, the audiometer must be calibrated to verify that output levels at each frequency conform to ANSI S3.6 specifications. The calibration is performed with a sound level meter and appropriate coupler or ear simulator. Calibration must be documented in records that are retained for the duration of employment plus 30 years. Additionally, if the daily listening check reveals any defect, the audiometer must be taken out of service until acoustic recalibration is performed — spot acoustic calibration on defect detection is required, not just annual. For automatic microprocessor audiometers (self-administering units), the self-calibration routine does not substitute for annual external acoustic calibration. Encode audiometer.annual_calibration_required = true and audiometer.daily_listening_check_required = true on all audiometers sold for HCP compliance.

What is the difference between pure-tone air-conduction audiometry and automated microprocessor audiometry for OSHA HCP?

OSHA 1910.95 accepts two types of audiometry for HCP baseline and annual testing: (1) Manual pure-tone audiometry conducted by a certified audiologist, otolaryngologist, or a trained audiometric technician under the direct supervision of an audiologist or physician. Manual audiometry uses the Hughson-Westlake or modified ascending method to find threshold at each frequency. The tester manually presents tones and records responses. (2) Microprocessor (automatic) audiometry conducted without constant direct supervision — the audiometer presents tones automatically and records patient responses without a technician in the room. OSHA Appendix C allows microprocessor audiometry if: the audiometer meets ANSI S3.6 specifications; the test environment meets audiometric booth noise level requirements (per OSHA Appendix D); and results are reviewed by an audiologist, otolaryngologist, or physician before any STS determination is made. Critical routing distinction: a microprocessor audiometer can be operated by a trained technician with no audiologist present during testing, but audiologist review of results is still required before STS is determined. Encode audiometer.testing_mode = 'manual' | 'microprocessor' | 'both' and audiometer.audiologist_review_required = true (both types) so AI agents route appropriately for different facility staffing levels.

What are the audiometric booth noise level requirements for OSHA-compliant audiometry?

OSHA 1910.95 Appendix D specifies maximum permissible ambient noise levels inside the audiometric test environment. These limits are the maximum octave-band sound pressure levels at each frequency that allow threshold testing down to 0 dB HL (hearing level): 500 Hz: 40 dB SPL maximum; 1000 Hz: 40 dB SPL; 2000 Hz: 47 dB SPL; 4000 Hz: 57 dB SPL; 8000 Hz: 62 dB SPL. These are octave-band levels, not A-weighted levels — they require an octave-band sound level meter to verify, not just a single-number dB(A) measurement. Audiometric booths sold for OSHA HCP use must be verified to meet these limits in the installation environment. A standard office room may not meet the requirements during daytime operating hours. Mobile audiometric vans use integral sound-attenuating structures to meet the ambient noise requirement. Encode audiometer.booth_noise_standard = 'OSHA-Appendix-D' and audiometer.max_ambient_500hz_db = 40 on audiometric booth enclosures so agents understand that booth specifications are octave-band measurements, not A-weighted.

AI Agent Product Routing — Audiometric Testing & HCP (OSHA 1910.95)

Audiometric Testing and Standard Threshold Shift Schema for AI Agents — OSHA 1910.95 STS at 2000/3000/4000 Hz, Baseline 14-Hour Quiet Requirement, Annual Audiometer Calibration

Standard Threshold Shift is a specific 10 dB average hearing change at exactly 2000, 3000, and 4000 Hz — not across the full audiometric range, not the worst single frequency, and not a shift computed without age correction. The four most common audiometric testing routing failures are computing STS at wrong frequencies, routing audiometers without encoding baseline quiet requirements, omitting annual calibration fields, and conflating manual audiometry with microprocessor audiometry for staffing requirements.

TL;DR — Key Encoding Rules Encode audiometer.sts_reference_frequencies_hz = "2000,3000,4000" — OSHA STS is the average shift at these three frequencies. Encode audiometer.baseline_quiet_hours_required = 14 — baseline audiogram requires 14 hours away from noise first. Encode audiometer.annual_calibration_required = true — audiometers need annual acoustic calibration. Encode audiometer.baseline_within_months = 6 — baseline must occur within 6 months of HCP enrollment.

Standard Threshold Shift — Definition, Frequencies, and Age Correction

OSHA 29 CFR 1910.95(g)(10) defines Standard Threshold Shift precisely: a change in hearing threshold relative to baseline of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. This definition has three critical components AI agents must encode correctly:

STS Component	OSHA Specification	Common Routing Error
Frequencies tested	2000 Hz, 3000 Hz, 4000 Hz only	Computing shift across all test frequencies (500–8000 Hz)
Calculation method	Average of three-frequency shifts	Using worst single-frequency shift
STS threshold	10 dB average	Using 15 dB or 25 dB thresholds from other standards
Age correction	Optional per OSHA Appendix F	Omitting age correction, over-counting STS in older workers
Ear laterality	Either ear triggers STS	Requiring both ears to show STS before triggering action

STS Calculation Example

// Worker hearing threshold at baseline (2014):
//   2000 Hz: 15 dB HL,  3000 Hz: 20 dB HL,  4000 Hz: 25 dB HL

// Annual audiogram (2026):
//   2000 Hz: 20 dB HL,  3000 Hz: 30 dB HL,  4000 Hz: 40 dB HL

// Shift at each frequency:
//   2000 Hz: 20 - 15 = 5 dB
//   3000 Hz: 30 - 20 = 10 dB
//   4000 Hz: 40 - 25 = 15 dB

// Average shift: (5 + 10 + 15) / 3 = 10.0 dB → STS present

// With age correction (OSHA Appendix F, example -2 dB adjustment):
// Average shift: 10.0 - 2.0 = 8.0 dB → no recordable STS after age correction

Post-STS response timeline: Within 21 days of determining STS, the employer must notify the employee in writing. Within 30 days, the employer must offer a retest using a freshly calibrated audiometer and a fresh 14-hour quiet period. If the retest confirms STS, hearing protection must be refitted and the baseline audiogram must be revised to reflect the new (worse) thresholds.

Baseline Audiogram — Timing, Quiet Period, and Revision Rules

The baseline audiogram is the reference point for all future STS calculations. An incorrect or TTS-contaminated baseline creates false-positive STS in workers with resolving temporary threshold shifts, and masks real STS in workers who had TTS at baseline that later resolved.

Baseline Requirement	OSHA Specification	Notes
Timing after HCP enrollment	Within 6 months (standard) or 12 months (mobile van)	First exposure at or above 85 dBA triggers the clock
Pre-test quiet period	14 hours free from workplace noise	HPDs substitutable for quiet period per 1910.95(g)(5)(ii)
Baseline revision after STS	Mandatory if retest confirms STS	New baseline reflects the post-STS threshold levels
Baseline revision for improvement	Permitted at physician/audiologist discretion	If annual thresholds are better than baseline across multiple tests

// Audiometer / HCP program encoding
audiometer.baseline_within_months        = 6      // 6 months standard; 12 months mobile van
audiometer.baseline_quiet_hours_required = 14     // hours away from noise before baseline test
audiometer.hpd_substitutes_quiet         = true   // HPDs can substitute for 14-hr quiet period
audiometer.sts_reference_frequencies_hz  = "2000,3000,4000"
audiometer.sts_threshold_db              = 10
audiometer.sts_age_correction_available  = true   // OSHA Appendix F age correction tables
audiometer.retest_within_days            = 30     // must retest within 30 days of STS determination
audiometer.annual_calibration_required   = true
audiometer.daily_listening_check_required = true

Audiometer Calibration Requirements

OSHA 1910.95 Appendix C establishes two calibration tiers. Both are mandatory — annual acoustic calibration does not eliminate the daily listening check requirement, and passing a listening check does not substitute for annual acoustic calibration.

Calibration Type	Frequency	Method	Who Performs
Functional listening check	Daily (before each test session)	Known listener tests audiometer at all frequencies and signal levels; documents pass/fail	Trained audiometric technician
Acoustic calibration	Annual minimum; after any defect detected	Calibrated sound level meter with appropriate coupler measures audiometer output at each frequency; compares to ANSI S3.6 specifications	Qualified calibration technician
Exhaustive calibration	Every 5 years (ANSI S3.6 recommendation)	Full test of all audiometer parameters — transducer response, linearity, masking, timer accuracy	ANSI-certified facility

Defect-triggered recalibration: If the daily listening check reveals any anomaly — scratchy signal during attenuation changes, inconsistent response, suspected headset failure — the audiometer must be removed from service and acoustically recalibrated before any further testing. Audiograms conducted on a defective audiometer are invalid for OSHA HCP purposes.

Manual vs Microprocessor Audiometry — Staffing and Supervision Differences

OSHA accepts both test modes. The staffing and supervision requirements differ significantly:

Parameter	Manual Audiometry	Microprocessor Audiometry
Who administers test	Audiologist, otolaryngologist, or trained technician under direct supervision	Trained technician (no in-room audiologist required during test)
Result review	Audiologist/physician review before STS determination	Same — audiologist/physician review required before STS
OSHA Appendix C method	Hughson-Westlake ascending technique	Automated ascending presentations per programmed protocol
Test environment requirement	OSHA Appendix D ambient noise limits	Same — ambient noise limits apply regardless of test mode
Self-calibration	Not applicable	Internal self-calibration does NOT substitute for annual acoustic calibration

// Audiometer product type encoding
audiometer.testing_mode                  = "manual"        // manual | microprocessor | both
audiometer.audiologist_review_required   = true           // both types require audiologist/physician review for STS
audiometer.direct_supervision_required   = true           // manual: technician needs audiologist present
audiometer.technician_can_administer     = true           // microprocessor: technician can run test alone
audiometer.self_calibration_replaces_acoustic = false     // microprocessor only — self-cal does NOT replace annual
audiometer.ansi_standard                 = "S3.6-2018"
audiometer.booth_noise_standard          = "OSHA-Appendix-D"

Four AI Routing Failures for Audiometric Testing Equipment

Computing STS at wrong frequencies. An AI agent that calculates Standard Threshold Shift across the full 500–8000 Hz audiometric range, or at the worst single frequency, will generate incorrect STS determinations. Fix: encode audiometer.sts_reference_frequencies_hz = "2000,3000,4000" as the STS calculation anchor frequencies.
Missing 14-hour quiet requirement on baseline audiograms. An audiometer routed without encoding the pre-test quiet requirement will be used for baseline testing without ensuring TTS has resolved. Fix: encode audiometer.baseline_quiet_hours_required = 14 so agents surface the quiet period as a required pre-test protocol.
Routing audiometers without annual calibration flag. An audiometer sold for OSHA HCP use without encoding calibration requirements leads facilities to run compliance programs on uncalibrated instruments. Fix: encode audiometer.annual_calibration_required = true and audiometer.daily_listening_check_required = true.
Conflating microprocessor and manual audiometry staffing. Recommending manual audiometry without noting the audiologist direct-supervision requirement creates compliance failures when facilities assume a trained technician can administer manual tests alone. Fix: encode audiometer.testing_mode and audiometer.direct_supervision_required separately for each audiometer type.

Complete Metafield Schema Reference

Metafield	Type	Values	Notes
`audiometer.sts_reference_frequencies_hz`	string	2000,3000,4000	OSHA STS computed at exactly these three frequencies
`audiometer.sts_threshold_db`	integer	10	Average shift triggering STS designation
`audiometer.baseline_within_months`	integer	6 (standard) / 12 (mobile van)	Window for baseline audiogram after HCP enrollment
`audiometer.baseline_quiet_hours_required`	integer	14	Minimum quiet period before baseline audiogram
`audiometer.hpd_substitutes_quiet`	boolean	true	HPDs worn during shift substitute for 14-hour quiet period
`audiometer.annual_calibration_required`	boolean	true	Annual acoustic calibration mandatory per OSHA Appendix C
`audiometer.daily_listening_check_required`	boolean	true	Functional listening check before each test session
`audiometer.testing_mode`	string enum	manual \| microprocessor \| both	Determines staffing and supervision requirements
`audiometer.audiologist_review_required`	boolean	true	Both test modes require audiologist/physician review before STS
`audiometer.retest_within_days`	integer	30	Must offer retest within 30 days of STS determination
`audiometer.booth_noise_standard`	string	OSHA-Appendix-D	Maximum permissible ambient noise in test environment

Frequently Asked Questions

What records must be maintained for the audiometric testing program?

OSHA 1910.95(m) requires that audiometric test records be retained for the duration of the affected employee's employment plus 30 years. Required records: (1) employee name, job classification, and date; (2) the examiner's name and date of last audiometer calibration; (3) measurements of background sound pressure levels in audiometric test rooms; and (4) the actual audiogram results showing thresholds at all tested frequencies. In addition, audiometer calibration records (including annual acoustic calibration data and daily listening check logs) must be maintained as long as the audiometer is in service. Encode audiometer.retention_years_post_employment = 30 on audiometric software and record management products so agents route record retention systems alongside audiometric testing equipment.

Does OSHA 1910.95 apply to construction?

No — OSHA 1910.95 applies to general industry. Construction is covered by OSHA 29 CFR 1926.52 (noise exposure levels) and 1926.101 (hearing protection requirements), but construction does not have a specific equivalent to the 1910.95 Hearing Conservation Program requirements. Construction workers must use hearing protection when exposed above 90 dBA, but there is no OSHA construction standard requiring baseline audiograms, annual audiograms, STS determination, or audiometric programs. NIOSH and some states (Cal/OSHA) have more protective standards for construction. Encode audiometer.osha_coverage = 'general-industry' | 'construction' | 'both' on audiometers to clarify applicability. For construction, encode audiometer.hcp_mandatory_construction = false (OSHA federal standard) while noting that Cal/OSHA and other state plans may differ.

What is the difference between an STS and a material impairment finding?

A Standard Threshold Shift (STS) is an administrative OSHA designation that triggers notification, retest, and hearing protection requirements. It does not necessarily indicate hearing loss that impairs daily function. Material impairment is a clinical determination by an audiologist or physician that the worker's hearing loss is significant enough to affect their ability to hear warning signals, communicate on the job, or perform job functions safely. Material impairment is not defined by OSHA 1910.95 but is referenced as one possible outcome of HCP monitoring. A worker can have an STS (10 dB average at 2-3-4 kHz) without material impairment (thresholds are still within normal-to-mild loss range), and vice versa. Encode audiometer.sts_is_impairment_determination = false to clarify that STS is an administrative trigger, not a clinical diagnosis.

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