OSHA 29 CFR 1910.95 Appendix I — Definitions — Appendix I to OSHA 1910.95 provides definitions for key terms used throughout the hearing conservation standard. Understanding these definitions precisely — not in colloquial terms — is essential for correctly applying the standard's requirements. This plain-language guide explains what Appendix I requires, what it means in practice for EHS managers, and how it connects to the broader OSHA 1910.95 hearing conservation program requirements. According to CDC/NIOSH, approximately 22 million U.S. workers are exposed to hazardous occupational noise annually.
Soundtrace delivers audiometric testing and noise monitoring that meets the technical requirements of all 1910.95 appendices — including ANSI S3.1 ambient noise validation per audiogram and ANSI S3.6 audiometer specifications.
Action Level
Definition: An 8-hour time-weighted average of 85 decibels measured on the A-scale, slow response, or equivalently a dose of fifty percent.
Plain language: The noise exposure threshold that triggers the full OSHA 1910.95 hearing conservation program. Not the PEL. The program starts at 85 dBA TWA, not 90 dBA. This is the most commonly misunderstood definition in the standard — many employers incorrectly believe the program only applies above the 90 dBA PEL.
Criterion Sound Level
Definition: A sound level of 90 decibels.
Plain language: The reference level used to calculate noise dose. A worker exposed to exactly 90 dBA for 8 hours accumulates a 100% dose. The criterion level is the denominator in the permissible exposure time calculations.
Decibel (dB)
Definition: Unit of measurement of sound level.
Plain language: Sound pressure levels are measured on a logarithmic scale. A 3 dB increase represents a doubling of acoustic energy (NIOSH); a 5 dB increase doubles the permissible duration under OSHA's exchange rate. A 90 dBA workplace is not twice as loud as 45 dBA — it is many orders of magnitude more intense.
Hertz (Hz)
Definition: Unit of measurement of frequency, numerically equal to cycles per second.
Plain language: The pitch of a sound. Low frequencies (125–500 Hz) are bass sounds; high frequencies (2000–8000 Hz) are treble sounds. Occupational NIHL primarily damages the 4000 Hz range — the frequency most vulnerable to mechanical damage in the cochlea.
Noise Dose
Definition: The ratio, expressed as a percentage, of (1) the time actually spent at a given noise level to (2) the time permitted at that level.
Plain language: A percentage of the permissible noise exposure. 100% dose = one full PEL. 50% dose = the action level. Doses above 100% trigger mandatory engineering controls; doses above 50% trigger the full HCP. See: Appendix A noise exposure computation guide.
Representative Exposure
Definition: Measurement of an employee's noise dose or 8-hour time-weighted average sound level that the employer reasonably believes, based on information available to the employer, is representative of the exposures of other employees in the workplace.
Plain language: OSHA allows monitoring a sample of workers rather than every individual, as long as the sample accurately characterizes all workers' exposures. The employer's judgment about representativeness must be reasonable and defensible.
Time-Weighted Average Sound Level (TWA)
Definition: That sound level, which if constant over an 8-hour exposure, would result in the same noise dose as is measured.
Plain language: The equivalent steady-state noise level that would produce the same accumulated exposure as the actual varying noise levels the worker experienced. A worker who spends 2 hours at 95 dBA and 6 hours at 85 dBA does not have a simple average TWA of 90 dBA — the TWA is calculated using the dose formula in Appendix A.
Professional Supervisor
OSHA 1910.95(g)(3) reference: OSHA requires audiometric testing programs to be supervised by a licensed or certified audiologist, otolaryngologist, or other physician.
Plain language: The licensed professional responsible for reviewing all audiograms, making STS determinations, deciding medical referrals, and overseeing the clinical quality of the audiometric testing program. EHS managers are not qualified to serve as PS; technicians may administer tests but not supervise the program clinically. See: audiometric testing for employers: complete guide.
OSHA 1910.95 compliant audiometric testing — every appendix requirement met
Soundtrace delivers audiometric testing that meets ANSI S3.1 test environment requirements (Appendix D), uses ANSI S3.6 calibrated audiometers, and is supervised by a licensed audiologist — fully compliant with every 1910.95 appendix requirement.
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