A noise dosimeter is the instrument OSHA 29 CFR 1910.95 requires for measuring individual worker noise exposure. Unlike a sound level meter that captures noise at a fixed point, a personal noise dosimeter is worn by the worker throughout a full shift, integrating cumulative dose across every task and location. The result — an 8-hour time-weighted average (TWA) — is the data employers need to determine which workers must be enrolled in the hearing conservation program and which are below the 85 dBA action level. According to the CDC, 22 million U.S. workers are exposed to hazardous occupational noise annually — and most of them work in environments where personal dosimetry is the only way to know their actual exposure.
Soundtrace integrates personal noise dosimetry with real-time facility monitoring in a single platform — automatically identifying workers at or above 85 dBA and triggering HCP enrollment and audiometric testing scheduling.
- What is a noise dosimeter?
- How a noise dosimeter works
- OSHA-required dosimeter settings
- Personal vs. wearable noise dosimeters
- Noise dosimeter vs. sound level meter
- Calibration requirements
- Noise dosimeter rental vs. owned equipment
- Using dosimeter results for OSHA compliance
- Frequently asked questions
What Is a Noise Dosimeter?
A noise dosimeter is a small, body-worn instrument that measures an individual worker's cumulative noise exposure over time. It uses a microphone to sample sound pressure levels continuously, applies the appropriate frequency weighting and time integration, and calculates the 8-hour TWA and noise dose percentage against the OSHA criterion level.
The key distinction from other noise measurement instruments is that a dosimeter measures personal exposure — the actual noise reaching the worker's ear — rather than the noise level at a fixed point in the facility. A worker who spends part of their shift in a quiet office and part on a loud production floor will have a personal TWA that neither a fixed measurement nor a spot-check with a sound level meter would accurately capture.
How a Noise Dosimeter Works
The dosimeter microphone is clipped to the worker's clothing within 30 cm of the ear — typically on the collar or shoulder epaulette. Throughout the shift, the instrument continuously samples sound pressure, applies A-frequency weighting (which approximates human hearing sensitivity), and accumulates noise dose using the configured exchange rate and criterion level.
At the end of the measurement period, the dosimeter calculates:
- Noise dose (%): The ratio of actual exposure time to permissible exposure time at the measured level, expressed as a percentage. A dose of 100% equals one full PEL.
- 8-hour TWA: The equivalent steady-state noise level that would produce the same cumulative dose over an 8-hour shift.
- Peak level: The highest instantaneous sound pressure recorded during the session.
Modern dosimeters also store time-history data — showing noise levels at each interval throughout the shift — which is useful for identifying specific tasks, machines, or locations that drive the majority of the noise dose.
OSHA-Required Dosimeter Settings
The instrument settings determine whether dosimeter data is legally valid for OSHA compliance purposes. A dosimeter configured with NIOSH settings produces a different TWA than one configured to OSHA settings for the same exposure — and only OSHA-configured data can be used to make HCP enrollment determinations under 1910.95.
| Parameter | OSHA Setting | NIOSH Setting | Why It Matters |
|---|---|---|---|
| Instrument standard | ANSI S1.25, Type 2 or better | ANSI S1.25, Type 2 or better | Accuracy requirement for legal defensibility |
| Frequency weighting | A-weighting (dBA) | A-weighting (dBA) | Approximates human hearing sensitivity |
| Time response | Slow (1-second averaging) | Slow | Required for continuous and fluctuating noise |
| Exchange rate | 5 dB | 3 dB | Key difference — affects every TWA calculation above criterion |
| Criterion level | 90 dBA | 85 dBA | Key difference — reference level for 100% dose |
| Integration threshold | 80 dBA | 80 dBA | Minimum level included in dose accumulation |
Verify and document instrument settings before every monitoring session. A dosimeter left on NIOSH settings from a previous survey will produce results that overstate dose relative to OSHA's standard — potentially triggering unnecessary HCP enrollment. Conversely, using NIOSH-configured data for OSHA enforcement defense is not valid. Both errors are avoidable.
Personal vs. Wearable Noise Dosimeters
The terms "personal noise dosimeter" and "wearable noise dosimeter" refer to the same type of instrument — a body-worn device that measures individual worker exposure. The distinction is primarily one of form factor and marketing rather than function.
Traditional personal dosimeters are small electronic units worn on the belt or clipped to clothing, with a cable running to a microphone near the ear. They have been the standard for OSHA personal noise monitoring for decades and are accepted in all regulatory contexts.
Modern wearable dosimeters are smaller, often wireless, and designed for less obtrusive wear during active industrial work. Some integrate with software platforms for real-time data transmission and automated reporting. The measurement principles are identical to traditional units — the wearable designation refers to the reduced physical profile.
Regardless of dosimeter type, the microphone must be placed within 30 cm of the ear canal in the worker's hearing zone. Placement on the belt, in a pocket, or on the non-dominant shoulder produces data that may not accurately represent ear-level exposure — particularly in directional noise environments.
Noise Dosimeter vs. Sound Level Meter
A sound level meter (SLM) measures instantaneous noise levels at a fixed point in space. A noise dosimeter measures cumulative personal exposure over time. They serve different purposes and are appropriate for different monitoring tasks.
| Application | Preferred Instrument | Why |
|---|---|---|
| Individual HCP enrollment determination | Noise dosimeter | Captures actual personal exposure across variable tasks and locations |
| Facility noise mapping | Sound level meter | Efficient for characterizing noise levels across areas |
| Engineering control effectiveness | Sound level meter | Before/after spot measurements at equipment |
| Peak/impulse noise assessment | Sound level meter (peak-hold) | Captures instantaneous peak levels dosimeters may miss |
| Shift-long exposure documentation | Noise dosimeter | Produces legally defensible TWA for OSHA compliance records |
OSHA permits area monitoring for representative sampling when it accurately characterizes individual exposure. However, for workers with variable tasks or mobile work patterns, personal dosimetry is required to produce defensible exposure data. For a full comparison, see noise dosimeter vs. sound level meter: which does OSHA require?
Calibration Requirements
Calibration is a documentation requirement, not just a quality control step. Uncalibrated dosimeter data is not legally defensible for OSHA compliance purposes.
- Pre- and post-use acoustic calibration: Using a calibrator at a known sound pressure level (typically 94 dB SPL at 1 kHz), confirm the dosimeter is reading within ±1 dB before and after each monitoring session. Document both readings. A drift of more than 1 dB may invalidate the session data.
- Annual calibration: A formal electroacoustic calibration by a qualified technician per ANSI S1.25, checking frequency response, linearity, and time-weighting accuracy. Required at minimum annually or after any repair.
Calibration records must be retained as part of the noise monitoring documentation package. OSHA inspectors regularly request calibration certificates alongside dosimetry results.
Noise Dosimeter Rental vs. Owned Equipment
The choice between renting and owning noise dosimeters depends on monitoring frequency, workforce size, and program complexity.
| Factor | Rental | Owned |
|---|---|---|
| Upfront cost | None | $500–$2,000+ per unit |
| Cost per survey | $50–$200/day per unit | Near zero after purchase |
| Best for | Infrequent surveys, small employers, initial baseline surveys | Regular monitoring, large workforces, frequent re-monitoring needs |
| Calibration responsibility | Rental provider | Employer |
| Availability | Must schedule in advance | On-demand |
| Re-monitoring flexibility | Scheduling delay after process changes | Immediate re-monitoring after trigger events |
Noise dosimeter rental costs $50–$200 per unit per day depending on model and provider. For an employer conducting one noise survey per year with 5 workers to monitor, rental is economical. For an employer who needs to re-monitor after every equipment change or who has 50+ workers to characterize individually, the rental cost per survey can exceed the purchase cost of owned equipment within 2–3 survey cycles.
Using Dosimeter Results for OSHA Compliance
The dosimeter output — TWA and dose percentage — drives several compliance decisions under OSHA 1910.95:
- At or above 85 dBA TWA: Worker must be enrolled in the hearing conservation program. Baseline audiogram required within 6 months (or 12 months with HPD in the interim). Annual audiograms required thereafter.
- At or above 90 dBA TWA (PEL): HPD use is mandatory. Engineering controls required where feasible. Worker cannot opt out of HPD use regardless of preference.
- Below 85 dBA TWA: No HCP enrollment required. Document the result and retain for 2 years.
Always document the dosimeter make, model, serial number, settings, calibration readings, and monitoring date alongside the TWA result. This documentation is the noise monitoring record that OSHA inspectors request during audits and that you will need if a worker's HCP enrollment decision is ever challenged.
Real-time noise monitoring without manual dosimetry coordination
Soundtrace combines continuous area noise sensors with personal dosimetry data in one platform — automatically identifying workers at the 85 dBA action level and triggering HCP enrollment and audiometric testing scheduling.
Get a Free Quote See Soundtrace noise monitoring →Frequently Asked Questions
A noise dosimeter is a body-worn instrument that measures an individual worker's cumulative noise exposure over a work shift, calculating the 8-hour time-weighted average (TWA) and noise dose percentage. Under OSHA 29 CFR 1910.95, personal dosimetry is the required method for determining individual employee exposure for hearing conservation program enrollment decisions.
OSHA 1910.95(d) requires dosimeters meeting ANSI S1.25 (Type 2 or better), set to A-weighting, slow time response, 5 dB exchange rate, 90 dBA criterion level, and 80 dBA integration threshold. Data collected with NIOSH's 3 dB exchange rate or 85 dBA criterion level cannot be used directly for OSHA compliance determinations.
The terms refer to the same type of instrument — a body-worn device measuring individual worker noise exposure. "Wearable noise dosimeter" emphasizes a smaller, more modern form factor designed for active industrial use. Both measure cumulative noise dose and calculate 8-hour TWA using the same ANSI S1.25 methodology.
Rental is appropriate for infrequent surveys or initial baseline assessments. Owned equipment is more cost-effective for employers who monitor regularly, have large workforces, or need on-demand re-monitoring after process changes. Rental costs of $50–$200/day/unit can exceed the purchase price of a basic dosimeter within a few survey cycles for employers with ongoing monitoring needs.
Acoustic calibration is required before and after every monitoring session using a calibrator at a known sound pressure level (typically 94 dB at 1 kHz). If pre- and post-use readings differ by more than 1 dB, the session data may be invalid. Annual electroacoustic calibration by a qualified technician is also required. All calibration readings must be documented and retained with the monitoring records.
A sound level meter measures instantaneous noise levels at a fixed point in space. A noise dosimeter measures cumulative personal exposure over time as the worker moves through different noise environments. For OSHA individual HCP enrollment determinations, personal dosimetry is preferred because it captures the worker's actual TWA regardless of movement between quiet and noisy areas.