A noise dosimeter is a small, wearable device that measures a worker’s total noise exposure over the course of a shift. Unlike a handheld sound level meter that captures noise at a single moment, a dosimeter clips onto a person’s body and continuously records sound levels for hours, calculating a cumulative “dose” of noise the worker has absorbed. It’s the standard tool for determining whether someone’s workplace is loud enough to damage their hearing.
How a Noise Dosimeter Works
The device uses an omnidirectional microphone that responds to pressure variations in the air, converting them into an electrical signal proportional to the sound pressure. That signal passes through a filtering network that applies frequency weighting, most commonly A-weighting, which adjusts the measurement to match what the human ear actually perceives. We don’t hear all frequencies equally well, so A-weighting filters out the very low and very high frequencies that our ears are less sensitive to. This gives a reading that reflects how loud the environment truly feels to the person wearing it.
After weighting, the dosimeter applies a threshold. Under OSHA’s settings, sound levels below 90 dBA are filtered out so they don’t accumulate in the dose calculation. The device then continuously integrates what remains, compiling a running total of noise exposure throughout the day. At the end of a shift, the stored data produces two key outputs: a percentage dose and a time-weighted average (TWA).
What the Numbers Mean
The percentage dose tells you how much of the allowable daily noise exposure a worker has received. A dose of 100% means the worker hit the maximum permitted level. A dose of 50% is OSHA’s “action level,” the point at which employers must start a hearing conservation program that includes audiometric testing and providing hearing protection.
The TWA converts that dose into a single decibel number representing the average noise level across an eight-hour shift. If noise stayed perfectly constant all day, the TWA would simply equal the measured level. In reality, noise fluctuates, so the dosimeter does the math. OSHA’s formula for converting dose to TWA is: TWA = 16.61 × log₁₀(D/100) + 90, where D is the accumulated dose as a percentage.
OSHA’s permissible exposure limit is 90 dBA over an eight-hour day. The action level that triggers a hearing conservation program is 85 dBA as an eight-hour TWA. NIOSH, the research arm of the CDC, recommends a stricter limit of 85 dBA as the maximum exposure. The difference matters because the two agencies also use different exchange rates.
The Exchange Rate Difference
The exchange rate determines how quickly permissible exposure time drops as noise gets louder. OSHA uses a 5 dB exchange rate: for every 5 dB increase, the allowed exposure time is cut in half. So at 90 dBA you get 8 hours, at 95 dBA you get 4 hours, and at 100 dBA you get 2 hours. NIOSH uses a 3 dB exchange rate, which is more protective. Under the NIOSH standard, every 3 dB increase halves the allowed time. This means a 3 dB jump (say, from 85 to 88 dBA) cuts your safe exposure from 8 hours to 4.
When configuring a dosimeter, the exchange rate setting fundamentally changes the dose calculation. A dosimeter set to OSHA parameters will read a lower dose for the same noise environment than one set to NIOSH parameters. Many modern dosimeters can run both calculations simultaneously, but it’s important to know which standard your readings reflect.
Where the Microphone Goes
The microphone is typically clipped to the worker’s shoulder, positioned on the outer edge near the ear. OSHA requires the microphone to be placed within a two-foot sphere surrounding the worker’s head, in what’s called the “hearing zone.” This positioning captures what the ear would actually receive, including reflections off the body and nearby surfaces. If the worker wears head coverings or respiratory hoods, the microphone goes outside the covering unless the employer is specifically trying to measure the noise attenuation the covering provides.
Proper placement matters because even a few inches of difference can change readings. Clipping the microphone to a belt, for instance, would underrepresent exposure since the body would shield it from overhead noise sources.
Dosimeters vs. Sound Level Meters
A sound level meter is a handheld instrument that gives you a snapshot of how loud a specific location is at a given moment. It works well when noise is steady and workers stay in one place all day. You can take readings at various spots around a facility, map the noise levels, and estimate exposure from there.
A dosimeter becomes necessary when those conditions don’t hold. If a worker moves between quiet offices and loud production floors, or if noise levels fluctuate throughout the shift, spot measurements can’t capture true exposure. The dosimeter follows the worker everywhere, recording the full picture. It’s also better suited for environments with impulse noise (sudden bangs, hammering, pneumatic tools) layered on top of continuous background noise, since it captures all of it in a single integrated measurement.
In practice, many workplaces use both. Sound level meters identify problem areas during an initial survey. Dosimeters then go on individual workers to confirm whether their personal exposure crosses regulatory thresholds.
Frequency Weighting Options
Most occupational noise monitoring uses A-weighting because it approximates how humans perceive sound at moderate levels. Our ears are less sensitive to very low frequencies, and A-weighting reflects that by reducing their contribution to the reading. Regulatory standards from both OSHA and NIOSH specify A-weighted measurements.
C-weighting is sometimes used alongside A-weighting, particularly for measuring peak sound levels from sudden loud events like explosions or stamping presses. At very high volumes, human hearing becomes more sensitive to low frequencies, and C-weighting accounts for this. The World Health Organization recommends a maximum workplace peak of 135 dB measured with C-weighting. Z-weighting applies no filter at all, capturing the full unmodified sound spectrum. It’s used more for analyzing the sound source itself rather than its effect on hearing.
What Triggers Regulatory Action
When a dosimeter reading shows a worker’s eight-hour TWA reaches or exceeds 85 dBA (a dose of 50%), OSHA requires the employer to enroll that worker in a hearing conservation program. This includes baseline and annual hearing tests, access to hearing protection, training on noise hazards, and ongoing monitoring. If exposure hits the permissible limit of 90 dBA (a dose of 100%), the employer must take steps to reduce noise through engineering controls, administrative changes, or both.
Dosimeter data also serves as legal documentation. If a worker later develops noise-induced hearing loss, the dosimetry records show whether the employer was monitoring exposure and whether protective measures were in place. For this reason, many safety programs keep detailed logs of dosimetry results by worker, date, and job task.
Who Uses Noise Dosimeters
Construction workers, factory employees, airport ground crews, miners, musicians, military personnel, and anyone working near heavy machinery or loud equipment are common candidates for dosimetry. Mining operations were among the earliest adopters, and agencies like the Mine Safety and Health Administration have long required personal noise monitoring in that industry.
Beyond traditional industrial settings, dosimeters are increasingly used in entertainment venues, dental offices, schools, and call centers where noise may not seem obviously dangerous but can accumulate over a full shift. A dental hygienist exposed to high-speed drills for hours, or a teacher in a noisy cafeteria, may be closer to exposure limits than expected. The dosimeter’s value is that it replaces assumptions with data.