Measuring workplace noise requires a sound level meter or a personal noise dosimeter, proper calibration, and a systematic approach to surveying your facility. The key threshold to know: OSHA sets a permissible exposure limit of 90 dBA over an 8-hour workday, while NIOSH recommends a stricter limit of 85 dBA. Your measurements will determine whether you need hearing protection programs, engineering controls, or both.
The Two Regulatory Standards You Need to Know
Two federal agencies set noise exposure limits, and they differ in important ways. OSHA’s permissible exposure limit (PEL) is 90 dBA as an 8-hour time-weighted average. It uses a 5 dBA exchange rate, meaning for every 5 dBA increase above the limit, your allowable exposure time is cut in half. At 95 dBA, for example, a worker can only be exposed for 4 hours.
NIOSH uses a stricter standard: 85 dBA over 8 hours with a 3 dBA exchange rate. Under this rule, every 3 dBA increase doubles the noise energy and halves the safe exposure time. Most occupational health professionals consider the NIOSH standard more protective, and it’s the benchmark many employers aim for. The 85 dBA level is also OSHA’s “action level,” the point at which employers must start a hearing conservation program that includes monitoring, audiometric testing, and hearing protection.
Sound Level Meters vs. Noise Dosimeters
You have two main instruments to choose from, and they serve different purposes.
A sound level meter (SLM) is a handheld device you carry through a facility to measure noise at specific locations. It’s ideal for creating a noise map of your workplace, identifying which machines or areas produce the most noise, and taking spot measurements. You hold it near ear level, about 30 centimeters from where a worker’s ear would be, making sure not to stand between the noise source and the microphone.
A noise dosimeter is a small device worn by an individual worker throughout a shift. The microphone clips to the top of the shoulder, midway between the neck and the edge of the shoulder, pointing straight up. Dosimeters continuously record exposure over the entire workday and calculate the time-weighted average automatically. They’re the better choice when workers move between areas, operate different equipment throughout the day, or have noise exposure that fluctuates unpredictably.
In steady, consistent noise environments, both instruments produce nearly identical results as long as they meet Type 2 accuracy standards. In environments where noise fluctuates rapidly, dosimeters tend to read slightly higher because they capture short bursts of noise more effectively. Most researchers consider the dosimeter reading more accurate in those situations.
Choosing the Right Equipment Class
Sound level meters and dosimeters come in two performance classes. Class 1 instruments have tighter accuracy tolerances and work across a wider temperature range. Class 2 instruments have slightly wider acceptance limits but are still compliant for workplace measurements. For OSHA compliance purposes, a Type 2 (or Class 2) instrument is the minimum requirement. Class 1 meters are typically used in laboratory settings or when you need the highest precision, but for most workplace noise surveys, Class 2 is sufficient and considerably less expensive.
How to Conduct a Noise Survey
A noise survey gives you a complete picture of sound levels across your facility. Start by walking through during a typical operating period when all equipment is running at normal capacity. Measurements taken during downtime or partial operations won’t reflect real exposure.
Move through the facility with your sound level meter, taking readings every few feet. At each location, measure for at least 30 seconds to capture any variation in the noise. Record the reading on a floor plan or facility map that shows equipment locations and where workers are stationed. This creates a noise map that visually identifies problem zones.
Key practices to follow during your survey:
- Calibrate before and after. Check your instrument with an acoustic calibrator at the start and end of every measurement session. This confirms your readings weren’t affected by instrument drift.
- Use the correct settings. Set the meter to A-weighting (dBA), which filters sound to approximate human hearing sensitivity. Use slow response for general area measurements.
- Use a windscreen. The foam cover that fits over the microphone reduces interference from air movement, even indoors near fans or HVAC systems.
- Hold the meter away from your body. Your body can block or reflect sound waves, skewing the reading. Extend the meter at arm’s length or use a tripod.
- Don’t stand between the source and the microphone. Position yourself so the microphone has a clear path to the dominant noise source.
Running Personal Dosimetry
When you need to know an individual worker’s actual exposure over a full shift, personal dosimetry is the standard method. Before attaching the dosimeter, explain to the worker what the device does and why it’s being used. Clip the microphone to the shoulder on the side most likely to face the loudest noise source, pointing the microphone upward. If the dosimeter is a single integrated unit without a separate microphone, clip it to the collar or upper chest area per the manufacturer’s instructions.
Set the dosimeter to the appropriate exchange rate and threshold for the standard you’re measuring against (5 dBA exchange rate for OSHA compliance, 3 dBA for NIOSH). Let it run for the full work shift. Afterward, download the data and check the time-weighted average. Take notes on what tasks the worker performed during the measurement period, as this context helps you identify which activities drive the highest exposure.
Using a Smartphone App as a Screening Tool
NIOSH developed a free sound level meter app for iOS that can serve as a useful preliminary screening tool. A 2025 validation study published in Scientific Reports found that the app running on an iPhone without calibration or an external microphone showed strong correlation with professional-grade sound level meters for average noise levels and 8-hour time-weighted averages. The app demonstrated high validity and moderate-to-good reliability across measurements.
There are limitations, though. The app tends to read higher than conventional meters at levels above 90 dBA, and one phone model in the study showed a mean difference of 6.4 dBA compared to the professional meter. Peak noise measurements were less reliable than averages. A smartphone app is useful for quick checks and identifying areas that need formal measurement, but it shouldn’t replace a calibrated, standards-compliant instrument for official compliance monitoring.
Calculating the Time-Weighted Average
The time-weighted average (TWA) represents a worker’s cumulative noise exposure normalized to an 8-hour day. Most dosimeters calculate this automatically. If you’re using a sound level meter and need to calculate it manually, you’ll need to know the noise level and duration at each level the worker was exposed to throughout the shift.
OSHA provides a formula: TWA = 16.61 × log₁₀(D/100) + 90, where D is the noise dose expressed as a percentage. A dose of 100% equals the permissible exposure limit. A dose of 50% means the worker received half the allowable exposure. If a dosimeter reads a dose of 200%, the worker received twice the allowable exposure, and the TWA would be above 90 dBA under OSHA’s standard.
What Your Measurements Trigger
Your results determine what actions you’re required to take. At 85 dBA TWA (OSHA’s action level), you must implement a hearing conservation program. This includes ongoing noise monitoring, annual hearing tests for exposed workers, providing hearing protection at no cost, and training on noise hazards. Workers get to choose from at least two types of hearing protection.
At 90 dBA TWA (OSHA’s PEL), you must use engineering or administrative controls to reduce noise. Engineering controls include adding sound barriers, enclosing noisy equipment, installing vibration dampening, or replacing loud machinery with quieter alternatives. Administrative controls involve rotating workers out of noisy areas or scheduling loud operations when fewer people are present. Hearing protection is required for any worker exposed above the PEL while controls are being implemented.
Recordkeeping and Documentation
Every noise measurement session should be documented thoroughly. Record calibration results before and after measurements, the specific instruments used (including serial numbers), measurement locations, dates and times, which workers were monitored, the duration of each sample, and the tasks being performed during measurement. Note any unusual conditions like equipment malfunctions or non-typical production levels that might affect the readings.
NIOSH recommends maintaining noise measurement records for the duration of each worker’s employment plus 30 years. This long retention period exists because noise-induced hearing loss develops gradually, and historical exposure records may be needed decades later. Keep records of equipment calibration, noise measurements, area noise levels, audiometric test results, training dates, hearing protection assignments, and any noise control projects you’ve implemented. Over time, this data lets you track whether your noise controls are actually working or whether exposure levels are creeping upward as equipment ages.