Noise Monitoring & Dosimetry

You can't judge noise by feel. To know whether workers are over the limit, someone has to measure. A noise dosimeter is a small device worn on the body that records a worker's noise exposure over a shift and calculates a dose. Understanding how it works helps workers and managers use the results to protect hearing.

• Safety Managers
• Industrial Hygienists
• Noisy Trades
• Supervisors

Instruments

• Sound level meter: measures the sound level at a moment, in decibels (dB). Good for finding noisy machines and mapping areas.
• Noise dosimeter: a body-worn meter with a microphone clipped near the ear. It records noise over time and reports dose, time-weighted average (TWA) and peaks. Best for workers who move around.
• Octave band analyzer: breaks noise into frequencies for engineering controls and selecting hearing protectors.
• Instruments should meet ANSI S1.25 or IEC standards and be calibrated before and after each use with an acoustic calibrator. See Noise.

Key terms

• dBA: decibels measured with A-weighting, a filter that mimics human hearing. OSHA limits use dBA.
• OSHA permissible exposure limit (PEL): 90 dBA as an 8-hour time-weighted average. Higher levels allow less time using a 5 dB exchange rate: every 5 dB increase cuts the allowed time in half.
• Action level: 85 dBA (8-hour TWA) triggers a hearing conservation program. See Audiograms & Hearing Tests.
• Dose: the percent of allowed exposure. 100 percent dose equals 90 dBA for 8 hours. A 50 percent dose equals the 85 dBA action level.
• TWA: the steady level that would give the same dose over 8 hours.

NIOSH recommends a stricter 85 dBA limit with a 3 dB exchange rate (every 3 dB halves the time). Many employers use NIOSH criteria for better protection. Impact or impulse noise should not exceed 140 dB peak. See Hearing Protection.

Using a dosimeter

• Set the instrument correctly. For compliance with the PEL: A-weighting, slow response, 90 dBA criterion level, 5 dB exchange rate, and 90 dB threshold. For the hearing conservation program, set a 80 dB threshold so noise from 80 to 90 dBA counts toward the action level.
• Calibrate before and after sampling, and record it.
• Place the microphone on the shoulder or collar, within about 10 inches of the ear, with the cable tucked in and the body not blocking it. Monitor a full shift or a representative period.
• Explain to the worker: don't tap or shout into the microphone, don't remove it, and act as normal. Tampering ruins results.
• Select workers who represent each job or similar exposure group, and sample on typical days. Repeat when processes change.
• Note what the worker did and the noise sources so results can be understood.

Using results

• Tell workers their results. Doses of 50 percent or more (85 dBA TWA) require a hearing conservation program, including hearing tests and hearing protectors. See Audiograms & Hearing Tests.
• If exposures exceed 90 dBA, employers must use feasible engineering or administrative controls such as quieter equipment, enclosures, mufflers and maintenance, and rotation. Hearing protectors fill the gap. See Hearing Protection Fit Testing.
• Keep records of measurements. Workers and their representatives can observe monitoring and access results.
• Repeat monitoring when equipment, processes or production change.

Noise monitoring checklist

• The dosimeter is calibrated before and after use and set for the correct criteria
• The microphone is placed on the shoulder near the ear and not tampered with
• Samples cover a representative shift for each job
• Workers are told the results and enrolled in hearing conservation if at or above 85 dBA TWA
• Engineering controls are used and monitoring is repeated after changes

Last updated: October 8, 2026