Measurements · Noise
TWA / Noise Dose
Also known as noise dose, noise dosimeter reading
Time-weighted average / noise dose over the work shift.
What it is
What is TWA and noise dose?
Time-weighted average (TWA) is a worker's noise exposure expressed as a single level normalized to a standard eight-hour working day. It takes the varying noise received across the shift and states the steady level that would give the same exposure over eight hours. Noise dose expresses the same exposure as a percentage of the permitted daily amount, where 100 percent equals the limit, so a dose of 50 percent means half the allowable exposure has been used. Both are computed from the average level using a defined exchange rate and a criterion level (the level that, sustained for eight hours, equals 100 percent dose). Under US rules the exchange rate is 5 dB and the average is Lavg; on the equal-energy basis used elsewhere it is 3 dB and the average is Leq.
TWA and dose are two views of one exposure (a level and a percentage), and both determine compliance in hearing conservation.
Health and operational effects
Occupational noise-induced hearing loss is among the most common and most preventable workplace injuries, and TWA and dose are the metrics that decide what must be done about it. When exposure reaches the action level, an employer is typically required to provide hearing protection, information, and audiometric testing. At the higher exposure limit, exposure must be reduced by engineering or administrative controls regardless of the protection worn. Because these duties are triggered by the daily exposure, TWA and dose sit at the center of every hearing-conservation program.
For employers, this is a legal obligation with real liability attached: the dose is what an inspector checks and what a hearing-loss claim turns on, so measuring it accurately is both a duty and a defense.
Limits
TWA / Noise Dose Limit Values
The published values, by averaging period; each links to the standard that sets it.
8-hour TWA (OSHA PEL)
8-hour TWA (OSHA action level)
8-hour TWA (NIOSH REL)
8-hour TWA (PEL)
8-hour TWA (action level)
- OSHA / NIOSH noise 90 dBA: 5 dB exchange rate
- OSHA / NIOSH noise 85 dBA: 5 dB exchange rate
- OSHA / NIOSH noise 85 dBA: 3 dB exchange rate
- MSHA noise 90 dBA: 5 dB exchange rate; integrates 90 to at least 140 dBA
- MSHA noise 85 dBA: hearing conservation program; integrates 80 to at least 130 dBA
TWA is reported in decibels, normally dB(A), and dose as a percentage, both stated with their exchange rate and criterion level. Interpret 100 percent dose as the daily limit reached, with the action level at a defined lower percentage. The applicable action and limit values appear in the table on this page. Because a short spell of very loud work can dominate a whole shift's dose, the value shows which tasks drive exposure and where controls or job rotation would help most.
How it is measured
How TWA / Noise Dose Is Measured
TWA and dose are measured with a personal noise dosimeter (IEC 61252; ANSI S1.25 in the US) or a sound level meter with dosimetry built to IEC 61672, which integrates the sound level continuously through the shift, applying the configured exchange rate and criterion level to produce the TWA and the accumulating dose. Because this is a personal, configurable, occupational measurement rather than a simple ambient reading, it uses an external dosimeter logged on a Thiamis gateway, which can also report the anchor metric Leq for context. Getting the exchange rate, criterion level, and measurement duration right is essential, as each directly changes the result.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing noise dosimeters
A dosimeter is worn by the worker, with the microphone mounted on the shoulder within a few centimeters of the ear, so that it samples the noise the person actually receives as they move between tasks and locations. It should be worn for a full and representative shift, since a dose measured over a partial or atypical period does not represent the real daily exposure. The worker must be briefed not to shout into or shield the microphone, as tampering distorts the reading.
Every device and option that measures TWA / Noise Dose 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 TSI · SP-SE-1, SP-SE-1-1/1, SP-SE-1-1/3… | Noise Monitors | Thiamis |
Where it is monitored
Solutions That Monitor TWA / Noise Dose
Each one links to the solution page showing what that industry measures it for.
Cabin Air Quality
Often addedRespirable dust, gases, and blast monitoring
Noise Monitoring
Often addedEnvironmental and occupational sound levels
Occupational Safety & Industrial Hygiene
Often addedWorker exposure, confined space, and heat stress
Related
More in Noise
Questions
TWA / Noise Dose FAQ
What is the difference between TWA and dose?
They are two expressions of the same exposure: TWA states it as an eight-hour-equivalent level in decibels, while dose states it as a percentage of the permitted daily amount, with 100 percent being the limit. One is a level, the other a proportion, and instruments usually report both.
What are the criterion level and action level?
The criterion level is the sustained eight-hour level that equals 100 percent dose: the daily limit expressed as a level. The action level is a lower exposure at which duties such as providing hearing protection and audiometric testing begin. The exact figures depend on the regulation and appear in the limit table on this page.
How does TWA relate to Leq?
TWA is Leq (or Lavg under a 5 dB exchange rate) normalized to a standard eight-hour day. On the equal-energy 3 dB basis, an eight-hour Leq and the TWA are effectively the same quantity; the difference arises when a 5 dB exchange rate is used for US compliance.
How do I monitor noise dose?
Noise dose needs an external IEC 61672 sound level meter with dosimetry on a Thiamis gateway, worn for a full shift and configured with the correct exchange rate and criterion level. Use the configurator to combine noise exposure with dust and the other parameters your workplace assessment requires.
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