How to choose a noise dosimeter

A noise dosimeter answers one question: how much sound energy did this person receive during this shift, expressed in the terms the regulation uses. Everything else on the datasheet is there to make that number trustworthy and to help you explain it. This guide takes a buyer through the standard, the exposure arithmetic, the two physical designs, hazardous-area certification and the logging features that turn a compliance figure into a useful survey.

The standard: IEC 61252

IEC 61252 specifies personal sound exposure meters. It defines the measured quantity (A-weighted sound exposure, from which L_EX,8h, dose and TWA follow), the tolerances on frequency weighting, level linearity and time averaging, and the test conditions. Its tolerances correspond broadly to class 2 of IEC 61672-1. ANSI/ASA S1.25 is the US equivalent and adds the exchange rate and threshold features used by OSHA. A datasheet should state conformance to IEC 61252 and, for the US market, to ANSI S1.25; a few models also claim IEC 61672-1 class 2 as sound level meters, which is useful when the same instrument is used for spot measurements.

Exchange rates, criterion levels and thresholds

Exposure arithmetic differs between jurisdictions, and a dosimeter must be set to match. The exchange rate is the increase in level that halves the permitted exposure time. A 3 dB exchange rate follows the physics of energy: twice the sound energy is twice the dose. A 5 dB exchange rate was adopted historically to allow for intermittent exposure and remains in the US OSHA permissible exposure limit. The criterion level is the level that gives 100 % dose over eight hours. The threshold is the level below which sound is ignored in the dose calculation; it is a feature of OSHA practice and is normally switched off for ISO 9612 work.

Values are quoted as commonly applied; the regulation in force where the measurement is made governs. Most current dosimeters run two or more virtual dosimeters simultaneously, so a single shift can be reported under both 3 dB and 5 dB rules. Check that the datasheet lists the exchange rates and that the criterion and threshold are freely adjustable rather than fixed presets.

Badge or cable

Two physical designs exist. A badge dosimeter is a single sealed unit, microphone and electronics together, clipped to the shoulder. There is no cable to snag, nothing in a pocket, and the wearer can rarely tamper with it. The price is a small display or none, so results are read through a docking station or app, and the microphone position is fixed by where the badge sits.

A cable dosimeter puts a small microphone on the shoulder and the instrument in a pocket or on a belt. It usually has a display, more memory, longer battery life and room for more processing, such as octave bands or audio recording. The cable is the weak point: it can be caught by machinery, and it gives the wearer something to pull. Both designs are compliant with IEC 61252. Choose by workplace: badges for mobile workers, confined spaces and anywhere clothing is changed during the shift; cable units where the extra data matter and the environment is less physical.

Intrinsic safety

An intrinsically safe dosimeter is designed so that it cannot release enough electrical or thermal energy to ignite an explosive atmosphere. Certification is to ATEX (EU), IECEx (international), UL 913 and CSA (North America) and MSHA (US mines). The marking on the certificate states the equipment group (I for mining, II for surface industry), the category or equipment protection level, the gas group (IIC covers hydrogen and acetylene and is the most demanding) and the temperature class (T4 means a maximum surface temperature of 135 °C). An “Ex ia IIC T4 Ga” marking, for example, permits use in zone 0 for most gases.

Intrinsic safety is a property of the certified configuration. Only the certified battery, charger and accessories may be used, and a non-IS dosimeter cannot be converted. The instrument page for each IS variant quotes the marking the manufacturer publishes; where the manufacturer does not publish it the field is blank.

Logging, octave bands and audio

A single shift figure tells you whether a person is over or under a limit; it does not tell you why. Three features change that.

• Time history logging at 1 s or 1 min shows when the exposure occurred, so that tasks and locations can be matched to the profile. Check the logging interval, the parameters stored at each interval and the memory in shifts.
• 1/1-octave bands allow hearing protectors to be selected by the octave-band method of ISO 4869-2 rather than by the cruder SNR or NRR approach. Filters should be stated to IEC 61260-1 class 2 or better.
• Audio event recording captures a few seconds of sound when a level is exceeded, so that a spike can be identified as a genuine source or as the wearer tapping the microphone. Recordings are compressed and are not measurement-grade, but they settle disputes.

Motion sensors that flag when the dosimeter was taken off are also worth having for unsupervised shifts.

Practical points

1. Battery life must cover the longest shift plus a margin; look for at least 24 hours of logging.
2. Windscreens are essential outdoors and in air streams; buy spares, they are lost.
3. Check the calibrator adaptor and the docking or charging arrangement for a fleet of ten or twenty units.
4. Software should export per-second data in an open format, not only a PDF report.
5. Dosimeters used as sound level meters need IEC 61672-1 class 2; the category table shows which models state it.

Put candidate instruments side by side in the comparison tool; it highlights every differing specification, including intrinsic-safety markings and logging intervals.