Meter Class
Sound level meters are sold as type 1 or type 2. The numbers describe how far the instrument is allowed to be wrong, and knowing the tolerance is more useful than knowing the label.
When acoustic measurement is part of a wider workplace review, this guide can add operational context, but it should not be treated as a substitute for calibrated sound data.
What the classes are
A specification of permitted error across frequency, defined in the international standard IEC 61672.
Type 1 permits smaller deviations from the ideal response and holds them over a wider frequency range.
For standards, background, or a second perspective, consult IEC standards resources.
Type 2 permits larger deviations, with the allowance widening at the extremes of the frequency range.
Neither is "accurate" and the other "inaccurate". Both are specifications with stated tolerances, and a type 2 instrument performing within its tolerance is working correctly.
Which one a task needs
Regulatory and legal work generally specifies type 1, or accepts type 2 for particular purposes depending on jurisdiction.
Workplace surveys are commonly done with type 2 instruments, which is adequate for identifying where a problem is.
Deciding whether a room is noisy needs neither — a phone will place you in the right range if you know what it can and cannot do.
The class matters when the number will be contested. If nobody will argue with the figure, the class is not the constraint.
What the class does not cover
Whether it is calibrated. A type 1 meter out of calibration is worse than a calibrated type 2, and the class says nothing about the state of the individual unit.
Whether it was used correctly. Position, orientation, the operator's body, wind, and averaging period all affect the result more than the class does.
And whether the right quantity was measured. A spot reading is not a dose, and no instrument class fixes measuring the wrong thing.
The NIOSH app case
The NIOSH Sound Level Meter app, with a calibrated external microphone, meets type 2 requirements of IEC 61672 part 3. (NIOSH.)
Which is a specific and limited claim: the system — a particular iOS device plus an external microphone — was tested against a defined tolerance.
NIOSH states it is not for compliance purposes, and that statement sits alongside the type 2 claim rather than being contradicted by it. Meeting a tolerance and being acceptable as a legal record are different questions.
What to check when buying
The class, and whether the task actually requires it.
Whether a calibrator is included or available for the same instrument.
The laboratory calibration interval and what it costs, which is a recurring expense that rarely appears in the purchase decision.
And whether it logs. A dosimeter measuring a full shift answers a different question from a meter taking readings, and the difference matters more than the class for most workplace questions.
The short version
- Type 1 and type 2 are permitted-error specifications under IEC 61672, not quality grades
- Type 1 permits smaller deviations over a wider frequency range; a type 2 within tolerance is working correctly
- Regulatory work generally specifies type 1, workplace surveys commonly use type 2, and deciding whether a room is noisy needs neither
- The class says nothing about calibration state, correct use, or whether the right quantity was measured
- The NIOSH app with an external calibrated microphone meets type 2, and is stated by NIOSH not to be for compliance
- When buying, check the class against the actual task, calibrator availability, the laboratory interval and its cost, and whether it logs