Class 1 vs class 2 sound level meters
By soundlevelmeter.review editors, practising acousticians. Published . Neutral by policy: no brand pays for placement.
The class marking is the first thing a buyer reads on a sound level meter and the first thing a reviewer of a noise report checks. It is also widely misunderstood. This guide explains what IEC 61672-1 actually specifies for each class, what the tolerance figures mean in practice, where each class is typically required, and the situations in which the class marking does not tell you what you need to know.
What IEC 61672-1 specifies
IEC 61672-1 is the international specification for sound level meters. It defines the frequency weightings (A, C and Z), the time weightings (F, S and the optional I), the time-averaged level Leq, sound exposure level and C-weighted peak, and then sets tolerance limits around each design goal. The same design goals apply to both classes. What differs is the size of the tolerance, the frequency range over which it applies, and the environmental conditions within which the instrument must remain inside it.
The standard also defines two groups, X and Z, which describe how the instrument is to be tested, and it requires the manufacturer to publish a range of information in the instruction manual, from the reference sound pressure level to the effect of the windscreen. The class is therefore a statement about the complete instrument as tested with a particular microphone, preamplifier and firmware, not a label on the display.
The tolerances in numbers
The acceptance limits at the 1 kHz reference frequency are about ±1.1 dB for class 1 and about ±1.4 dB for class 2. These figures include the expanded uncertainty of the laboratory test, which is why they are slightly wider than the design tolerance alone. Away from 1 kHz the limits open up for both classes, and they open up faster for class 2: at the extremes of the audio range, class 2 limits are several decibels wide and effectively one-sided at the highest frequencies. The table summarises the differences a buyer can act on.
| Property | Class 1 | Class 2 |
|---|---|---|
| Acceptance limit at 1 kHz | about ±1.1 dB | about ±1.4 dB |
| Frequency range with tight limits | Specified from 10 Hz to 20 kHz; limits stay narrow across the audio band | Limits widen markedly below about 50 Hz and above about 8 kHz |
| Operating temperature | −10 °C to +50 °C | 0 °C to +40 °C |
| Typical microphone | ½-inch condenser, often externally polarised or high-stability prepolarised | ½-inch prepolarised (electret) condenser |
| Pattern approval | Available for many models; required for legal measurements in several countries | Rarely sought |
| Matching calibrator (IEC 60942) | Class 1 or LS | Class 2 or better |
| Typical use | Environmental, planning, legal, laboratory, building acoustics | Workplace surveys, screening, education, facilities management |
Class 2 is not a poor instrument: at mid frequencies and comfortable temperatures it is held to limits only a few tenths of a decibel wider. But the situations where the classes diverge are exactly those of environmental work: low-frequency plant noise, high-frequency tones, winter nights and summer afternoons.
Where each class is typically required
The following is a description of common practice, not legal advice. Requirements are set by the standard, regulation or authority a measurement is made for, and they change.
Environmental and planning measurements
ISO 1996-2 and most national environmental noise procedures expect a class 1 instrument. Several European countries go further and require pattern approval by the national metrology institute, with periodic verification, for measurements that have legal effect: Germany, France, Switzerland, Austria and others operate such schemes. Planning authorities elsewhere usually accept a class 1 instrument with a current laboratory calibration certificate.
Workplace noise
ISO 9612, the EU Physical Agents (Noise) Directive as implemented nationally, and the US OSHA noise standard all accept class 2 (or the older type 2) sound level meters for occupational exposure measurement. Noise dosimeters follow IEC 61252, whose tolerances correspond broadly to class 2. Many practitioners still buy class 1 for workplace work so that one instrument serves every job.
Building acoustics and laboratory work
ISO 16283 and ISO 3382 require class 1 instruments and class 1 filters to IEC 61260-1. Product noise testing to ISO 3744 and similar standards also expects class 1.
What the class does not tell you
- The class applies to one configuration. A meter approved to class 1 with its standard microphone may be class 2, or unclassified, with a different microphone, a long extension cable or an outdoor kit the manufacturer has not tested. Check the manual.
- Filters have their own class. Octave-band performance is specified by IEC 61260-1, not IEC 61672-1. A class 1 meter can have class 2 filters, and a datasheet that says only “class 1” has not told you which.
- The class says nothing about the noise floor, battery life, logging or software. Two class 1 meters can differ by 10 dB or more in inherent noise and by a factor of ten in memory. Those are the specifications that decide whether an instrument suits a job; see Understanding sound level meter specifications.
- Conformance is a manufacturer’s claim unless tested. Pattern approval means an independent institute has tested a sample of the design against IEC 61672-2. Periodic testing to IEC 61672-3 shows that your particular instrument still meets the limits. A datasheet alone shows neither.
Deciding
Buy class 1 if any measurement might be assessed against an environmental standard, used in a dispute, or made below 0 °C. Buy class 2 if the work is occupational exposure, screening or teaching and the budget matters. If in doubt, several manufacturers sell the same instrument in both classes with the microphone as the only difference; the category table can be filtered by class to find them, and the comparison tool shows what else differs.
Frequently asked questions
- Is a class 1 meter more accurate than a class 2 meter?
- It is held to tighter tolerances, which is not quite the same thing. Two instruments measuring a steady 1 kHz tone at room temperature may agree within a fraction of a decibel whatever their class. Class 1 is guaranteed to stay closer to the design goal at low and high frequencies and temperatures, where the difference for tonal or low-frequency sound can be several decibels.
- Can a class 2 meter be used for environmental noise measurement?
- Some national procedures permit it for indicative or screening measurements, a few with a stated additional uncertainty. Measurements submitted to an authority, used in a planning condition or presented in a dispute are generally expected to be class 1. Read the standard the measurement will be assessed against before buying.
- What does group X or group Z mean on a datasheet?
- IEC 61672-1:2013 introduced the two groups to describe how an instrument's electrical and acoustical tests are carried out during pattern evaluation and periodic testing. The designation tells a laboratory which procedures apply; for a buyer it rarely affects the choice.
- Does class 1 or class 2 apply to the calibrator as well?
- Yes, under IEC 60942, which classifies acoustic calibrators as LS, 1 or 2. A class 1 meter should be checked with a class 1 or LS calibrator; a class 2 calibrator introduces a larger uncertainty than the meter itself.
- Is 'type 1' the same as 'class 1'?
- No. Type 1 and type 2 were the grades of the older IEC 60651 and IEC 60804 standards and of ANSI S1.4-1983. IEC 61672-1 replaced them and tightened several requirements, so a meter stating only IEC 60651 type 1 has not been shown to meet class 1.
Related tables: Handheld sound level meters, Noise monitoring stations. Terms are defined in the glossary; the standards referred to are explained under Standards.
More guides
- How to choose a sound level meter · Class 1 vs class 2, integrating vs non-integrating, parameters by application, options and licences, calibration.
- How to specify a noise monitoring station · Autonomy, comms, weatherproofing, outdoor microphone kits, remote calibration checks, cloud platforms, data ownership.
- Sound level meter calibration and verification · Field calibration vs laboratory calibration, IEC 61672-3, ISO 17025, intervals, reading a certificate.