Sound level meter calibration and verification

Every noise measurement standard requires that the instrument be calibrated, and most reports contain a sentence saying it was. Behind that sentence sit two quite different activities with different purposes, different standards and different costs. This guide separates them, explains what IEC 61672-3 and ISO/IEC 17025 add, discusses how often laboratory calibration is needed, and shows how to read the certificate that comes back.

Field calibration with an acoustic calibrator

An acoustic calibrator is a small sound source that produces a stated sound pressure level, usually 94 dB or 114 dB, at a stated frequency, usually 1 kHz, inside a cavity that fits over the microphone. Calibrators are specified by IEC 60942 in classes LS, 1 and 2; the calibrator class should match the meter class. The field check is simple: fit the calibrator, switch it on, read the meter, and compare with the calibrator’s certified level corrected for atmospheric pressure where the manufacturer says so. Repeat at the end of the measurement. The two readings and the difference between them go into the field notes.

The field check establishes three things: that the microphone, preamplifier and meter are working; that the sensitivity has not shifted since the last laboratory calibration; and that nothing changed during the measurement. It does not test the frequency response, the time weightings, the linearity or the self-noise, and it cannot detect a fault at any frequency other than the one it produces. That is why it is a check and not a calibration in the laboratory sense, however the menu item is labelled.

Dosimeters are checked the same way using an adaptor for the small microphone, or with a reader unit that contains a calibrator. Outdoor stations are checked with a calibrator at installation and at each site visit, and between visits by an electrostatic actuator or charge injection check; see How to specify a noise monitoring station.

Periodic laboratory calibration

A laboratory calibration tests the instrument against its specification across the range of conditions the standard describes. For sound level meters the procedure is IEC 61672-3, Periodic tests. It covers the indication at the calibration check frequency, the acoustical and electrical frequency weightings, level linearity over the full range, time weightings, tone-burst response, peak response, overload indication and the self-generated noise, together with the response to the acoustic calibrator and the effect of the windscreen. Each result is reported with its measurement uncertainty, and the instrument is then stated to conform, or not, to the class 1 or class 2 limits of IEC 61672-1.

IEC 61672-3 is a subset of the pattern-evaluation tests of IEC 61672-2: it tests what can drift or fail in service, not the design. Filters are tested to IEC 61260-3 and calibrators to IEC 60942; dosimeters to the manufacturer’s procedure or IEC 61252.

ISO/IEC 17025 accreditation

A certificate is only as good as the laboratory behind it. ISO/IEC 17025 accreditation is granted by a national accreditation body after assessment of staff, equipment, methods, traceability and uncertainty budgets, and is specific to a scope: check that calibration of sound level meters to IEC 61672-3 is actually on it. Certificates from laboratories accredited by signatories to the ILAC arrangement are accepted across borders. Manufacturers’ service centres are often accredited; independent laboratories are an alternative when turnaround or location favours them.

How often

IEC 61672 does not prescribe a calibration interval. The interval is set by the user’s quality system, and a defensible one takes account of three things: what the measurement standards and regulations you work under require, since some name a maximum interval for the meter and a shorter one for the calibrator; what the manufacturer recommends, usually 12 months; and the instrument’s own history, since a meter that has returned within tolerance for five consecutive years justifies a longer interval than one that was adjusted last time. Twelve to twenty-four months covers most practice. Pattern-approved instruments used under a national verification scheme follow that scheme’s period, which is a legal requirement rather than a quality choice. Several instruments in the database publish a recommended interval; it appears in the specification table where the manufacturer states it.

What a certificate shows

A calibration certificate from an accredited laboratory should contain, at a minimum:

• Identification of the instrument, microphone and preamplifier by model and serial number, and of the firmware version, since the class applies to the combination.
• The procedure used (for example IEC 61672-3:2013) and the environmental conditions during the tests.
• The measured values for each test, with the expanded measurement uncertainty (normally at 95 % coverage) for each.
• The applicable tolerance limits and a statement of conformance or non-conformance, together with the decision rule used to account for uncertainty, as ISO/IEC 17025:2017 requires.
• Any adjustment made before the final measurements, with the as-found values.
• The traceability statement, naming the national measurement institute at the head of the chain, and the accreditation body’s mark with the laboratory’s number.

If the sensitivity was adjusted, the as-found value tells you how far the instrument had drifted, and whether measurements made late in the interval need a comment. If a result conforms but its uncertainty bar crosses the limit, the decision rule explains how that was decided.

Putting it into practice

1. Buy the calibrator with the meter, in the same class, and send both for calibration together.
2. Record field-check readings before and after every measurement, with the calibrator serial number.
3. Set an interval in your quality system, diarise it, and keep certificates for the life of the instrument.
4. Keep a drift log from the as-found values; it is the evidence for extending or shortening the interval.
5. Check that every component, including a new microphone or outdoor kit, is covered by the certificate.