To calibrate or not and how often ?

Calibration of microphones, sound level meters and calibrators !

Why calibrate?

Unfortunately, the word calibration has ended up having two meanings; to describe the use of a sound calibrator in connection with a measurement and to describe periodic verification which is carried out in a Calibration Laboratory.

Calibrating using a Sound Calibrator

When you are going to perform noise measurements, you will need to ensure that you measure the sound pressure level correctly. The procedure of making your sound level meter measure correctly is called calibration.

For sound measurements, calibration is no less than paramount since sometimes legal action will be taken based on the sound and noise levels measured!

The use of calibrators date back to those days when it was easier to design a stable calibrator than a stable sound level meter. Fortunately, this is no longer the case, sound level meters are as stable as calibrators. However, the microphone is a delicate device designed to fulfil all specifications requested. Hence they are vulnerable and easily subject to damage unless great care is taken.

One may therefore say that a calibrator is just as much a device for verification of appropriate operation as it is a device for re-adjustment of the sensitivity of the sound level meter.

Many standardised measurements require that you calibrate your sound level meter before and after the measurement session. In this way you are able to ensure that all data are correctly acquired and that nothing has changed during the measurement session.

To calibrate a sound level meter we use what is called a sound calibrator — such as the Norsonic sound calibrator Nor1251, Nor1252 or Nor1253 and the new series Nor1255, Nor1256, Nor1257 and Nor1258.
A sound calibrator is designed to produce a known exact and stable sound pressure level.

The actual calibration is then carried out by mounting the calibrator onto the microphone as shown in the picture and then switch the calibrator on.

If the sound level meter fails to indicate the correct sound pressure level, its sensitivity is adjusted until it indicates the correct value (some countries put restrictions on your right to adjust the meter, if your measurements shall comply with certain standards).

The sound level meter is then said to be calibrated (for a particular measurement).

In case the level deviates significantly from earlier or nominal values and/or it is not possible to adjust the instrument to produce a stable and correct results, a thorough check of the sound level meter will be needed.We have talked about the need to measure correctly. As we shall see this is strictly speaking not true, since a measuring device be it a speedometer or a sound level meter can only estimate the true, real value of the parameter it is measuring.

An uncertainty will always be present. The scope of the calibration is to bring this uncertainty to within given limits or tolerances. For sound level meters the width of this acceptable interval of estimates will depend on which type or class it belongs to.

Instrument Classes

According to the international standard called IEC 60651 or IEC 61672, sound measuring instruments are divided into different types, depending on their accuracy. A lower number indicate better quality and accuracy.

National and/or international standards applicable in your country may impose restrictions on which instrument types you may use for a given measurement task.

Obviously, the sound level meter is not the only part of this which is “infected” by uncertainties. The problem applies to sound calibrators as well. Hence, even these have been divided into several types (called classes) depending on their level accuracy and ability to maintain a stable level. This is to ensure that measurements made with high-quality precision sound level meters are not ruined by inaccurate calibration.

Periodic Verification

Periodic verification takes the above a step further. On a regular basis (e.g. every year, every second year etc.) sound level meters, microphones and sound calibrators are examined and measured to determine their sensitivity, frequency response, accuracy in level etc.

Periodic verification is always carried out by a calibration laboratory.

All instrumentation for acoustics should be subject to periodic verification to ensure proper, reliable operation. The Norsonic Calibration Laboratory Asia can do this for you — even if your sound measuring equipment is not manufactured by us!

Yearly calibration of microphones, calibrators and soundlevelmeters is recommended.