Calibration of sound level meter

The characteristics of sound level meters are specified in the international standards IEC 61672, IEC 60651 and IEC 60804. Measurement procedures and tolerances to verify that the characteristics of the sound level meter are correct are also stated in these standards. Examples of characteristics which are tested: frequency response of microphone and filter, the different time weightings (Fast, Slow etc.), level linearity, overrange indication etc.

Sound level meters are divided into different classes of accuray, where class 1 is normal for precision measurements and class 2 for simple measurements. Approximate accuracy for a sound level meter of class 1 is 1 dB and 2 dB for class 2. However, the accuracy is dependent on the character of the sound.

Sound level meters are often equipped with external or built-in octave band filters. Characteristics for these are specified in a separate standard, IEC 61260 or IEC 225.

FAQs:

How often shall I calibrate (verify) my sound level meter?

In acoustic measurement standards, two different intervals of verification occur; 1 and 2 years respectively. Instrument manufacturers often recommend annual calibration.

I use a sound calibrator prior to each measurement. In that case I do not need to send the sound level meter for calibration, or do I?

The sound calibrator calibrates the sound level meter at one single frequency and level. It says nothing about how the sound level meter works at other frequencies and levels or at other types of sound, e.g. impulse noise. The most common defects in sound level meters cannot be discovered with a sound calibrator. Thus, a sound calibrator does not replace a regular verification of the sound level meter.

Which sound level meter shall I buy?

The requirements of accuracy of the measurement decide which class to choose, normally class 1 or 2. Be careful with indistinct specifications. There are examples of specifications where it is said that the sound level meter is "manufactured to meet" or "designed to fulfil" a certain class without certification that it really does that. Furthermore, it is important to choose a sound level meter which can be calibrated (verified). As the verification is done mainly with electric signals, it must be possible to unscrew the microphone.