A new set-up for type approval testing and calibration of sound calibrators

A new set-up allows the automatisation of tests for the type approval and the calibration of sound calibrators.

Sound calibrators generate acoustic signals with known sound pressure levels at defined frequencies. Because of the high accuracy of the level sound calibrators are an excellent transfer standard for the unit of sound pressure. They are used as a reference for the determination of the sensitivity of acoustic measuring devices, especially for the testing and calibration of sound level meters. The latter, if used for legal metrology purposes, need to be periodically tested (verified), and they have to be type-approved. Sound calibrators as reference sound sources shall fulfill the requirements according to appendix 21 of the verification regulations and need also a type approval.

Type approval tests of sound calibrators are performed on the basis of the requirements, test procedures and tolerances for the sound pressure level, the frequency and the harmonic distortion of signal generated by the devices as specified in the International Standard IEC 60942. This is to ensure that the performance specifications are met, even if external factors (e.g. different climatic conditions, changes of the supply voltage, influence of electromagnetic fields) act upon the calibrators. A periodic re-calibration (normally in an interval of two years) is needed to get a stable and reliable reference sound pressure.

A new set-up has been established to test sound calibrators according to the recent International Standard issued in the year 2003 (see figure 1). Computer software was developed which controls the measurements according to the procedures described in the standard. The measurement data are automatically registered and checked on the tolerances.

Figure 1: Set-up for measuring sound calibrators

The sound pressure level is determined by the insert-voltage technique: the acoustic signal generated by the sound calibrator which is coupled to a microphone (see figure 2) is transferred to an electrical voltage. The microphone voltage cannot be measured directly, so an AC calibrator is used for the generation of an AC voltage which is equal to the microphone output voltage. The sound pressure level of the sound calibrator output is then calculated from this voltage and the known sensitivity of the microphone.

Figure 2: Sound calibrator on reference microphone

In a transition period both, the "old set-up" and the "new set-up" had been used for the measurements, and the results agreed very well. The best uncertainty of the sound pressure level was 0,05 dB.

The new set-up was designed to be portable, so it can quickly be moved to other rooms and can be controlled by a notebook computer. This was required due to the fact that some tests for the type approval are performed in other buildings of the PTB.

The automated recording and processing of the measurement data provided the following major advantages:
- improved reproducibility of the measurement results
- errors in the transfer of measurement data are avoided
- faster testing.
This new set-up provides the basis for a reliable and effective testing and calibration of sound calibrators according to IEC 60942 in the future.

Contact person:

Manfred Brandt, FB 1.6, AG 1.61, E-mail: Manfred.Brandt@ptb.de