APMP Interlaboratory Comparison of Sound Level Meters: Investigation of the Instability of the Free-Field Response Level

The National Metrology Institute of Japan (NMIJ) organized and conducted an interlaboratory comparison of sound level meters to technically support emerging National Metrology Institutes in the Asia Pacific Metrology Programme (APMP). Two class 1 sound level meters were circulated among the participating institutes. Each participant was asked to determine the A-weighted free-field response level of the sound level meters using a normal calibration method at frequencies from 31.623 Hz to 19.953 kHz. This paper presents analysis results and discussion of this comparison. We examined the stability of the sound level meters using NMIJ’s calibration results for monitoring and revealed that both sound level meters changed their characteristics during circulation. We confirmed that for each sound level meter, the time-dependent change in the free-field response level was caused by the instability of the microphone attached to the sound level meter. Although neither sound level meter had an acceptable level of stability during circulation, we evaluated the variations in the free-field response level calibrated by each participant, considering the objective of this comparison. We found that the variations among the participants were greater than expected at several frequencies, and in most cases, the unexpected variations surpassed the time-dependent change in the free-field response level due to the microphone’s instability. Preparing for the next stage, in which the participants would improve their calibration procedure and/or uncertainty estimation, we proposed measures to mitigate the instability of a microphone supplied with a sound level meter and verified their efficacy.