A BIPM/CIPM key comparison on microphone calibration—defining the state of the art

The formation of the Consultative Committee on Acoustics, Ultrasound and Vibration (CCAUV) now gives acoustical quantities the same status as more established metrics like mass or voltage. The principle work of CCAUV is to establish degrees of equivalence between member states of the Metre Convention. This is achieved through key comparisons and subsequent regional comparison. For sound‐in‐air, the subject of this key comparison is most appropriately the calibration of laboratory standard microphones. The first such project considered the pressure calibration of IEC type LS1P microphones in the frequency range from 63 Hz to 8 kHz. Twelve national laboratories took part in the key comparison, piloted by the UK’s National Physical Laboratory. The project has now been completed. The results for the measured pressure sensitivity level have a standard deviation of around 0.02 dB at frequencies below 1 kHz, rising to a maximum of 0.04 dB at 8 kHz. The mean of these results normalized to 0 dB at each frequency is considered as the key comparison reference value (KCRV) and the standard deviation provides an estimate of its standard uncertainty. The KCRV then defines the datum which enables the performance of all laboratories to be related.