Primary measurement standards for sound in air:

• NPL provides primary measurement standards appropriate for the wide range of acoustical measurements in air. These standards support sound pressure measurements for manufacturing and product quality assurance (QA), health, safety and hearing conservation, environment noise measurement and monitoring, and many other applications.
• Dissemination of the measurement standards is undertaken to a degree through NPL's own calibration service. However the support provided to UKAS laboratories by this project is also vital for their effective operation, enabling traceability to be spread far more widely than NPL could manage alone.
• The facilities require continual incremental development to keep pace with (or even drive) the understanding of the physics underlying the calibration process and embodied in specification standards. From time to time the facilities also need significant re-development to keep pace with changes in IT and to replace instrumentation before it becomes obsolete.

A cornerstone of measurement infrastructures around the world is the CIPM Mutual Recognition Arrangement (MRA) that assures equivalence of measurement standards between all signatories of the agreement. This is as true for sound pressure as it is for the base units of the SI system. For the MRA to work in practice, it is necessary to demonstrate this equivalence and the instruments for doing so are so-called key comparisons.

NPL is currently leading the key comparison on primary calibration microphones, designated CCAUV.A-K5. Twelve participating National Measurement Institutes (NMIs) from around the world have now completed their measurements and the data is being analysed by NPL. The results will show the degrees of equivalence between the different participants, who will then act as linking laboratories to enable this examination of equivalence to be extended to all NMIs in their respective areas that claim a capability to conduct such measurement.

For more information, please contact Richard Barham