Traceability of the unit watt in airborne sound: an overview

Sound power is a key acoustic parameter, which is widely used for the characterisation and labelling of sound sources. The up-to-date determination methods are valid for frequencies above 50 Hz and for broadband sources. To extend the lower frequency limits and to include sources with narrow band spectral content an extensive study for the establishment of traceability of the unit watt in airborne sound has been performed and an overview of the results is presented in this contribution. The traceability concept is introduced and the dissemination process is explained. The investigation included theoretical models, measurements under calibration conditions and in situ, and results are presented. After exploring and validating the dissemination parameters, the really emitted sound power of sources under test in realistic acoustic environments was referred to their free field sound power. For the inclusive establishment of traceability, the related uncertainty was also determined in a transparent budget, which allows the decomposition of the factors contributing to the uncertainty.