Dissemination of the unit Watt in airborne sound: aerodynamic reference sound sources as transfer standards

In the process aiming at the establishment of both traceability and transparent uncertainty budgets for airborne sound power, the dissemination of the unit Watt ensures the correct relation between the sound power of a stationary primary standard and the sound power of transportable transfer standards. The paper reports on a detailed study to qualify aerodynamic reference sound sources (RSSs) as transfer standards. The temporal stability of RSSs has been determined for both narrow and broad frequency bands. Specially designed scanning apparatuses have provided directivity results. Remaining room reflections and near field effects have also been studied. The dissemination also requires the prediction of sound power under meteorological (static pressure, ambient temperature) and operating (rotational speed) conditions different from calibration conditions. These influences have been investigated and a correction is proposed. Finally, interlaboratory measurement results of the sound power of an aerodynamic reference sound source are reported.