Primary Sound Power Sources for the Realisation of the Unit Watt in Airborne Sound

At present, airborne sound power is not a directly measurable quantity. It is calculated from sound pressure measurements performed in special environments, e.g. free or diffuse fields or from sound intensity measurements using 2-microphone probes. All the different methods are lacking traceability and transparent uncertainty budgets. These deficiencies are finally based on the lack of a primary standard for sound power as the metrological basis. In the joint research project funded by the European Metrology Research Program, primary sound power standards were established on the base of the Rayleigh Integral Method where the sound power is calculated from measured velocities on a baffled vibrating body. Different reference sound power sources were designed in different Institutes. The construction of the primary sources was mainly focused on the piston; its material, shape and friction during its movement. Primary sound power sources were installed in hemianechoic and reverberant environments, and characterisation measurements were performed to see the effect of different environments on the sound power output. Comparison measurements were performed to compare the new and present methods for the primary sound power source.