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Research project on traceability for the quantity sound power started


The sound power is the major descriptor for the total amount of the sound radiated from a source. In contrary to the acoustic field quantities like sound pressure, it is independent of the acoustic environment and of the distance to the source. The acoustic properties of technical products e.g. a sound emission or a sound insulation, are therefore described in terms of sound power or quantities derived thereof. Due to its outstanding importance in noise protection and other fields, several European Directives refer directly to the concept of sound power which are the Outdoor Directive, the Machinery Directive and the Eco-Design Directive. A new European funded joint research project started in June 2013 with the aim to establish traceablity for the quantity sound power.

The experimental determination of sound power and quantities derived thereof is currently based on the measurement of field quantities like sound pressure. Acoustic power can be calculated from the field quantities under restricting assumptions only. This approach is the basic reason why different standardised procedures to determine sound powers give different results for the same measurand and why uncertainties easily reach several dB to date. One exemplary consequence following from this is, that it is often impossible to resolve discrepancies between defined legal requirements for noise protection and measurement results. The current situation is thus described by a lack of traceability and an absence of transparent uncertainty budgets for the quantity sound power.

To improve the situation, a joint research program (JRP) funded by the European Metrology Research Programme (EMRP) started in June 2013 with the aim to establish traceability for the measurand sound power. Starting point is a primary standard for the realisation of the unit watt in airborne sound. It will be based on a vibrating baffled solid body. The sound power output of this device can be determined from the vibration velocity of the body’s surface and several additional quantities like static pressure and temperature. In particular, measurements in the airborne sound field as well as restricting assumptions on the nature of the sound field are not required. An uncertainty of 0.5 dB for the realisation of the unit watt is thus aimed at. The feasibility of this approach could already be demonstrated by first experimental and numerical investigations with a prototype of such a primary source.

A further main objective of this JRP is to develop a system for the dissemination of the unit watt using appropriate transfer standards. It will be investigated whether existing aerodynamic reference sound sources may serve as transfer standards. Furthermore, a tonal transfer standard will be developed and tested. The uncertainty of the sound power emitted by the transfer standards will be determined. The aim is that this uncertainty is only slightly larger than the uncertainty of the primary standard.
A final goal of the JRP is the application of the transfer standards in machinery noise. One application to be developed is the qualification of complete measurement setups for sound power determinations. These setups are combinations of the acoustic field properties and the measurement equipment. The determination of the sound power of real sources by comparing them to a transfer standard is another application aimed at. Here, the determination of the uncertainty of the sound power is a major topic.


[1] Wittstock, V.; Schmelzer, M.; Bethke, C.:  Establishing traceability for the quantity sound power. Proceedings of Internoise 2013, on CD-ROM, Innsbruck, Austria, September 2013

[2] Schmelzer, M.; Wittstock, V.; Bethke, C.:  Numerical computation and experimental verification of the emitted sound power of a vibrating baffled piston into a hemi-anechoic room. Proceedings of AIA-DAGA 2013, on CD-ROM, Meran, Italy, March 2013

Contact person:

Volker Wittstock, Dept. 1.7, WG 1.72, volker.wittstock@ptb.de