The sound power level is the major descriptor for the noise emission of sources. It is considered to be independent of the outer sound field and it is determined by several different standardised procedures. Uncertainty estimates are obtained by round robin tests. This current concept does not work for frequencies below 50 or 100 Hz (depending on the method) and for tonal sources and it is lacking a transparent uncertainty budget. To tackle these problems, a European project has focused on establishing traceability for the quantity sound power. The contribution introduces the concept and theoretical background. It is especially investigated to which extent the sound power of a source with a given volume flow depends on the outer sound field and whether a power substitution method can be used to determine the free-field sound power in ordinary sound fields.