Corrections of Incident Power in Sound Reduction Index Measurements

When the airborne sound insulation in building acoustics test facilities is determined, the sound power incident on the test element and the transmitted power are compared. The problem is, that the incident power is not a measurand, but just follows from the underlying model. Application of both, the conventional method according to ISO 140-3 and the intensity method (ISO 15186-1) implies that the power incident on the partition in the source room is calculated from the sound pressure level prevailing there. According to ISO 140-3 the regions in the vicinity of walls, where the level is increased, are excluded from the measurement. The question is whether, for this reason, at low frequencies a correction should be applied with regards to the relation between pressure and intensity derived for the diffuse field. It will be shown mathematically, that in the vicinity of side walls the intensity incident on the partition is increased. The correction resulting from this for the incident power in the source room is not identical to the Waterhouse correction. But if the correction of the incident power as well as the Waterhouse correction in the receiving room are applied, nearly the same values as without correction result for the sound reduction index for large specimen that cover the total partition wall.