Remarks on the definition of airborne sound insulation and consequences for uncertainties

Airborne sound insulation is explicitly defined as the ratio between incident and transmitted sound power. Since sound power cannot be measured directly, field quantities like sound pressure are measured to derive the desired sound power. The relation between sound pressure and sound power depends on the nature of the sound field, i.e., to which extent it is a diffuse sound field. This is the main reason why it is impossible to derive an analytic equation for the measurement uncertainty of a sound power and thus of a sound insulation. The current practice is to define standardized test facilities for the measurement of airborne sound insulation. The uncertainty of measured sound insulations is then approximated by the standard deviation of reproducibility determined by interlaboratory tests. This is equivalent to changing the definition of airborne sound insulation. It is no longer the sound power ratio but the mean value of the sound insulation measured in very many or all thinkable laboratories meeting the required specifications. Thus, laboratory specifications become part of the definition of airborne sound insulation. The contribution highlights the background of the different definitions and shows consequences for the uncertainty of airborne sound insulation.