Sound pressure-based ratings are preferred for evaluation of insitu sound isolation

Abstract

The International Building Code has recently been revised to explicitly specify the ratings to use for field (in situ) measurements of airborne and impact sound isolation in residential properties. We review the differences between the various ratings, which (on the surface) are defined by their normalization procedure. Two normalization options are defined in current ASTM and ISO standards: a standard reverberation time of 0.5 s, and a standard amount of absorption of 10m2. It is not always appreciated that the choice of normalization is a choice of the type of measurement being performed. Normalization to a standard reverberation time is based on sound pressure and hence is a measurement of the acoustic isolation between spaces, while normalization to a standard amount of absorption is based on apparent sound power and hence is a measurement of the performance of the separating assembly. It is sometimes assumed the sound power-based ratings are representative of the sound isolation experienced by occupants, but they often provide an inaccurate assessment of the resultant isolation. Analysis and examples are presented, and it is demonstrated that the sound pressure-based ratings are preferred.