This test method provides a means of measuring the sound isolation between the interior of a closed room and locations outside the room, and also the background noise levels at the locations outside the room. The results can be used to rate the degree of speech privacy, or to estimate the probability of speech being intelligible or audible at each receiving point.

People speak at different levels and vary their voice level in reaction to room noise and other acoustical factors. Consequently it is not possible to say definitely whether a room is protected against eavesdropping. One can only assign a probability of being overheard. The owners or managers of the closed room under consideration must set criteria for this probability according to their specific goals and circumstances. The non-mandatory appendix gives an approach to setting criteria.