Concerning airborne sounds, the procedure for characterizing the sound insulation of building partitions is first based on obtaining a description of the sound transmission losses between spaces.

The sound insulation index is determined by comparison with the conventional reference description in standard EN ISO 717-1. For this purpose, this description is overlaid on the diagram of the values of the difference of sound pressure levels between the emitting and receiving enclosures, corrected in such a way that the average value of the unfavourable deviation (as shown in the figure), calculated by dividing the sum of the unfavourable deviations by the total number of frequency bands, considered in the test, must be as high as possible, but without exceeding the value of 2 dB. The insulation index is defined as the value of the ordinate of the conventional reference description for the frequency of 500 Hz.