The Sound Reduction Index of a partition can be measured according to the method described by ISO 140-3. This measurement technique does not make it possible to discern between direct transmission through the partition and transmission through the lateral structures of the rooms. The paper reports the preliminary results of a research project aimed at comparing three different methods of SRI measurement: the traditional method, the sound intensity method and a method based on the measurement of the vibration velocity of the partition. The comparison makes it possible to explain the drop in the performance of a specific double partition at certain frequencies.