Actually acoustic performance of building components are analysed with reference to standardised conditions, in laboratories with suppressed flanking transmission. Laboratory results often show values of the performances quite different from those measured in real buildings, as a consequence of flanking transmission and of different conditions of realisation in situ. Moreover, in spite of all efforts aimed to standardise laboratory test conditions, often we find different certified performances of same building components obtained in different laboratory. In some cases, these differences may be very relevant. Besides, the Italian decree on acoustic requirement of buildings prescribes limiting values to be measured in situ. For these reasons, many producers of building materials and components more frequently presents in their depliants both laboratory values and in situ values of acoustic performances of their products. The paper shows the preliminary results of an research aimed to verify acoustic performances of partition walls and floors in a laboratory which respect conditions of ISO 140-1, but with the presence of relevant flanking transmission, similar to real buildings. The relevance of the flanking transmission has also been modified by means of a lining applied to the ceiling of the receiving room of the laboratory.