Close examination of the state of preservation of the mosaics of the Basilica of Santa Maria Assunta in Torcello has revealed widespread areas where the fabric of the wall structure has become detached. Before proceeding with restoration, in addition to investigating the causes of alteration it was necessary to determine the extent and position of the altered zones, in order to take different steps according to the gravity and extent of these areas. The present work analyses the results of stethoscopic research on the central apse and the Last Judgement of the basilica. The location of the detached areas was found by listening to the variation in tone of a low frequency signal (440 Hz) applied to the surface. The deterioration has been classified into three grades as follows: a. areas of serious detachment and areas with moving surfaces; b. areas of medium detachment; c. solid and homogeneous areas. Data from the stethoscopic survey of the wall surfaces were plotted on a diagram in relation to a photographic report previously carried out, in order to obtain a two dimensional map of the altered areas.