Andrea Truppin writes: 

"In his later films, Andrei Tarkovsky develops a compelling language based on sound's potential  for ambiguity and abstraction. He probes sound's ability to function both literally - attached to an object - and abstractly - independent of any recognizable source. In these films, sound moves beyond its traditional role as secondary support for the image, at times surpassing the visual in its ability to convey certain types of meaning.

In Tarkovsky's use of sound, meaning is produced as much through the synnergism of narrative and formal elements, both aural and visual, as through the audience efforts to establish coherence among these elements. These efforts by the audience represent internal struggles akin to those experienced by the characters." ( page 234)