One image, and sometimes more than one, placed over another to yield a double or multiple exposure.
Superimposition
Layers of sound added to the general soundtrack.
Split screen, divided screen, multi image
Frame divided into two or more parts so that different space or action can be presented simultaneously.
Masking
Sound completely or partially covered by another.
Engulfment
Use of excessive volume of sound to put the recipient in a state that cannot be controlled cognitively
Drone
Persistence of a constant sound layer with no apparent change in pitch and intensity.