Narrow band(s) of magnetic oxide usually coated towards the edges of the base side of motion picture film for accepting audio signal recordings in the form of magnetic impulses.

After the picture is printed, narrow stripes of iron oxide material similar to the coating on magnetic recording tape are applied to the film (see below). The sound is then recorded on the magnetic stripes in real time. In the theatre, the film is played back on projectors equipped with magnetic heads, similar to those on a tape recorder, mounted in a special soundhead assembly called a ‘penthouse’.