A photographic soundtrack in which the signal is represented by a line of varying density. No longer used in motion picture production.
Whereas on a variable area sound track any one part of the track is totally opaque or totally transparent, with a variable density sound track it is the same density at any point across the width of the track, but as you proceed along the track the density changes. Thus, there is very dense lines and clear lines along with lines of varying shades of grey in between, but the appearance of the track is a series of lines drawn across the width of the track.
Similarly, when the track passes the slit of the reproducer, the slit will be obscured by lines of varying shades of grey. Similarly light falling on the photo cell is varying and the current produced is varying and produces the sound in the loud speaker.
Brown, H., 1985, Basic Film Handling, in FIAF Technical Manual First Edition, FIAF Preservation Commission, Federation Internationale des Archives du Film, Brussels, Belgium