This is the position on a film analogous to cinematographer, only for the aural dimension instead of the visual. Sometimes credited as "supervising sound editor", also responsible for overseeing dialogue editors and foley artists. Masters in the field include Ben Burtt (Star Wars and its sequels), Gary Rydstrom (Saving Private Ryan, Toy Story), and Walter Murch, who coined the term to describe his work on Apocalypse Now, containing by far the most complex soundtrack at that time.
Much of the sound designer's best work, unlike that of the cinematographer's, goes unnoticed by the conscious mind. Altering and even inventing sound effects is the flashy side of the job; painstakingly creating ambiances to fit the emotional mood of the moment is much trickier. The craft has been revolutionized in the past decade by the introduction of software like ProTools alongside the Avid. The previous method, cutting and splicing sound on magnetic tape (think reel-to-reel, only with sprocket holes) was far more difficult to organize and operate, and hence more time-consuming. Low budget filmmakers may still, unfortunately, be forced to work that way.