Tinnitus is the term for the perception of sound when no external sound is present. It is often referred to as "ringing in the ears," although some people hear hissing, roaring, whistling, chirping, or clicking. Tinnitus is not a disease but a symptom of another underlying condition – of the ear, the auditory nerve, or elsewhere. Tinnitus can be intermittent or constant, with single or multiple tones. Its perceived volume can range from very soft to extremely loud. 50 million Americans experience tinnitus to some degree. Of these, about 12 million have tinnitus which is severe enough to seek medical attention. Of those, about two million patients are so seriously debilitated by their tinnitus, that their day to day functioning is affected.