Tinnitus is the sensation of hearing a sound, such as a ringing or roaring, that does not come from a person's surroundings (nobody else can hear it). The sound may be continuous or come and go, it may keep time with the person's heartbeat, or it may coincide with the person's breathing.
To the person who is affected with tinnitus, the sound seems to come from one ear or from inside the head. In rare cases, clicking or crackling sounds or other noises in the ear can be heard by the doctor as well as by the person who has tinnitus.
Normal sounds that come from a person's surroundings are "heard" when sound waves strike the eardrum. The eardrum vibrates, and those vibrations enter the inner ear, where they stimulate nerve cells to create signals that travel along the acoustic nerve to the brain. The brain then translates the signals into patterns that a person recognizes as sounds.
Tinnitus occurs when there is no external source of sound waves. For reasons that are not understood, the brain receives signals, either from inside the head or from within the ear, that cause the sensation of hearing a sound.
Tinnitus is most noticeable (and bothersome) when the affected person is in a quiet environment. The condition is often treated by using background noise to mask the ringing or roaring that is caused by tinnitus.