Tinnitus is the medical term for the perception of sound in one or both ears or in the head when no external sound is present. It is often referred to as "ringing in the ears." It can be intermittent or constant - with single or multiple tones - and its perceived volume can range from subtle to shattering. The exact physiological cause of tinnitus is not known. There are, however, several likely sources, all of which are known to trigger or worsen tinnitus.
SOURCES: American Tinnitus Association; "The Human Body," edited by John O.E. Clark. GRAPHIC: Brenna Maloney and James Smallwood - The Washington Post