Tinitis resulting from exposure to loud sounds is permanent. Most of the 'background noise' we experience is caused by the vibrations in tense muscles. Next time you're in a quiet place, try clenching your jaws. You will har the noise level increase.

Our ears contain many small hairs (called cilia ?), which act to tune out a lot of this background noise. Loud sounds, or other physical shock, can break, or damage these hairs, destroying their ability to reduce the background noise. The result is intense, self-inflicted noise. As Asrael has mentioned, because this sound is always there, it can become unbearable, possibly leading to madness and/or suicide.