Tremolo is not a type of vibrato!
Tremolo is a voice movement that indicates dysfunction. It appears when there is incorrect tension and misplaced energy. A tremolo sort of mimics vibrato, but is very fast – like a machine gun – and uneven in its vibratory pattern.
Other negative symptoms will accompany a tremolo such as shaking of the jaw or head, tightness in the jaw, constriction in the throat, and problems with pitch.