Of the three broad categories of commonly recognized tonal oscillation, the vibrato, the tremolo and the wobble, the only one possessing positive elements in relation to the mechanical functioning of the vocal mechanism is the vibrato.  The oscillating pattern of the tremolo is too rapid and fluttery, the wobble is too erratic in design and without relevance to changes of intensity.  On the other hand, the vibrato possesses a regularity of both its periodicity and amplitude of vibration directly proportional to increases and decreases in tonal volume, the effect on listening being that the total mechanism functions in a state of equilibrium.  Thus, the vibrato is not an acquired skill, but a state and condition of the mechanism whose presence announces itself after all the physical and conceptual areas of the voice are in place and the vocal mechanism itself operating in a state of balanced tension.