Whenever a feeling of hesitancy exists in the mind of the performer or listener as to the type and classification of voice movements the following rule may be applied to guide the ear: if one is UNAWARE of any voice movements taking place, and the tone is ‘alive,’ resonant and vibrant, then it is certain that a vibrato action is present. Conversely, whenever there is a consciousness of a voice movement then that movement cannot be classified as a vibrato, but is a form of tremolo, wobble and the ‘straight’ tone, or some combination thereof. A correct vibrato action is inconspicuous, while all faulty movements are very noticeable.

Reid, Cornelius L. Bel canto: principles and practices. Coleman-Ross, 1950.