Because the ultimate vibrato consists of five to seven waves or oscillations per second, each oscillation requires a half-step. Anything wider (like a whole step), sounds more like a warble or tremolo which sounds like an aged and labored voice.
Listening to singers that Brett has trained like Keith Urban, Miley Cyrus and Taylor Swift, You hear a common thread in all of their voices: a well-mastered and controlled vibrato.
Good vibrato beautifies the voice and even sets the mixed voice free from excess tension. Bad vibrato kills musical careers. In this course, the singer learns to control the rate and depth of oscillation for each musical style.