We have applied high-resolution vocal frequent analysis to a population of singing voices. Two important elements have become apparent: (1) Confirmation that the singing formant originates in the resonators. This is observed especially on a low fundamental, and it is acquired through technical skill and experience. (2) Observation of the vibrato, which, isolated from the clinical study, regarding only its graphic presentation, could have been interpreted as 'abnormal'.
Morsomme, D., Remacle, M., & Millet, B. (1993). High-resolution frequency analysis as applied to the singing voice. Folia Phoniatrica, 45(6), 280-287. https://hdl.handle.net/2078.5/130496 (Original work published 1993)