So, what, exactly, is a fully functional voice?
A fully functional voice covers approximately two octaves of range. (It can be more but it shouldn’t be much less). It is even throughout that range and can go from piano to forte through 3/4 of it without problem. The vowel sounds are undistorted and clear, with the exception of any modification, which is a conscious choice, for the sake of tonal color. Consonants are easily pronounced, with the exception of those in the high soprano range, where they can be softened (or eliminated occasionally) for the sake of tonal color. Vocal quality is clear, and vowel sounds can be changed or adjusted for expressive purposes. The sound has a steady vibrato, at approximately 1/4 tone above and below the sustained pitch (frequency) at a rate of about 5.5 to 6.5 Hz or cycles per second, except in phrases of great emotion when the pitch range can functuate by as much as 1/2 step and the rate can increase to 7 Hz. The voice should be sturdy enough to stand up to at least an hour of moderately loud singing and to some amount of stress from other factors such as a mild illness, mild physical fatigue, ambient noise, or psychological stress. It is housed in strong body with good posture that can inhale deeply without excess movment and is connected to strong but moveable abdominal muscles.
Variations on this depend upon professional need. For instance, a jazz vocalist does not necessarily need to sing a clear tone nor one with constant vibrato, nor does the rate need to be as mentioned above. A Broadway vocalist or someone doing gospel may not have an evenly developed two octaves, but could sing primarily in one register, making the voice considerably shorter and less even, but still very functional and healthy. Rock singers can live with a certain amount of “raspiness” or “roughness” provided it does not become debilitating physically or cause musical problems such as flatting. Barbershop vocalists do not sing with vibrato at all, but that doesn’t mean they are not healthy.
Functionality depends on only two things. One is health and the other is satisfaction. If the artist singing is happy with how they sing, how they sound, and what they can do with the voice, and the voice remains healthy except in times of illness such as a cold, then whatever function is there is sufficient to do the job, at every level from amateur to professional. If the person has never had any training but fits this description, they are technically skilled, even if they don’t really know what they are doing well enough to explain or describe it. If the person has had lots of training but does not have these capacities, no matter how much training they may have, they are BEGINNERS with POOR TECHNIQUE, and should not teach anyone anything ever. If there is a medical reason why the voice does not fit into the category of fully functional, and the artist knows and understands why he or she has limitations, there is no reason why that person shouldn’t sing and teach in whatever way strikes their fancy.
I hope this is helpful.