Uncle Dave is restricting "artificial" to just one of its meanings when it has several (see OED).
Applicable here are the definitions "not natural in manner," "displaying special art or skill (allied with the word artifice)," etc.

Perhaps a better word is forced (vide OED):Falsetto- "A forced voice of a range or register above the natural."

In any case, the definition is subjective; what is natural for one may be forced or artificial in another.