“Onset” is another word for attack. It is used by voice scientists because it has less violent meanings than does the word, attack.
A glottal onset occurs when one can hear and feel the initial closing of the vocal folds after their initial opening/closing, as found in the disapproving expression in English, “uh, uh”.
A breathy onset occurs when the passage of breath is heard over the partially open vocal folds prior to the initial opening/closing of the folds.
These are the two extremes of vocal onset. Each can be taken to greater extremes. The glottal attack can be so tense and loud that it is almost painful both for the listener’s ears and the singers throat. The breathy onset can be maintained to the exclusion of a phonated tone.
The balanced onset is in the very mid-point between these extremes. It is neither glottal nor breathy in sound or feel. It represents a near perfect coordination of the flow of breath with the beginning of vocal fold oscillation. This can only be accomplished if the control of breath flow is coordinated with the precise closing of the vocal folds such that the initial pressure sustained by the vocal folds is exactly that of the degree of breath flow. A perfect match occurs between breath flow and initial pressure required by the vocal fold closure, which is referred to as the “threshold pressure” of the vocal folds.
The idea is a bit daunting and researchers have spent no little amount of time trying to determine exactly how such a beginning of tone can occur. In fact, at one time a well respected voice scientist (Husson) proposed an unusual hypothesis known as the neurochronaxic theory that postulated the vocal folds are set into oscillation by neural impulses and thus are not dependent on air flow at all!
For singers there is no need to be so archly concerned. The simultaneous beginning of breath and tone is their goal if they are to obtain the most efficient and healthy oscillation of the vocal folds for singing.
Singers who habitually use the glottal onset will usually produced a phonation that is representative of the extreme medial pressure necessary for the glottal onset, a tone that is usually referred to as a “pressed” tone.
Singers who habitually use the breathy onset will usually produce a phonation that is representative of the lack of sufficient medial pressure and continue to produce a breathy tone. An interesting corollary occurs when the breathy onset singer attempts to reduce the breathiness in their resultant tone; they usually will crossover to a pressed phonation.
However, the balanced onset will produce a tone that is a good balance between breath flow and oscillation. This is a tone that is free from excessive interference oscillations in the motions of the vocal folds. It is, for most singers, a learned response because it requires awareness of coordination moments but it quickly becomes habitual. Because it is more a coordination exercise than a strength exercise it does not require great energy to develop or maintain.