In its most general definition, phonation is the making of phones or vocal sounds. This general definition includes voiced sounds, which include quasi-periodic oscillations of the vocal bands, and unvoiced sounds, which do not. The focus of this post will be on the voiced sounds produced during exhalation, which is the subset of the general definition most often meant when singers talk about 'phonation'. From here forward, when the word is used, it will be used in that sense.
Basic Motions of the Vocal Bands
During breathing, the posterior ends of the vocal bands are moved apart, making a triangular-shaped opening for the passage of air. This motion is called abduction. Here is a labeled, magnified picture of the vocal bands, taken from above, with the vocal bands abducted. The inverted white V shape is formed by the vocal ligaments, which are also called the 'vocal cords'. They are on the inner edges of the vocal bands.
The opening between the bands is called the glottis.
When the person is about to phonate, the posterior ends of the vocal bands are moved together, narrowing the glottis, often to the point of closing it. This motion is called adduction. Here is a magnified picture of the vocal bands of a different person, from above, with the vocal bands adducted to the point of glottal closure.
The Start of Phonation
Phonation starts when the vocal bands are adducted enough that they cause air pressure to build up below the glottis, and that air pressure is sufficiently high (when compared with ambient air pressure) to cause the glottis to open and narrow (or shut) repeatedly. This repeated motion produces pulses of air pressure to be released into the spaces just above the vocal bands, called the glottal pulse waves, or taken together, the phonated tone. (For the time being, we will not include vocal tract resonance in the discussion.)
How Adduction Affects Phonation
The phonated tone is directly affected by the amount of glottal closure, which occurs in each cycle of motion. If the glottis does not completely close, (because the adduction is incomplete) then:
- the voiced phonation sound will be mixed with the sound of air turbulence passing through the larynx;
- the glottal pulses will not be very intense; and
- the listener will hear 'breathiness' to more or less extent, inversely related to the amount of glottal closure.
If the glottis closes completely in each cycle, then:
- there will be very little sound of air turbulence;
- the glottal pulses will be more intense; and
- the listener will not hear breathiness.
The percentage of the total glottal cycle time during which the glottis is closed is called the closed quotient.
Vocal Pitch Control
The pitch of the phonated tone is influenced by multiple factors, but is mostly the result of the actions of two muscle groups:
- muscles in each vocal band, which when they flex, shorten and thicken the vocal bands, tending to produce lower frequency glottal cycles; and
- muscles on the outside of the larynx, which, when they flex, lengthen and thin the vocal bands, tending to produce higher frequency glottal cycles. The two sets coordinate to produce the full range of frequencies that can be sung. The activity of these muscles is often called registration.
Inter-relationship of Adduction and Registration
The amount of adduction that occurs is affected by the thickness of the vocal bands. When they are stretched long and are thin, the muscles which adduct the bands must move them farther toward the middle in order to get the same amount of glottal closure as is achieved with less motion when they are short/thick. If this additional adduction does not occur as the pitch ascends, the closed quotient becomes less as the vocal bands thin, and eventually the glottis does not close at all during the cycle. Thus, the progression is heard by the listener, as a weakened vocal tone.
Conversely, as the singer goes lower in the range, the vocal bands shorten and thicken, and progressively less adduction motion is needed to bring the glottis to closure. If this lesser adduction does not occur as the pitch descends, the closed quotient rises as the bands thicken, and eventually the glottis does not open and shut with a constant frequency. This latter situation is called vocal fry.
The Influence of Breath Energy on Phonation
The air pressure below the glottis during the phonation cycle is called subglottic pressure. At the beginning of a phonated tone, the energy of exhalation is resisted by the vocal folds during the closed phase, and causes a specific level of subglottic pressure to occur which, as we have seen, varies based on the adduction and the registration used for a note. If adduction and registration remain consistent, but breath energy (force of exhalation) increases, the closed quotient will progressively lessen. If adduction and registration remain consistent, but breath energy decreases, the closed quotient will progressively rise. In the former case, the voice eventually becomes breathy, and in the latter, the periodic oscillations cease, and a vocal fry results.
By training the voice to correctly balance and coordinate the laryngeal muscle actions and breath energy, the singer can achieve consistency of vocal tone and power throughout the entire range of the voice. With this balance, the singer is free to vary dynamics to suit the music being sung, and is also free to produce subtle tone changes by varying the closed quotient at will, in ways suited to the artistic expression desired.
This essay was first published February 7, 2009 on The Modern Vocalist.com the Internet's #1 community for vocal professionals, voice health practitioners and pro-audio companies worldwide since November 2008.