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voice & speech: journey of the voice: phonation: physiology

Laryngeal Physiology: How it works

Understanding how the structures of the larynx work is a complicated process as it works aerodynamically, in response to the breath stream flowing through the glottis. Once you understand how the stream of air is chopped up to make the waves of sound, it is quite simple to grasp the different ways in which that sound can be modified to change the pitch (i.e., singing high or low), the intensity (volume) or to switch registers.

How Vibration Happens

The Bernoulli Effect
is the scientific principle the draws the vocal folds together. The Bernoulli Effect is all around us. It is the main principle of lift, which causes airplanes to fly, and baseballs to spin.

A simple example of how the Bernoulli effect works is experienced by a bicycle commuter everyday: riding along, a large truck passes her. Its speed creates an area of lower pressure, it draws in the surrounding air as it passes the cyclist, and she feels as if she is being sucked toward the truck -- in fact she is! Another example is found in the tap in a highschool science lab. The flow is constricted in a very narrow nozzle. Above the nozzle is a small hole, which draws in air to create a vacuum in experiments.

The vocal folds are also drawn in by the Bernoulli Effect. The intrinsic muscles of the larynx bring the vocal folds together, they "approximate" them, so that the space between the folds, the glottis, is essentially closed off. Once they are closed, the air stream creates a pressure against the closed vocal folds until they are blown apart. As the air rushes through the very narrow, constricted opening, it must accelerate to get through. This high speed air, much like the truck in the example above, creates suction perpendicular to the direction of its flow -- it draws the side of the opening in.

"The Wave" - the simultaneous actions of the vocal folds and the mucosal wave
The vocal folds move in a wave-like manner, opening and closing in three dimensions:

• the glottis opening from back to front
• the folds undulating back to front
• the folds undulating vertically, bottom edges open first and close first

Pitch Change

• Fundamental Frequency: when speaking or singing there is always an underlying "note" to each sound. That pitch is defined as the average rate of vocal fold vibration and is measured in cycles per second, or Hertz.
• Males & females are roughly an octave apart.
• For pitch to rise, the vocal folds must vibrate more quickly. To do this, the folds get thinner by being stretched longer.
• Extrinsic muscles manipulate the cartilages to make the folds tense and raise the pitch.
• Pitch can be lowered by the muscles drawing the cartilages closer together, relaxing the folds.

Intensity Change
If frequency (pitch) is the number of waves of high and low pressure, intensity is the size of the wave, which controls "volume". As we get louder:

• the folds don't open any further than usual, but they stay closed longer, creating more distinct "puffs of air" - a greater difference in the low and high pressure levels
• the folds are pressed together more firmly
• we create high, sub-glottic pressure against the folds to compensate
• articulation affects the potential pressure difference - e.g. it is impossible to shout on /m/ because the restriction of the airflow (the air must go through the nose on /m/) and so there is a limit to intensity one can make.

Modes of Vibration
Regular or Modal: sometimes called chest-tone

• this is the normal vocal fold vibration we have discussed so far

Falsetto: sometimes called head-tone

• only the free edges of the folds vibrate
• posterior part doesn't vibrate but is held tense
• result is often slightly breathy because the folds don't necessarily touch

Glottal Fry:

• folds are held together tightly
• free edges bubble the sound out
• wave pattern is unusual
• idling chainsaw or popcorn sound

Registers

Registers tend to mean the type of voice, especially in singing, though the name may refer to the mode of vibration in some writings. The Registers Chart helps to explain and visualize the various traditional voice types and to compare where the modes shift.

Attacks
There are three kinds of attacks (or beginning of the each voiced sound):

Simultaneous:

• breath and vibration occur at the same time (this is the preferred mode)

Glottal:

• vocal folds held together before abruptly "popping" apart to begin vibration (often heard to accentuate a word that begins on a vowel)

Breathy:

• breath occurs first, then folds begin to vibrate

 

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