Theories of phonation

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Here is a thorough account of the theories of phonation, drawn from Cummings Otolaryngology, Scott-Brown's Otorhinolaryngology, and K.J. Lee's Essential Otolaryngology.

Theories of Phonation

Background

The role of the larynx in sound production has been recognized for centuries, but the precise mechanism by which it generates sound from exhaled air was not understood until the mid-20th century. Two major competing theories emerged, with the current understanding refined by the body-cover/mucosal wave model.

1. Neurochronaxic (Neurochroaxic) Theory - Husson, 1950

Proposed by Raoul Husson in 1950, this theory held that:

Each single vibration (oscillation) of the vocal folds is triggered by a separate, discrete nerve impulse from the recurrent laryngeal nerve
Glottic vibrations are caused by rhythmic nerve impulses that are synchronous with the frequency of sound produced
The vocal folds would therefore contract actively with each cycle

Why it was rejected: This is considered a physiologically impossible hypothesis. The human voice can reach frequencies of 1000 Hz or more (during falsetto), meaning the recurrent laryngeal nerve would have to fire 1000 times per second - far beyond the maximum firing rate of any motor nerve. Nerve conduction velocity and chronaxie measurements made this untenable. The theory has been largely discredited and is now of historical interest only.

(Cummings Otolaryngology, p. 1023)

2. Myoelastic-Aerodynamic Theory - Van den Berg, 1950s (Current Accepted Theory)

Proposed by Janwillem van den Berg in the 1950s after using high-speed motion pictures to document vocal fold motion, this is the currently accepted and widely endorsed theory of phonation.

The theory holds that vocal fold vibration results from the interaction of aerodynamic forces and the mechanical (myoelastic) properties of the laryngeal tissues. No active muscular contraction occurs with each vibratory cycle; the folds vibrate passively.

The Vibratory Cycle (Step by Step)

Pre-phonatory phase: The vocal folds abduct briefly for inspiration, then are rapidly adducted by contraction of the lateral cricoarytenoid muscles. The folds are positioned near the midline with a small gap.
Subglottic pressure builds: Exhaled air is forced against the adducted vocal folds. Subglottic pressure rises until it overcomes the resistance (myoelastic tension) of the adducted folds.
Inferior separation: The vocal folds are blown apart, beginning at their inferior border - the folds "peel apart" from the bottom upward.
Bernoulli effect: As air passes through the narrowed glottis, its velocity increases and lateral pressure drops (Bernoulli's principle). This negative pressure sucks the vocal fold mucosa back toward the midline.
Elastic recoil: The myoelastic forces (elastic tension in the vocal fold tissue) also contribute to the return of the folds to midline.
Closure: The inferior margins close first. When the vocal folds return to midline, subglottic pressure builds again and the cycle repeats.

Each cycle produces a puff of air released into the supraglottic vocal tract, generating the acoustic signal.

(Cummings Otolaryngology, p. 1023; K.J. Lee's Essential Otolaryngology, p. 916; Scott-Brown's Otorhinolaryngology, p. 945)

Requirements for Normal Phonation (Box 53.1 - Cummings)

Adequate breath support
Approximation of vocal folds (appropriate gap)
Favorable vibratory properties (mucosal pliability)
Favorable vocal fold shape (3D contour)
Control of length and tension

3. Body-Cover Theory and the Mucosal Wave Model

A refinement of the myoelastic-aerodynamic theory, proposed by Hirano and elaborated further by others, including the two-mass model (Ishizaka and Flanagan).

The key insight is that the vocal fold is not a homogeneous structure and vibrates in three dimensions:

Body: The vocalis muscle (thyroarytenoid) forms a firm, relatively static inner core
Cover: The overlying mucous membrane (epithelium + superficial lamina propria) is loose and pliable, blown by the expiratory airstream

The mucosal wave is a traveling wave that propagates from the inferomedial surface upward (inferiorly to superiorly) along the vocal fold with each cycle:

As the inferior edges separate, a puff of air is released
The superior edges remain in contact briefly longer, creating a temporal lag
This traveling wave can only be visualized using laryngostroboscopy or high-speed photography

The two-mass model explains why the superior and inferior edges of the vocal folds are out of phase: as the superior edges separate, divergent glottic airflow generates greater negative pressure at the lower edge, accelerating closure of the inferior glottis first.

(Cummings Otolaryngology, p. 1023; Scott-Brown's Otorhinolaryngology, p. 945)

The Vibratory Cycle - 4 Phases (Box 59.1 - Scott-Brown's)

Phase	Description
Closing phase	Vocal folds begin to close rapidly from their lower margin
Closed phase	Medial edges in full contact
Opening phase	Folds separate from lower margin upward; superior margin remains last
Open phase	Folds are separated (the longest part of a normal cycle)

This 9-frame diagram shows the full vibratory cycle with the mucosal wave traveling from the inferior to superior aspect of each fold:

Vocal fold vibratory cycle showing 9 phases of mucosal wave propagation

(Scott-Brown's Otorhinolaryngology, Fig. 59.4 - Vocal fold vibratory cycle)

Variations by Register

Register	Mechanism
Modal (chest voice)	Full mucosal wave; Bernoulli effect dominant in closing phase; body and cover move somewhat independently
Falsetto	Increased tension + decreased thickness; mucosal wave reduced/absent; elastic recoil is primary closing force; only superior edges make contact; short closing phase
Low pitch	Vocalis muscle relaxed; "body" participates in oscillation

Summary Comparison

Feature	Neurochronaxic Theory	Myoelastic-Aerodynamic Theory
Proposed by	Husson (1950)	Van den Berg (1950s)
Mechanism	Active nerve impulse per cycle	Passive aerodynamic + elastic forces
Role of muscles	Active contraction each cycle	Set tension; passive during vibration
Bernoulli effect	Not incorporated	Central to closure
Status	Discredited	Accepted (current standard)

References:

Cummings Otolaryngology Head and Neck Surgery, Section on Phonation, p. 1023
Scott-Brown's Otorhinolaryngology Head & Neck Surgery, "The Biomechanics of Phonation" and "The Vibratory Cycle," p. 945
K.J. Lee's Essential Otolaryngology, "Phonation," p. 916

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