True Vocal Folds – Voice Science

Definition

The true vocal folds are paired multilayered structures within the Larynx that vibrate to produce the sound source for voice. Each vocal fold comprises five histological layers functioning as three mechanical units: the cover (epithelium plus superficial lamina propria), vocal ligament (intermediate and deep lamina propria), and body (Thyroarytenoid Muscle). The term “vocal folds” reflects modern understanding that these are complex tissue folds rather than cord-like strings, superseding the historical term “vocal cords” based on outdated violin string analogies.

Context

Relevance to Singing

For singers, the true vocal folds represent the primary sound source determining pitch, loudness, and timbre. Understanding their layered structure explains why vocal health depends critically on preserving tissue pliability—particularly in the superficial lamina propria (Reinke’s space), which enables the mucosal wave essential for vibration. Register changes from chest to head voice reflect alterations in fold thickness, length, and tension controlled by intrinsic laryngeal muscles.

Singers routinely produce fundamental frequencies from 80 Hz (bass low notes) to over 1000 Hz (soprano high notes) through precise control of fold dimensions. Different singing styles create distinct configurations: belting maintains thick folds with extended closure phase; falsetto produces thin, elongated folds with minimal closure; classical technique balances cricothyroid and thyroarytenoid activation for consistent timbre across range. Research shows vocal fold length correlates with voice classification—sopranos average 14.9 mm, tenors 18.4 mm, basses 20.9 mm (Roers and Pestel, 2002).

Common vocal pathologies affecting singers—nodules, polyps, cysts—result from phonotrauma mechanisms including repeated high-impact collision, excessive contact pressure, and tissue dehydration. Musical theater singers show 40% vocal fold abnormality prevalence in first-year students versus 0% in classical singers, highlighting style-specific injury risks (Kochis-Jennings et al., 2012).

Historical Discovery and Development

Ancient physician Galen of Pergamon (129-216 AD) first described recurrent laryngeal nerves through pig vivisection, though reliance on animal anatomy created inaccuracies persisting 1,500 years. Andreas Vesalius’s 1543 De humani corporis fabrica revolutionized understanding through human cadaveric dissection, defining the glottis and arytenoid cartilages. Julius Casserius’s 1600-1601 De Vocis Auditusque Organis Historia Anatomica provided what contemporaries called “the most accurate description to date” with magnificent copper plate illustrations.

Antoine Ferrein coined “cordes vocales” (vocal cords) in his 1741 memoir, comparing vocal structures to violin strings—an analogy dominating thinking for two centuries despite fundamental inaccuracy. The transition to “vocal folds” reflects 20th-century recognition of their complex multilayered nature. 

Friedrich Berthold Reinke identified and characterized the subepithelial space in 1895-1897, describing its “well-defined limits” and “major role in natural oscillatory function”—observations foundational to modern laryngology. This space, now termed Reinke’s space, corresponds to the superficial lamina propria critical for mucosal wave propagation.

Scientific Basis

Five-Layer Histological Structure

Minoru Hirano’s landmark 1974 publication transformed understanding by establishing five histological layers functioning as three mechanical units (Hirano, 1974): 

Epithelium: Stratified squamous, non-keratinizing tissue approximately 0.05 mm thick forming the protective surface layer. 

Superficial lamina propria (Reinke’s space): Loose fibrous tissue with hyaluronic acid, fibronectin, and proteoglycans—the pliable layer enabling mucosal wave propagation. Described as “soft gelatin” mechanically. 

Intermediate lamina propria: Primarily elastic fibers (collagen Type III), functioning as “soft rubber bands” providing recoil properties. 

Deep lamina propria: Dense collagenous fibers (collagen Types I and III), described as “cotton thread” providing structural stability. The intermediate and deep layers together form the vocal ligament. 

Thyroarytenoid muscle: Deepest layer with both passive elastic and active contractile properties, constituting the “body” in body-cover theory. 

This layered structure explains vocal fold versatility—a single anatomical structure produces three-octave frequency ranges through differential layer engagement.

Biomechanical Oscillation Mechanism

Vocal folds oscillate through myoelastic-aerodynamic principles formulated by Janwillem van den Berg in the late 1950s, displacing earlier neural pacing theories. Subglottal pressure builds below adducted folds until sufficient to push them apart (phonation threshold pressure typically 2-3 cm H₂O). Inferior margins separate first due to subglottal pressure, followed by superior margins—creating vertical phase difference essential for sustained oscillation (Ishizaka and Flanagan, 1972). 

During opening, convergent glottal shape generates negative pressure via the Bernoulli effect in the constriction, aiding closure. Elastic recoil of stretched tissue returns folds toward midline. This cycle repeats 100-1000 times per second, generating quasi-periodic pulsatile glottal flow. The mucosal wave—tissue displacement propagating from inferior to superior margins with ~1 mm amplitude—provides visible manifestation of this vertical phase difference (Titze, 1994). 

Cadaveric studies quantify biomechanical properties: longitudinal Young’s modulus measures 30 kPa at low strain, increasing 10-15× to 390-450 kPa at high strain. This nonlinear stress-strain relationship enables three-octave frequency range. Transverse Young’s modulus (1.0 kPa) proves 30× less stiff than longitudinal, allowing differential strain patterns during vibration (Chan and Titze, 1999).

Acoustic Properties and Voice Quality

The voice source spectrum exhibits a harmonic series with fundamental frequency and integer multiples, with spectral roll-off of approximately 12 dB per octave from the glottal source. The relationship between first and second harmonics (H1-H2) provides critical information about glottal closure: large positive H1-H2 indicates thin folds with brief or absent closure producing breathy voice; negative H1-H2 indicates thick folds with extended closed phase producing pressed voice (Titze, 1994). 

Three acoustic measures quantify voice quality: jitter (cycle-to-cycle frequency variation, normal <0.5%), shimmer (amplitude perturbation, normal 0.19-0.23 dB), and harmonic-to-noise ratio (HNR, normal 9.6-11 dB). Values exceeding normal ranges indicate irregular vibration contributing to perceived hoarseness, roughness, or breathiness. 

Adult male fundamental speaking frequencies average 120-125 Hz conversationally, females 200-210 Hz—differences explained by vocal fold length (males 17-21 mm, females 11-15 mm). Trained singers extend this range across three octaves through precise control of fold length, tension, and thickness (Titze, 1988).

Pedagogical Considerations

Register Production and Control

Laryngeal configurations reflect thyroarytenoid/cricothyroid muscle activation patterns: 

TA/Thick/Modal/Chest: Moderate balanced activation produces unchanged length with 8% thinning and 77% adduction. Complete approximation with moderate tension creates regular periodic vibration perceived as normal conversational quality. 

CT/Thin/Head/Falsetto: High cricothyroid with low thyroarytenoid produces 8.4% lengthening, 10.5% thinning, and 10.6% abduction. Thin elongated folds (up to 50% elongation) with often incomplete closure generate fundamental frequencies exceeding 500 Hz with weak harmonics perceived as light and flute-like. 

Belt: High thyroarytenoid with low cricothyroid creates 2.5% shortening, 6.6% thickening, and 60.5% adduction. Tight approximation with long closed phase (>60%) produces strong higher harmonics perceived as tense and bright (Hirano, 1988). 

Recent computational studies by Zhang (2016-2023) demonstrate through 300,000+ simulations that vertical thickness dominates voice quality control more than traditionally emphasized parameters. Closed quotient increases 0.105 per millimeter thickness increase; H1-H2 decreases 4.3 dB per millimeter. This challenges decades of clinical practice focused on two-dimensional superior view, suggesting critical parameters may be hidden from standard examination (Zhang, 2023).

Style-Specific Considerations

Classical operatic singing: Balanced thyroarytenoid-cricothyroid throughout range maintains consistent timbre. Closed quotient maintained at 50-60% for efficient acoustic coupling. Singer’s formant created by epilaryngeal narrowing produces 2-3 kHz energy cluster enabling projection over orchestras. Research shows vocal fold contact forces must remain below 20 kPa to prevent injury (Sundberg, 1987). 

Belting (musical theater/pop): Thyroarytenoid-dominant activation carries chest voice above typical passaggio. Longer closed phase (≥70%) with thick vocal fold configuration throughout range. Strong second harmonic dominance with first formant tracking second harmonic (F1/H2 tracking). Higher vocal fold contact forces require careful management to prevent phonotrauma (Kochis-Jennings et al., 2012). 

Falsetto production: High cricothyroid with low thyroarytenoid produces thin, elongated folds with primarily ligamental edge vibration. Often incomplete glottal closure with permanent gap. Nearly sinusoidal flow with weak harmonics. Countertenors train falsetto with sufficient closure for operatic projection using low subglottal pressure while maintaining closed phase (Sundberg and Högset, 2001).

Vocal Health and Phonotrauma

Optimal phonation requires barely-touching glottal configuration minimizing contact pressure while maintaining closure. Balanced thyroarytenoid-cricothyroid activation prevents excess force. Complete but brief closure without hyperadduction. Sufficient breath support without excessive driving pressure. Systemic hydration maintains mucosal lubrication and tissue pliability. 

Phonotraumatic mechanisms include repeated high-impact vocal fold collision creating cumulative microtrauma, shear forces during vibration straining tissue layers, and excessive contact pressure (>20 kPa) causing vascular compression and inflammation. Dehydration increases tissue viscosity and phonation threshold pressure, requiring higher driving pressure. 

Common pathologies and mechanisms: nodules (bilateral symmetric masses at mid-membranous portion from chronic high-impact collision, most common in belters and teachers); polyps (unilateral masses from acute forceful phonation causing vascular rupture, elevated in gospel/praise singers); scarring (disruption of layered structure from surgery, trauma, or chronic inflammation, severely impedes mucosal wave) (Zuim et al., 2023).

Common Misconceptions

Misconception: “Vocal folds are cord-like strings similar to violin strings” 

Reality: Antoine Ferrein’s 1741 term “cordes vocales” (vocal cords) based on violin string analogies persisted for two centuries despite fundamental inaccuracy. Vocal folds are complex multilayered tissue folds with five histological layers functioning as three mechanical units. Unlike strings vibrating through external excitation, vocal folds oscillate through self-sustained myoelastic-aerodynamic coupling. The mucosal wave propagates through tissue layers rather than vibrating as a unified string. Modern terminology reflects this understanding—”vocal folds” supersedes “vocal cords” in voice science (Lydiatt and Lydiatt, 2010).

Misconception: “Register changes result from different vibrating locations or ‘switching’ between mechanisms” 

Reality: Register transitions reflect alterations in vocal fold thickness, length, and tension controlled by intrinsic laryngeal muscle activation patterns, not discrete anatomical switches. Hirano’s body-cover theory (1974) established that the same five-layered structure produces all registers through differential layer engagement. Modal/chest voice involves moderate balanced thyroarytenoid-cricothyroid activation; falsetto results from high cricothyroid with low thyroarytenoid producing thin, elongated folds. Computational modeling demonstrates continuous parameter variation rather than discrete switching (Story and Titze, 1995). The perceived “break” between registers reflects biomechanical threshold conditions, not changing vibrating structures. 

Misconception: “Hyaluronic acid in vocal folds is just passive filler material” 

Reality: Hyaluronic acid (HA) plays essential biomechanical roles beyond space-filling. Chan, Gray, and Titze (2001) demonstrated through rheometry on excised human vocal folds that removing HA decreased elastic shear modulus by 35% while increasing dynamic viscosity by 70% at phonatory frequencies. HA maintains optimal balance between elastic and viscous properties for vibration—reducing tissue viscosity while preserving elasticity. This explains HA’s effectiveness as bioimplant material for vocal fold scarring and its critical role in normal phonatory function. Recent research identifies PIEZO1 mechanosensitive ion channels in vocal fold epithelia, suggesting HA may also mediate mechanotransduction pathways (Chan et al., 2001).

Related Terms

Also known as: Vocal Cords (historical), Vocal Lips, Phonatory Folds, Plica Vocalis (Latin) 

See also: False Vocal Folds (ventricular folds positioned superiorly), Reinke’s Space (superficial lamina propria critical for mucosal wave), Body-Cover Theory (framework explaining multilayered function), Vocal Registers (configurations produced through muscle activation patterns)

References

Chan, Roger W., Steven D. Gray, and Ingo R. Titze. 2001. “The Importance of Hyaluronic Acid in Vocal Fold Biomechanics.” Otolaryngology–Head and Neck Surgery 124(6): 607-614. https://doi.org/10.1177/019459980112400602. 

Chan, Roger W., and Ingo R. Titze. 1999. “Viscoelastic Shear Properties of Human Vocal Fold Mucosa: Measurement Methodology and Empirical Results.” Journal of the Acoustical Society of America 106(4): 2008-2021. https://doi.org/10.1121/1.427947. 

Hirano, Minoru. 1974. “Morphological Structure of the Vocal Cord as a Vibrator and Its Variations.” Folia Phoniatrica 26(2): 89-94. https://doi.org/10.1159/000263771. 

Hirano, Minoru. 1988. “Vocal Mechanisms in Singing: Laryngological and Phoniatric Aspects.” Journal of Voice 2(1): 51-69. https://doi.org/10.1016/S0892-1997(88)80058-4. 

Ishizaka, Kenzo, and James L. Flanagan. 1972. “Synthesis of Voiced Sounds From a Two-Mass Model of the Vocal Cords.” Bell System Technical Journal 51(6): 1233-1268. https://doi.org/10.1002/j.1538-7305.1972.tb02651.x. 

Kochis-Jennings, Karen A., et al. 2012. “Laryngeal Muscle Activity and Vocal Fold Adduction During Chest, Chestmix, Headmix, and Head Registers in Females.” Journal of Voice 26(2): 182-193. https://doi.org/10.1016/j.jvoice.2010.11.002. 

Lydiatt, Daniel D., and William M. Lydiatt. 2010. “The Historical Latin and Etymology of Selected Anatomical Terms of the Larynx.” Clinical Anatomy 23(8): 926-933. https://doi.org/10.1002/ca.20912. 

Roers, Frank, and Denis M. Pestel. 2002. “Measurement of Adult Vocal Fold Length.” Journal of Voice 16(4): 451-457. https://doi.org/10.1016/S0892-1997(02)00117-7. 

Story, Brad H., and Ingo R. Titze. 1995. “Voice Simulation with a Body-Cover Model of the Vocal Folds.” Journal of the Acoustical Society of America 97(2): 1249-1260. https://doi.org/10.1121/1.412234. 

Sundberg, Johan. 1987. The Science of the Singing Voice. DeKalb: Northern Illinois University Press. 

Sundberg, Johan, and Christian Högset. 2001. “Voice Source Differences Between Falsetto and Modal Registers in Counter Tenors, Tenors and Baritones.” Logopedics Phoniatrics Vocology 26(1): 26-36. https://doi.org/10.1080/14015430116949. 

Titze, Ingo R. 1988. “The Physics of Small-Amplitude Oscillation of the Vocal Folds.” Journal of the Acoustical Society of America 83(4): 1536-1552. https://doi.org/10.1121/1.395910. 

Titze, Ingo R. 1994. Principles of Voice Production. Englewood Cliffs, NJ: Prentice Hall. 

Zhang, Zhaoyan. 2016. “Cause-Effect Relationship Between Vocal Fold Physiology and Voice Production in a Three-Dimensional Phonation Model.” Journal of the Acoustical Society of America 139(4): 1493-1507. https://doi.org/10.1121/1.4944754. 

Zhang, Zhaoyan. 2017. “Effect of Vocal Fold Stiffness on Voice Production in a Three-Dimensional Body-Cover Phonation Model.” Journal of the Acoustical Society of America 142(4): 2311-2321. https://doi.org/10.1121/1.5008497.

Zhang, Zhaoyan. 2023. “Vocal Fold Vertical Thickness in Human Voice Production and Control: A Review.” Journal of Voice 37(5): 664-679. https://doi.org/10.1016/j.jvoice.2021.03.017.

Zuim, Paulo A. B., Mara Behlau, and Noemi Grigoletto De Biase. 2023. “Association of Genre of Singing and Phonotraumatic Vocal Fold Lesions in Singers.” Journal of Voice 37(6): 963.e1-963.e8. https://doi.org/10.1016/j.jvoice.2021.08.025.