Secondary Sound Sources in the Larynx

turbulence aspiration whisper glottal-stops transient
Last updated: 2025-01-19

Secondary Sound Sources in the Larynx

While vocal fold vibration provides the primary sound source for speech, several secondary mechanisms generate additional acoustic energy in the larynx. These sources contribute to voice quality, consonant production, and various phonatory behaviors, and can become pathologically prominent in voice disorders.

Turbulence-Generated Sound

Turbulence in the glottal airstream creates an additional source of sound with a characteristic “hissing” quality. This turbulent noise results from chaotic airflow patterns when air velocity exceeds critical thresholds or when airflow encounters irregular surfaces or narrow constrictions.

Aspiration

Aspiration is turbulence noise combined with vocal fold vibration. It occurs when:

  • The glottis is not fully closed during the closed phase
  • Air velocity through the glottal opening exceeds the threshold for turbulent flow
  • There is significant average (DC) flow component relative to the oscillating (AC) component

Aspiration is an important perceptual component of breathy voice quality. The acoustic signature includes:

  • Harmonic structure from vocal fold vibration (periodic component)
  • Broadband noise filling the spectrum between harmonics (aperiodic component)
  • Elevated baseline airflow (high u₀/uac ratio)

Clinical Note: Mild aspiration is common in normal relaxed phonation, but excessive aspiration may indicate incomplete glottal closure due to vocal fold paralysis, paresis, bowing, or muscle weakness. Acoustic analysis can quantify the harmonic-to-noise ratio to assess the degree of aspiration.

Whisper

Whisper is turbulence noise produced in the absence of vocal fold vibration. Key characteristics include:

  • The vocal folds are held in partial abduction
  • Airflow through the glottis is sufficient to generate turbulence
  • No periodic oscillation occurs—only aperiodic noise
  • The spectrum is continuous (not harmonic)
  • Spectral shaping still occurs through vocal tract resonances

Whisper demonstrates that audible speech is possible without vocal fold vibration, relying entirely on turbulent airflow as the sound source. The noise is then filtered by vocal tract configurations to produce recognizable (though whispered) speech sounds.

Transient Sound Sources

Sudden onset or offset of phonation produces transient sounds labeled glottal clicks or glottal stops. These sounds define the nature of vocal attack and release.

Glottal Attacks

The manner of voice onset involves several possible configurations:

Hard (Glottal) Attack:

  • Vocal folds are completely adducted before phonation begins
  • Subglottal pressure builds up behind the closed glottis
  • Sudden pressure release creates a sharp transient click
  • Perceived as effortful or forceful voice onset
  • May involve increased mechanical stress on vocal fold tissues

Soft (Breathy) Attack:

  • Vocal folds begin moving from a partially abducted position
  • Phonation onset is gradual with initial breathiness
  • No abrupt pressure transient
  • Perceived as gentle or relaxed voice onset

Balanced (Simultaneous) Attack:

  • Vocal fold adduction and airflow initiation are coordinated
  • Minimal transient sound
  • Perceived as smooth, efficient voice onset
  • Generally preferred for vocal health

Glottal Releases

Similarly, the manner of voice offset can be:

Hard Release:

  • Abrupt cessation of vocal fold vibration
  • Vocal folds may slam together
  • Creates perceptible transient sound

Soft Release:

  • Gradual decrease in vocal fold oscillation amplitude
  • Smooth transition to breathy flow then silence
  • Minimal transient sound

Pedagogical Note: Voice teachers often describe vocal attacks using the terms “H attack” (soft), “glottal attack” (hard), or “balanced attack.” These correspond closely to the acoustic and physiological descriptions above. Repeated hard glottal attacks may contribute to vocal fold trauma and should be approached cautiously in voice training.

Liquid-Generated Sounds

Secondary sound can be generated by the movement of liquids on or near the vocal folds. This sound is characterized by a “wet” or “gurgly” quality.

Normal versus Pathological Wetness

Normal Phonation:

  • Small amount of liquid movement is normal
  • Mucus and mucosal wave create some liquid-related sound
  • Usually imperceptible or negligible

Excessive Wetness:

  • Excessive mucus production
  • Secretions pooling on vocal folds
  • Liquid in the ventricles or laryngeal vestibule
  • Perceived as problematic voice quality

Clinical Implications

Excessive wet vocal quality may indicate:

  • Upper respiratory infection with increased mucus production
  • Gastroesophageal reflux causing laryngeal irritation and hypersecretion
  • Post-nasal drip with secretions accumulating in the larynx
  • Hydration issues affecting mucus consistency
  • Laryngeal lesions that trap or displace mucus

Clinical Note: While a small amount of “wetness” is tolerated in normal phonation, excessive liquid-generated sound is considered a voice disorder requiring medical evaluation and treatment.

Summary

Secondary sound sources in the larynx supplement the primary glottal source, contributing to voice quality, phonatory gestures, and speech sound production. Turbulence generates aspiration (combined with vibration) or whisper (without vibration), adding aperiodic noise components to the acoustic signal. Transient sounds from sudden pressure changes create glottal attacks and releases that characterize voice onset and offset patterns. Liquid movement on the vocal folds can produce wet or gurgly sounds that, when excessive, indicate pathological conditions. Understanding these secondary sources is essential for comprehensive acoustic analysis and clinical voice assessment.

Key Takeaways

  • ✅ Turbulence in the glottal airstream generates aspiration (with vibration) or whisper (without vibration)
  • ✅ Aspiration is a key component of breathy voice, resulting from incomplete glottal closure and elevated baseline flow
  • ✅ Hard glottal attacks involve abrupt pressure release and create transient clicks; soft attacks are gradual and breathy
  • ✅ Balanced vocal attacks coordinate adduction and airflow for smooth, efficient voice onset
  • ✅ Liquid-generated sounds produce “wet” quality; excessive wetness may indicate pathology
  • ✅ Secondary sources contribute to voice quality but should not dominate the acoustic signal in healthy phonation

Further Reading

  1. Titze, I. R. (2000). Principles of Voice Production (2nd ed.). National Center for Voice and Speech.
  2. Holmberg, E., Hillman, R., & Perkell, J. (1988). Glottal airflow and transglottal air pressure measurements. Journal of the Acoustical Society of America, 84, 511-529.
  3. Hillman, R., Holmberg, E., Perkell, J., Walsh, M., & Vaughan, C. (1989). Objective assessment of vocal hyperfunction. Journal of Speech and Hearing Disorders, 54, 456-460.
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