Phonation: Vocal Fold Physiology, Pitch, and Intensity Control

Anatomy and Physiology of Voice Production: Phonation

Glottal Shape During Activities

• The glottis, which refers to the true vocal folds, changes shape during various activities.

• Quiet Breathing: Vocal folds are fully abducted.

  • The posterior cricoarytenoid muscle is solely responsible for abduction.

• Forced Inhalation: A slight movement toward abduction can be observed.

  • Rapid airflow can cause a slight suction, pulling the vocal folds slightly closer together, even as they remain abducted for air intake.

• Normal Phonation: Vocal folds are touching.

  • A small opening, called a posterior chink, can normally be present in the posterior larynx, particularly in females.

  • This is a notable physiological difference between male and female larynges; it is rarely seen in males but is normal in females.

  • Therefore, a posterior chink in females is not indicative of incomplete or impaired glottal closure.

• Whispering: The glottis forms a 'Y' shape.

  • There can be some tension, but the vocal folds do not vibrate against each other.

  • There is minimal to no vocal fold vibration, similar to an exaggerated posterior chink.

• Factors Altering Glottal Shape:

  • Abduction or adduction of vocal folds.

  • Rotation or tilting of the arytenoid or thyroid cartilages.

  • The force of the airstream directed through the larynx.

Vocal Fold Vibration

• During vibration, vocal folds actively move from an abducted to an adducted position through muscular contraction.

• Adduction Mechanism:

  • Vocal fold adductor muscles become active.

  • They cause rotation of the arytenoid cartilages, pulling the vocal folds toward the midline.

  • The arytenoid cartilages approximate each other via a gliding and rotating motion.

  • The vocal folds move together.

• Visual Representation:

  • Abducted: Appears as an inverted 'V' (top is anterior, bottom is posterior).

  • Adducted: Appears as two parallel lines, like the number '$11$'.

• Pitch, loudness, and speech are all influenced by the shape and physiology of the vocal folds during phonation.

Prosody

• Prosody refers to the rhythm, stress, and intonation of speech.

• It involves the variability in fundamental frequency, syllable length, and loudness.

• Prosody conveys meaning and emotion (e.g., question, command, statement) and is crucial for effective communication.

• Stress on words typically results in a lengthened syllable and increased loudness.

Terminology: Physical Measures vs. Psychoacoustic Correlates

• Frequency:

  • A physical measure referring to the rate of vibration.

  • Expressed in Hertz ($Hz$), representing cycles of vibration per second.

• Pitch:

  • The psychoacoustic correlate of frequency.

  • Refers to the perception of how high or low a sound is.

  • As frequency ($Hz$) increases, perceived pitch gets higher.

• Intensity:

  • A physical measure of sound.

  • Measured in decibels ($dB$).

  • Correlates with the psychoacoustic phenomenon of loudness.

• Loudness:

  • The psychoacoustic correlate of intensity.

  • Describes the perception of how loud or soft a sound is.

  • As intensity ($dB$) increases, perceived loudness increases.

Sound as a Mechanical Disturbance

• Sound is a mechanical disturbance of a medium (gas, liquid, or solid).

• In our atmosphere, sound excites air particles.

• Vibrating vocal folds displace air, causing periodic condensation (compression) and rarefaction (reduction in molecules) of air molecules.

• This wave-like pattern propagates for a certain distance, similar to ripples from a stone in a lake, eventually dissipating.

Pitch Control (Fundamental Frequency, $f_0$)

• The fundamental frequency ($f_0$) of the voice is determined by:

  • Stiffness of the vocal folds.

  • Mass of the vocal folds.

  • Length of the vocal folds.

• General Observations:

  • Children and females often have higher pitched voices due to shorter, smaller, and less massive vocal folds than men.

  • Men, despite having lower typical $f_0$, can achieve higher pitches by stretching their vocal folds.

• Physiological Mechanisms:

  • Lower Pitch: Shorter, thicker, less stiff vocal folds vibrate more slowly.

  • Higher Pitch: Longer, thinner, stiffer vocal folds vibrate at a faster rate.

• Muscles Regulating Length and Stiffness:

  • Cricothyroid (CT) Muscle:

    • Primary muscle for increasing pitch.

    • Contracts to rock the thyroid cartilage forward, closer to the cricoid cartilage.

    • This stretches and elongates the true vocal folds, increasing their stiffness and reducing their vibrating mass (only the cover vibrates at maximum CT contraction).

  • Thyroarytenoid (TA) Muscle:

    • Primary muscle for decreasing pitch.

    • Contracts to shorten the vocal folds.

    • This allows the vocal fold cover to become less stiff and more flexible, increasing the vibrating mass (both cover and body vibrate at maximum TA contraction).

  • Antagonistic Pair: CT and TA usually work in opposition but are often co-active to achieve intermediate pitch changes.

• Role of Subglottic Pressure:

  • Increasing subglottic (lung) pressure (dynamic length change) can also increase $f_0$ by increasing vocal fold stiffness during vibration.

• Complex Interplay of Muscle Contraction:

  • Maximum TA contraction: Maximally relaxed cover, greater vibrating mass (cover and body), lower pitch.

  • Maximum CT contraction: Maximally stretched cover, increased stiffness, smaller vibrating mass (only cover), higher pitch.

  • Mid-range $f_0$: Both TA and CT are active, with stiffness influenced by their relative contraction and subglottic pressure.

• Differential TA Contraction:

  • Thyromuscularis (lateral TA): Contraction lowers stiffness, especially if the cover is the main vibrating area.

  • Thyrovocalis (medial TA): Contraction increases stiffness of the entire vocal fold tissue, especially if the body is the main vibrating area.

Intensity Control (Loudness)

• Intensity is measured in decibels of sound pressure level ($dB ext{ SPL}$).

• Formula: $SPL = 20 ext{ log}<em>{10} ( rac{p</em>1}{p_0})$

  • Where $p<em>1$ is the pressure of the sound, and $p</em>0$ is the standard reference pressure ($20 ext{ microcascals}$).

• Subglottic Pressure ($p<em>s$): The primary source of vocal intensity; as $p</em>s$ changes, intensity changes.

• Factors Affecting Vocal Loudness:

  • Radiation of Sound: Opening the mouth widely allows for better transmission of acoustic power and better radiation of sound.

  • Glottal Power: Comprised of two main factors:

    • Lung Pressure (Subglottic Pressure, $p_s$): The pressure generated by the lungs.

      • Doubling the subglottic pressure results in a $6 ext{ dB}$ increase in glottal source power (i.e., $6 ext{ dB}$ louder).

      • Clinical Correlation: Diseases affecting lung pressure (e.g., COPD, ALS) can reduce speech intensity and perceived loudness.

    • Glottal Adjustment: The way the true vocal folds act when air is expelled.

      • Tight Vocal Fold Adduction: Requires more $p_s$ to push the vocal folds apart, leading to increased intensity.

      • Weak Vocal Fold Adduction/Closure: Allows air escape, reducing $p_s$ and intensity.

      • Vocal Fold Thickening: Requires greater $p_s$ to initiate oscillation.

• Glottic Configurations Affecting Intensity:

  • Normal Adduction: Vocal folds approximate adequately.

  • Pressed Glottic Configuration: Vocal folds are tightly together and thickened bilaterally, requiring greater $p_s$ for vibration.

    • Can result in a 'pressed' vocal quality, often addressed in voice therapy.

Clinical Correlations

• Superior Laryngeal Nerve (SLN) Paralysis:

  • Damage to the SLN, which innervates the cricothyroid (CT) muscle.

  • Results in an inability to vary or significantly increase fundamental frequency (pitch) using the CT muscle.

  • Some fundamental frequency adjustments are still possible through subglottic pressure changes and thyroarytenoid muscle contraction.

  • Challenging to identify because abduction and adduction remain intact (recurrent laryngeal nerve innervates other intrinsic laryngeal muscles).

• Recurrent Laryngeal Nerve (RLN) Paralysis:

  • Damage to the RLN, which innervates abductor and adductor muscles of the larynx.

  • One vocal fold may be unable to adduct or abduct, often paralyzed in a paramedian position (close to midline but slightly lateral).

  • Consequences:

    • Vocal folds cannot close completely, leading to air escape during phonation.

    • Reduced subglottic pressure.

    • Weak, breathy voice quality.

    • Decreased number of words per breath group due to insufficient $p_s$ to sustain phonation.

    • Increased risk of aspiration due to incomplete airway closure during swallowing (though other redundant airway closure mechanisms exist).