Sources of Fluctuation and Perturbation

Vocal fluctuations and perturbations arise from multiple physiological systems, each contributing variations on different time scales and with distinct acoustic signatures. Understanding these sources is essential for interpreting acoustic analysis results, distinguishing normal from pathological voice production, and developing targeted interventions for voice disorders. No single source accounts for all observed variations; rather, the measured jitter, shimmer, or tremor in any voice represents the combined effects of neurological control, biomechanical properties, and aerodynamic forces.

Neurological Sources

The neural control of phonation involves complex coordination between cortical planning, brainstem pattern generation, and peripheral motor execution. Each level of this hierarchy introduces potential sources of variation.

Motor Unit Recruitment Variability

Laryngeal muscles, like all skeletal muscles, are controlled by motor units consisting of a motor neuron and the muscle fibers it innervates. The firing patterns of these motor units exhibit inherent variability due to:

Stochastic neural noise: Random fluctuations in membrane potentials and synaptic transmission create cycle-to-cycle variations in motor unit recruitment. Even during sustained phonation at constant pitch and loudness, the number and timing of motor units firing varies slightly from moment to moment.

Temporal summation effects: The integration of excitatory and inhibitory postsynaptic potentials at motor neurons is a probabilistic process. Small differences in the temporal pattern of incoming signals can lead to variations in motor neuron firing times of several milliseconds.

Motor unit synchronization: While some degree of synchronization between motor units improves force steadiness, perfect synchronization never occurs. The resulting partial asynchrony contributes to small force fluctuations, which translate to variations in vocal fold tension and stiffness.

Research using electromyography (EMG) of laryngeal muscles has demonstrated that even during phonation perceived as “steady,” motor unit activity shows continuous variation. This variation is greater in intrinsic laryngeal muscles than in larger limb muscles, possibly due to the smaller motor unit sizes and higher firing rates required for fine vocal control.

Central Pattern Generator Noise

The brainstem contains neural circuits that generate rhythmic motor patterns, including those underlying respiration and potentially certain aspects of phonation. These central pattern generators (CPGs) exhibit intrinsic oscillatory behavior, but this oscillation is not perfectly regular:

Intrinsic rhythm variability: CPGs for respiration show cycle-to-cycle variations in respiratory period and depth, even in isolated brainstem preparations without sensory feedback. These variations propagate to subglottal pressure fluctuations during phonation.

Neuromodulation effects: Various neurotransmitters and neuromodulators (serotonin, norepinephrine, acetylcholine) influence CPG activity. Fluctuations in these chemical signals introduce slower variations in respiratory and potentially laryngeal motor patterns.

Coupling between oscillators: Multiple CPGs interact during speech and singing (respiratory rhythm, laryngeal tension control, articulatory movements). Imperfect coupling between these oscillators can create complex patterns of variation in the acoustic output.

Pathological Tremor

Distinct from normal motor variability, pathological tremors represent abnormal, rhythmic oscillations originating from specific neurological dysfunctions.

Essential Tremor

Essential tremor is the most common movement disorder, affecting 4-6% of individuals over age 65. When affecting the voice, it produces:

Frequency characteristics: Typically 4-12 Hz, most commonly 6-8 Hz
Vocal manifestations: Rhythmic modulation of pitch and/or loudness, often irregular in extent
Exacerbation factors: Worsens with anxiety, fatigue, or physiological stress
Anatomical origin: Involves cerebello-thalamo-cortical circuits; exact pathophysiology remains debated

Essential voice tremor can affect the larynx directly (intrinsic laryngeal muscles), the respiratory system (diaphragm, intercostals), or both. The tremor typically worsens during sustained phonation and may be less apparent during connected speech due to the varying motor demands.

Parkinsonian Tremor

Parkinson’s disease produces characteristic tremor due to basal ganglia dysfunction:

Frequency characteristics: 3-7 Hz, typically slower than essential tremor
Rest vs. action: Classic Parkinsonian tremor is most prominent at rest but voice tremor typically manifests during phonation
Associated features: Often accompanied by reduced loudness (hypophonia), monopitch (reduced F0 variation), and breathy voice quality
Pathophysiology: Dopamine depletion in substantia nigra leads to altered motor control throughout the system

Voice tremor in Parkinson’s disease may result from tremor of the laryngeal muscles themselves, respiratory muscles, or pharyngeal/oral structures. Differential diagnosis from essential tremor can be challenging based on voice alone.

Spasmodic Dysphonia

Spasmodic dysphonia (SD) represents a focal laryngeal dystonia characterized by involuntary spasms of laryngeal muscles:

Adductor SD: Involuntary hyperadduction of vocal folds creates voice breaks, strain, and effortful phonation
Abductor SD: Involuntary abduction creates breathy breaks, particularly on voiceless consonants
Frequency: Spasms create irregular perturbations rather than regular tremor, though some patients exhibit both SD and tremor

While not a tremor in the classical sense, SD creates dramatic perturbations in both frequency and amplitude that are clearly neurological in origin.

Normal aging affects neural control of phonation even without specific neurological disease:

Motor unit loss: Age-related loss of motor neurons reduces the total number of motor units available, forcing remaining units to innervate more muscle fibers (motor unit enlargement)
Reduced neural processing speed: Slower conduction velocities and increased synaptic delays reduce the speed and precision of neuromuscular control
Sensory decline: Reduced proprioceptive and tactile feedback from laryngeal structures may impair closed-loop control of phonation

These changes contribute to the increased jitter and shimmer typically observed in elderly speakers, even those without diagnosed voice disorders.

Biomechanical Sources

The mechanical properties of the vocal folds and surrounding structures create inherent sources of variation in vibratory behavior.

Vocal Fold Asymmetries

Perfect symmetry between the left and right vocal folds never exists. Even in healthy voices, small asymmetries affect vibratory patterns:

Mass Asymmetries

Differences in vocal fold mass arise from:

Anatomical variation: Slight differences in vocal fold length, thickness, or layer composition
Hydration differences: Unequal tissue hydration can temporarily alter mass
Pathological additions: Unilateral nodules, polyps, or cysts add mass to one fold

A heavier fold vibrates more slowly, creating phase differences between left and right folds. If the mass difference exceeds approximately 10%, diplophonia (perception of two simultaneous pitches) may occur.

Stiffness Asymmetries

Stiffness differences result from:

Differential muscle activation: Slight imbalances in thyroarytenoid or cricothyroid activation between sides
Scar tissue: Unilateral scarring from surgery, trauma, or chronic irritation increases stiffness
Tissue pathology: Conditions like sulcus vocalis create localized stiffness increases

Stiffness asymmetries affect the vibratory amplitude and pattern, potentially creating irregular mucosal wave propagation and increased perturbation.

Tension Asymmetries

Even with symmetric neural input, muscle force production may differ between sides due to:

Muscle fiber composition: Differences in the proportion of fast vs. slow-twitch fibers
Fatigability differences: One side may fatigue more rapidly during sustained use
Postural factors: Habitual head/neck posture may create chronic tension differences

Mucosal Wave Irregularities

The mucosal wave—the wavelike motion of the vocal fold epithelium and superficial lamina propria during vibration—can exhibit various irregularities:

Traveling wave disruptions: The mucosal wave normally travels from inferior to superior along the medial surface. Stiff regions (scarring, sulcus) can interrupt this propagation, creating irregular vibratory patterns.

Left-right phase variations: The mucosal waves on left and right folds should be nearly 180° out of phase during normal vibration. Asymmetries shift this phase relationship, creating irregular collision patterns and acoustic perturbations.

Vertical phase differences: Within each fold, the inferior and superior margins should exhibit specific phase relationships (body-cover theory). Disruptions to this vertical phase coupling alter the waveform of acoustic output.

Tissue Viscoelasticity

Vocal fold tissue exhibits both elastic (spring-like) and viscous (dashpot-like) properties. The viscoelastic nature means that stress-strain relationships depend on the rate of deformation:

Rate-dependent stiffness: Faster vibration frequencies encounter effectively stiffer tissue
Energy dissipation: Viscous components dissipate energy as heat, creating damping that affects vibratory amplitude
Hysteresis: Loading and unloading stress-strain curves differ, creating cycle-to-cycle variations

These viscoelastic properties vary with:

Hydration status: Dehydrated tissue is stiffer and less viscous
Age: Aging tissue generally becomes stiffer with reduced viscosity
Pathology: Edema increases mass and viscosity; fibrosis increases stiffness

Collision Forces

Each glottal cycle involves collision between the vocal folds at the midline. The forces generated during collision:

Vary with amplitude: Larger vibratory amplitude creates stronger collisions
Depend on phase: If left and right folds are out of phase, collision forces vary across cycles
Affect subsequent cycles: High collision forces may alter tissue properties slightly (strain-stiffening), affecting the next cycle

The nonlinear relationship between collision force and vocal fold behavior contributes to cycle-to-cycle perturbations.

Aerodynamic Sources

The airflow through and around the glottis creates additional sources of variation.

Turbulent Flow

At the constrictions created by the vocal folds and supraglottal structures, airflow becomes turbulent:

Reynolds number: The transition from laminar to turbulent flow occurs when the Reynolds number exceeds approximately 2,000-3,000. During phonation, glottal airflow typically operates well within the turbulent regime.

Turbulence characteristics: Turbulent flow exhibits chaotic, unpredictable velocity fluctuations on multiple time scales. While the mean flow may be steady, instantaneous pressures and velocities vary substantially.

Acoustic consequences: Turbulence generates broadband noise (aspiration noise) and creates fluctuating pressures acting on the vocal folds, potentially contributing to amplitude and frequency perturbations.

Vortex Shedding

As air flows past the vocal folds, vortices (rotating regions of fluid) form and shed periodically:

Karman vortex street: Alternate shedding of vortices from upper and lower edges of the glottis creates oscillating forces on the vocal fold surfaces

Frequency dependence: Vortex shedding frequency depends on flow velocity and geometry; when it coincides with vocal fold vibratory frequency, resonant interactions may occur

Symmetry breaking: Asymmetric vortex patterns can drive asymmetric vocal fold motion, increasing perturbation

Subglottal Pressure Variations

The subglottal pressure—the air pressure in the trachea below the vocal folds—provides the driving force for phonation. Variations in this pressure translate directly to variations in vocal output:

Respiratory Variations

Tidal volume fluctuations: Natural variability in breath depth affects available air and pressure
Respiratory rhythm: Respiratory cycle creates slow modulations in subglottal pressure
Muscle fatigue: Respiratory muscles fatigue during extended phonation, reducing pressure stability

Cardiovascular Influences

Pulse pressure: Heartbeat creates pressure pulses in thoracic vasculature that may influence subglottal pressure
Cardiac cycle: Some research suggests very small F0 perturbations time-locked to heartbeat, though clinical significance is unclear

Glottal Area Variations

The instantaneous glottal area (opening between vocal folds) affects the transglottal pressure drop and flow rate:

Bernoulli pressure: The constriction at the glottis creates negative pressure (Bernoulli effect) that pulls the folds toward midline. Small variations in glottal shape create pressure variations.

Flow resistance: The flow resistance is highly sensitive to minimum glottal area. Small irregularities in vocal fold edge shape create cycle-to-cycle resistance variations.

Acoustic-Aerodynamic Interactions

The acoustic wave in the vocal tract interacts with the aerodynamic flow, creating additional complexity:

Source-tract interaction: Acoustic impedance of the vocal tract affects glottal airflow and vocal fold vibration (discussed in Source-Filter Theory, Chapter 6). Variations in vocal tract shape create variations in this interaction.

Formant-harmonic interaction: When a harmonic of the source frequency aligns closely with a formant frequency, energy transfer between source and filter is maximized. Small F0 perturbations alter this alignment, creating amplitude variations.

Normal vs. Pathological Levels

Distinguishing normal from pathological perturbation requires understanding typical ranges:

Normal Perturbation Ranges

In healthy adult voices during modal phonation at comfortable pitch and loudness:

Jitter percent: 0.2-1.0%, typically <0.6%
Shimmer percent: 1.0-3.0%, typically <2.5%
HNR: >13 dB, often 15-20 dB
RAP: <0.68%
PPQ: <0.84%

These values represent general guidelines; individual variation exists based on age, gender, pitch level, and loudness.

Pathological Indicators

Perturbation measures suggesting potential pathology:

Jitter percent >1.04%: Suggests potential vocal fold pathology or neural control problems
Shimmer percent >3.81%: May indicate incomplete glottal closure, mass lesions, or neuromuscular dysfunction
HNR <13 dB: Indicates increased noise component, suggesting breathiness or roughness
Dramatically elevated values: Jitter >3%, shimmer >10%, HNR <5 dB indicate severe pathology

Context Dependence

Interpretation requires considering:

Age: Elderly speakers show higher normal jitter/shimmer
Gender: Minimal differences in normalized measures between men and women
Vocal task: Soft phonation, high pitch, and vowel type affect measures
Recording quality: Poor signal-to-noise ratio artificially inflates perturbation

Age and Gender Effects

Age Effects

Perturbation measures change across the lifespan:

Children: Higher jitter and shimmer than young adults due to developing neuromuscular control and smaller, more compliant vocal folds

Young adults: Most stable period with lowest perturbation values

Middle age: Gradual increases begin around age 50-60

Elderly: Significant increases in jitter and shimmer, with men showing greater increases than women

These age-related changes result from tissue changes (reduced elasticity, atrophy), neural changes (motor unit loss), and respiratory changes (reduced vital capacity).

Gender Effects

When perturbation measures are properly normalized (percent rather than absolute), gender differences are minimal in healthy voices:

Jitter percent: No significant difference
Shimmer percent: No significant difference
Absolute jitter: Higher in men due to longer periods (lower F0)
HNR: Slightly higher in women, possibly due to tighter glottal closure

Observed gender differences in clinical populations may reflect different patterns of voice use, occupational vocal loading, or referral biases rather than inherent physiological differences.

Key Takeaways

✅ Vocal perturbations arise from neurological (motor variability, tremor), biomechanical (asymmetries, tissue properties), and aerodynamic (turbulence, pressure variations) sources
✅ Normal motor control exhibits inherent stochastic variability that creates small cycle-to-cycle perturbations in all voices
✅ Pathological tremors (essential, Parkinsonian) create regular oscillations distinct from normal perturbations
✅ Vocal fold asymmetries in mass, stiffness, or tension disrupt symmetric vibration and increase perturbation measures
✅ Turbulent airflow and vortex shedding contribute to both perturbations and noise in the voice signal
✅ Normal jitter values are <1%, shimmer <3%, HNR >13 dB; values exceeding these suggest potential pathology
✅ Age increases perturbation due to tissue changes, neural decline, and respiratory effects; gender effects are minimal when properly normalized
✅ Interpretation of perturbation measures requires considering age, vocal task, and recording conditions

Sources of Fluctuation and Perturbation

Sources of Fluctuation and Perturbation

Neurological Sources

Motor Unit Recruitment Variability

Central Pattern Generator Noise

Pathological Tremor

Essential Tremor

Parkinsonian Tremor

Spasmodic Dysphonia

Age-Related Neurological Changes

Biomechanical Sources

Vocal Fold Asymmetries

Mass Asymmetries

Stiffness Asymmetries

Tension Asymmetries

Mucosal Wave Irregularities

Tissue Viscoelasticity

Collision Forces

Aerodynamic Sources

Turbulent Flow

Vortex Shedding

Subglottal Pressure Variations

Respiratory Variations

Cardiovascular Influences

Glottal Area Variations

Acoustic-Aerodynamic Interactions

Normal vs. Pathological Levels

Normal Perturbation Ranges

Pathological Indicators

Context Dependence

Age and Gender Effects

Age Effects

Gender Effects

Key Takeaways

Related Topics

Further Reading