SHLD740

1
SHLD740

• Voice Resonance Disorders

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Anatomy Physiology of Phonation Resonance

• A. Vocal Tract Anatomy (A Review)

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Vocal Tract Anatomy

• At the base of the tongue, the pharynx opens into
  the larynx.
• The superior passageway of the pharynx and larynx
  are shared.
• Superior support for the larynx is the hyoid
  bone.
• Its inferior point of attachment is the trachea.

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Laryngeal Model Review

• Cartilages, ligaments, and muscles of the larynx
• Epiglottis
• Thyroepiglottic ligament
• Hyoepiglottic ligament
• Thyroid cartilage
• Laminae
• Superior thyroid notch
• Laryngeal (thyroid) prominence
• Superior cornu
• thyrohyoid ligament
• Inferior cornu
• ceratocricoid ligament

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Laryngeal Model Review

• Cricoid cartilage
• Lateral articular facets
• Posterior superior articular facets
• cricothyroid ligament
• cricotracheal ligament
• Arytenoid cartilages
• Muscular processes
• Vocal processes
• Corniculate cartilages
• Cuneiform cartilages

6
Laryngeal Model Review

• Extrinsic laryngeal muscles have one point of
  attachment external to the larynx.
• Most are responsible for either elevating or
  depressing the entire larynx, especially during
  swallow.
• In trained singers, the extrinsic muscles may
  help produce notes outside the normal singing
  range.

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Suprahyoid muscles

• Suprahyoid muscles all are laryngeal elevators
• digastric
• geniohyoid
• mylohyoid
• stylohyoid

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Infrahyoid muscles

• Infrahyoid muscles all are laryngeal depressors
• sternohyoid
• omohyoid
• Sternothyroid
• thyrohyoid

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Laryngeal Model Review

• Intrinsic laryngeal muscles have both points of
  attachment within the larynx.
• They are categorized according to their effects
  on the shape of the glottis (abduction vs.
  adduction) and on the vibratory behavior of the
  vocal folds (tension vs. relaxation).

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Laryngeal Model Review

• Glottal Abductor
• Paired posterior cricoarytenoid muscle

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Laryngeal Model Review

• Glottal Adductor
• Paired interarytenoid (arytenoideus) muscle
• Paired lateral cricoarytenoid muscle

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Laryngeal Model Review

• Glottal Tensor
• Paired cricothyroid muscle

13
Laryngeal Model Review

• Glottal Relaxer
• Paired thyroarytenoid muscles

14
Vocal Tract Anatomy

• The opening into the larynx is known as the
  laryngeal vestibule, and it is bounded by the
  epiglottis, aryepiglottic folds, and arytenoid
  cartilages.
• The laryngeal vestibule ends at the superior
  surface of the false vocal folds.
• The aryepiglottic folds are attached to the
  lateral margins of the epiglottis, forming the
  lateral walls of the laryngeal vestibule.

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Vocal Tract Anatomy

• The entire laryngeal structure is lined with
  mucous membrane.
• The underlining of the glottis, down to the
  trachea has a very rich, wet mucous membrane that
  constantly lubricates the vocal folds.
• The blood supply is from a branch of the common
  carotid artery.

16
Vocal Tract Anatomy

• Nerve supply to the larynx is provided by the
  Vagus (Xth) cranial nerve.
• Cell bodies of the Xth Vagus cranial nerve
  controlling laryngeal muscle movement are located
  in the nucleus ambiguus in the brainstem.

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Vocal Tract Anatomy

• The internal branch of the superior laryngeal
  nerve, which is derived from the X Vagus Nerve,
  provides sensory innervation of the supraglottic
  and glottic area.
• It also innervates the cricothyroid muscle.

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Vocal Tract Anatomy

• The recurrent laryngeal nerve, also derived from
  the Xth Vagus cranial nerve, provides sensory
  innervation to the subglottic mucosa.
• On the ipsilateral side, it innervates all of
  the intrinsic laryngeal muscles except the
  cricothyroid muscle.

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Vocal Tract Anatomy

• Only the interarytenoid muscles receive bilateral
  innervation from the recurrent laryngeal nerves.

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Vocal Tract Physiology

• The normal speaking voice is produced through the
  physiological interdependence of three component
  processes
• Respiration
• Phonation
• Resonance
• We will consider each component as it relates to
  the production of voice.

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Respiration

• Respiration is the energy source for voice
  production.
• Because the respiratory system moves air into
  (inspiration/inhalation) and out of the lungs
  (expiration/exhalation), it involves issues of
  aerodynamics.
• During normal respiration, inhalation takes up
  approximately 40 of the respiratory cycle, while
  expiration takes up about 60.

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Respiration

• Since we phonate only on expiration, without some
  specialized control we could only phonate for
  about 6 seconds before having to be silent while
  inhaling for the next 4 seconds
• Phonation for speech production requires major
  modification to the respiratory resting breathing
  cycle so that several words can be uttered before
  the next inspiration.
• When you breathe for speech, you actually spend
  only 10 of the respiratory cycle on inspiration,
  and about 90 on expiration.

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Respiration

• Therefore, you have to alter how long you spend
  in each stage of the process.
• To maintain phonation, you must maintain a
  reasonably constant subglottic air pressure,
  letting air out slowing using respiratory
  musculature to restrain airflow.
• This is called checking action--the impedance in
  the flow of air out of your inflated lungs by
  means of the muscles of inspiration.

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Respiration

• The checking action permits you to maintain the
  constant flow of air through the vocal tract
  while permitting accurate control the pressure
  beneath vocal folds.
• When you exhaust the expiratory airflow needed
  for speech and arrive at the resting pressure,
  you can still say more without inhaling by
  enlisting the muscles of expiration to push
  beyond that resting lung volume to the expiratory
  reserve volume.
• Use of checking action and expiratory reserve
  volume allows you to keep speech fluid and
  controlled.

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Phonation

• By definition, phonation is the physiological
  process whereby the energy of moving air in the
  vocal tract is transformed into acoustic energy
  within the larynx by means of vocal fold
  vibration
• The air from the lungs is of a relatively steady
  state or unmodulated stream as it passes into the
  trachea and finally into the larynx
• The larynx is the principal structure for
  producing a vibrating air stream and the vocal
  folds constitute the vibrating elements.

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Phonation

• The vibrating vocal folds convert the unmodulated
  breath stream from the lungs into a rapid series
  of puffs.
• Rapid opening and closing of the vocal folds
  periodically interrupt the air stream to produce
  a vocal or glottal tone within the pharyngeal,
  oral, and/or nasal cavities
• To initiate voicing, we use muscular action to
  bring the vocal folds close enough together to
  create a force of turbulence in the expiratory
  airstream to cause vocal fold vibration.

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Phonation

• Each time the vocal folds are blown apart by the
  elevated subglottic pressure, a burst of
  pressurized air is released into the vocal tract.
• The effect of these transient bursts of energy is
  to excite the dormant column of air above the
  larynx to vibrate for a short period of time.
• A rapid succession of energy bursts serves to
  keep the air column vibrating.

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Phonation

• These short-duration vibrations generated within
  the supraglottic air column constitute the
  glottal or laryngeal tone.
• Modifications to the configurations of the
  cavities in the vocal tract change the acoustical
  properties of these cavities transforming the
  relatively undifferentiated glottal tone into
  meaningful speech sounds.
• The vibration of the vocal folds is not the
  product of repeated adduction and abduction of
  the vocal folds.

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Phonation

• Instead, the vibration of the vocal folds is
  achieved by placing and holding the vocal folds
  in the airstream in a manner that permits their
  physical qualities to interact with the airflow.
• As the vocal folds are brought together,
  subglottic turbulence increases and vibration is
  initiated and sustained as long as the folds are
  approximate and there is sufficient subglottic
  air pressure.

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Phonation

• A laryngeal posture of tonic (sustained tensing)
  contraction of musculature is needed for
  sustained phonation.
• To terminate phonation, muscular action is
  initiated to abduct the vocal folds and pull them
  out of the airstream far enough to reduce the
  turbulence and to stop the vocal folds vibrating.

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Phonation Lets see how this looks!

• 1 Column of air pressure moves upward towards
  vocal folds in "closed" position
• 2, 3 Column of air pressure opens bottom of
  vibrating layers of vocal folds body of vocal
  folds stays in place

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Phonation Lets see how this looks!

• 4, 5 Column of air pressure continues to move
  upward, now towards the top of vocal folds, and
  opens the top
• 610 The low pressure created behind the
  fast-moving air column produces a Bernoulli
  effect which causes the bottom to close, followed
  by the top

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Phonation Lets see how this looks!

• 10 Closure of the vocal folds cuts off the air
  column and releases a pulse of air
• The escaping "puffs of air" are converted to
  sound which is then transformed into voice by
  vocal tract resonators.

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Resonance

• As sound waves generated by the vocal folds
  travel through the supraglottic air column, into
  the pharynx, oral and nasal cavities, and across
  more rigid structures such as the velum, palate,
  tongue, and teeth, the excitation of air
  molecules creates a phenomenon called resonance.
• Resonance is the reinforcing or prolongation of
  sound by reflection of waveforms off another
  structure

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Resonance

• Vocal resonance is the modification of the
  laryngeal tone by passage through chambers of the
  pharynx and head so as to alter its quality.
• The vocal tract has four or five prominent
  resonances called formants, and the shape and
  length of the vocal tract determine their
  frequencies.
• Well explore vocal resonance more thoroughly
  when we discuss the properties of sound waves.

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Properties of Sound Waves

• Sound originates as a disturbance of the
  positions of particles within a substance.
• The initial disturbance also moves all the
  neighboring particles, and these in turn move
  their neighbors.
• This passing along of pressure disturbances by
  the particles in a medium is called sound
  propagation.

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Properties of Sound Waves

• The disturbance propagates as a momentary change
  in position, in front of and in back, of each
  particle.
• The pressure disturbance is positive in front of
  each particleabove the average density of the
  mediumresulting in condensation in the mediuman
  increase in air density.

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Properties of Sound Waves

• The pressure disturbance is negative behind each
  particle, resulting in rarefaction, a decrease in
  air density.
• Sound waves travel in a longitudinal waveform
  composed of a succession of compressions and
  rarefactions of the mediums molecules.
• Each cycle of compression and rarefaction is an
  oscillation.

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Properties of Sound Waves

• The characteristics of a sound wave can be
  demonstrated with a tuning fork.
• When struck, the prong of a tuning fork will pass
  back and forth past the midline at a set speed.
• One complete journey from side to side is called
  a cycle.

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Properties of Sound Waves

• As the prong travels from side to side, the air
  particles are compressed in the direction in
  which the prong is traveling, leaving a drop in
  pressure behind.
• The vacuum immediately fills with air particles.
  
• This disturbance generates the periodic air waves
  which travel as pulses of compression and
  rarefaction radiating in all directions from the
  point of origin.

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Properties of Sound Waves

• A single regular waveform as produced by a tuning
  fork has simple harmonic motion.
• That is, it is the simplest smoothly connected
  back and forth movement possible.
• It vibrates at just one frequency, it natural or
  resonant frequency.

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Properties of Sound Waves

• A plot in time of harmonic motion is sinuous, so
  the wave is called a sine wave.
• Any sound wave can be analyzed into its sine wave
  components period, frequency, and amplitude.

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Properties of Sound Waves

• Period is defined as the time interval between
  repeating eventsthe time for one complete cycle.

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Properties of Sound Waves

• Frequency is dependent upon the number of
  oscillationscyclesin a unit of time.
• The measurement Hertz (Hz) is the number of
  cycles per second.

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Properties of Sound Waves

• The distance or extent of the oscillation from
  the original resting position is called the
  amplitude.

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Properties of Sound Waves

• When a sound is received by the ear, the extent
  of air particle motion determines the loudness of
  the sound heard.
• The rate of air particle disturbance determines
  the pitch of the sound heard and
• The form of the motion determines the timbre or
  quality of the sound heard.
• Sinusoidal waves generate pure tonessounds with
  frequency, amplitude, and phase.

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Properties of Sound Waves

• The air motions of speech sounds are complex in
  form because they are not just propagated in open
  spaces.
• Instead, they are propagated in a tube-like space
  in which there is a column of air pressure
  disturbance passing up the vocal tract, as well
  as air pressure disturbances passing from the
  center of the tube to the wall and then being
  reflected back and forth.
• In other words, theres a sine wave rising out of
  the vocal tract, falling back into the vocal
  tract, and bouncing off the walls of the vocal
  tract.

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Properties of Sound Waves

• Resonance in a tube is the constructive
  interference of waves experiencing multiple
  reflections.
• It is the essence of vocal tract acoustics.
• The human vocal tract resonates at certain
  special frequencies produced by the sound source.
• These frequencies depend on the size and shape of
  the tube, the size of its orifices, and the
  tension, density, and mass of its walls.
• If the surfaces are tense, the oscillations will
  be more frequent than those reflected from
  relaxed surfaces which absorb or damp sound
  waves.

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Normal Vocal Characteristics and Variants

• In nature, vibrators, such as the vocal folds, do
  not produce pure tones but create many
  sympathetic vibrations as well which are
  dependent upon the mode of vibration generated by
  the vibrator.
• The frequencies of the spectrum components depend
  on the pulsing rate, which determines the
  fundamental frequency (F0) and the frequencies of
  the other spectrum components.
• For example, if the fundamental frequency is 100
  Hz, then the harmonics multiplied by 2, 3, 4,
  etc., will be 200 Hz, 300 Hz, 400Hz.
• It is the different combination of harmonics
  which provide the individual quality of timbre of
  a sound.

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Normal Vocal Characteristics and Variants

• Fundamental frequency is dependent upon the size,
  shape, elasticity, and mass of the vibrator.
• As long as these factors remain constant, the
  number of oscillations per second never varies
  however strong the vibrator is set in motion
• The vibrating force, whether it is a blast of air
  or a blow, only changes the volume of the sound.

51
Vocal Fundamental Frequency

• The fundamental frequency of the voice is
  determined by vocal fold length, tension, and
  mass in combination with subglottic pressure.
• Vocal fold vibration increase as vocal fold
  tension is increased and mass is reduced.
• Vocal fold vibration decreases when the vocal
  folds are relaxed and the mass is bulky.

52
Vocal Fundamental Frequency

• The relative differences between men and women in
  vocal fold length (approximately 17-20 mm for men
  and 12-17 mm for women) and vocal fold thickness
  appear to be the primary determinants of
  differences in voice pitch.
• The typical fundamental frequency for men is
  around 125 Hz for women, around 225 Hz.
• When individuals phonate at increasingly higher
  pitch levels, they must lengthen the vocal folds
  to decrease their relative and mass and increase
  their tension.

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Vocal Fundamental Frequency

• Increases of pitch, therefore, appear to be
  related to lengthening of the vocal folds, with a
  corresponding decrease of tissue mass and an
  increase of fold tissue elasticity.
• Lowering the pitch is directly related to
  shortening the folds.
• There is a corresponding increase in tissue mass
  and a decrease of fold tissue elasticity.

54
Vocal Fundamental Frequency

• Maximum phonational range refers to the range of
  frequencies, from lowest to highest, that an
  individual can produce when the intensity of the
  tone is not being controlled.

55
Pitch Perturbation Jitter

• When consecutive vibratory cycles of the vocal
  folds vary in frequency so that there is pitch
  variation in a short-term speech signal, the
  phenomenon is referred to as pitch perturbation
  or jitter.
• This term is applied to frequency variability due
  to involuntary changes in the fundamental
  frequency as the result of instability of the
  vocal folds during vibration.
• Frequency perturbation reflects the biomechanical
  characteristics of the vocal folds, as well as
  variations of neuromuscular control.

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Pitch Perturbation Jitter

• Normal speakers have a small amount of jitter
  which may vary due to age, physical condition,
  and in some cases, gender.
• Although jitter occurs in the normal voice, there
  is a marked increase in dysphonic patients.

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Vocal Intensity

• Vocal loudness varies according to respiratory
  airflow and subglottic air pressure which affects
  the size of excursions executed by the vocal
  folds.
• Average intensity of conversational speech, three
  feet from the speaker is 60 dB.
• During quiet voiced speech, the average level is
  about 35-40 dB.
• During shouting, vocal intensity rises to about
  75 dB.
• Increasing vocal loudness can only be achieved by
  increasing vocal fold resistance to increased air
  flow.
• Normal speakers can usually produce maximum
  outputs in excess of 110 dB.

58
Vocal Intensity

• Maximum phonation duration (MPD) is the maximum
  time a person can sustain a tone in one
  continuous expiratory breath.
• It is often thought of as a measure of phonatory
  control and respiratory support.
• It is somewhat dependent upon gender, age, and
  the frequency at which phonation is produced.

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Amplitude Perturbation Shimmer

• During sustained vibration, the vocal folds will
  exhibit slight variation of amplitude from one
  cycle to the next.
• This is called amplitude perturbation or shimmer.
• Normal speakers will present a small amount of
  shimmer, depending upon the vowel used and the
  gender of the person.
• Shimmer has been found to be important in the
  perception of hoarseness.

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Vocal Registers

• The shape, length, density, and mass of the vocal
  folds alter constantly in production of notes of
  difference frequency.
• In other words, the vibratory pattern of the
  vocal folds and the acoustic parameters being
  produced can be changed over some ranges of pitch
  and loudness.
• Three perceptually distinct regions of vocal
  quality have been identified vocal fry (pulse),
  modal, and falsetto (loft).

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Vocal Registers

• Vocal fry is characterized by a long closed phase
  in the vibratory cycle.
• It is a fairly common occurrence in every day
  speech and is frequent in vocal strain and abuse.
• The frequency involved is within the 20-60 Hz
  range.

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Vocal Registers

• The modal register encompasses the range of
  frequency employed most frequently in normal
  phonation.
• The membranous portions of the vocal folds
  approximate to make complete contact in each
  closed phase to interrupt the pulmonary air flow
  briefly.
• This results in a train of glottal pulses which
  occur at about 100 Hz in adult males and in the
  region of 200 Hz in female adults and children.

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Vocal Registers

• The falsetto or loft register occurs in human
  vocal activities such as singing (i.e., the Beach
  Boys, the traditional Irish tenor), war cries,
  yodeling, giggling, and laughing.
• Falsetto is achieved by closure of the posterior
  two-thirds of the vocal folds with only the
  anterior one-third knife-thin edges vibrating.

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Multicultural Note

• Current knowledge about acoustical and perceptual
  parameters related to the normal voice of
  minority populations in the US is almost
  nonexistent.
• The few existing studies focus on the voice
  characteristics of African-Americans.
• For all ages of African-Americans studied, the
  fundamental frequency is lower than that of
  Whites.
• The lower modal frequency may be one factor
  leading to racial identification through voice.

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Multicultural Note

• Hudson and Holbrook (1982) also found that
  African-Americans tend to have greater
  flexibility above their mean modal frequencies,
  while Whites are just the opposite and show a
  greater range below their mean modal frequency.
• Intonational features of speakers of African
  American English include
• use of a wide range of pitches that frequently
  shift to falsetto register when points of
  emphasis are being made
• frequent use of level and rising final pitch
  contours on all sentence types

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Multicultural Note

• use of falling pitch contours when asking yes/no
  questions (demanding format) in formal and
  threatening contexts and
• The use of non-final intonation contours to
  express conditionality in a sentence.

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Vocal Attacks

• The hard or glottal attack occurs at the onset of
  phonation when the vocal fold edges make abrupt
  contact for an instant so that the breath stream
  is interrupted and then released explosively.
• This technique is used normally for emphasis in
  words with an initial vowel.
• Its use is also apparent in moods of fear, anger,
  and impatience.
• It is also the normal mode of articulation in
  German for vowels at the beginning of words.
• When its use is linguistic, it is innocuous.

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Vocal Attacks

• When it is a physiological symptom of laryngeal
  tension and incorrect methods of voice
  production, it can be harmful and result in
  mucosal changes of the vocal folds.
• For a soft attack the vocal folds do not fully
  adduct at the onset of phonation and some
  unvibrated air passes through the glottal chink.
• The breathy quality will be apparent at onset and
  volume will be radically reduced.
• Such phonation can be associated with emotions of
  joy and pleasure.

69
Vocal Attacks

• There are two types of whisper that result from
  different positions of the vocal folds.
• In a quiet whisper, the folds are slightly
  separated along the anterior two-thirds and a
  triangular aperture remains posteriorly as the
  arytenoids do not adduct
• In a strong whisper, the folds are adducted
  firmly along the anterior two-thirds and air is
  forced through the posterior triangular aperture
  with considerable friction.

70
Histology of the Vocal Folds

• Hirano (1977) has shown the vocal fold to be
  composed of five histologically distinct layers
  each with different mechanical properties.
• This multi-layered structure also has relevance
  in the development of benign vocal lesions.

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Histology of the Vocal Folds

• The true vocal folds have an epithelial lining
  that is composed of respiratory epithelium
  (pseudostratified squamous) on the superior and
  inferior aspects of the fold and nonkeratinizing
  (not structurally hard like nails or horns)
  squamous epithelium on the medial contact
  surface.

72
Histology of the Vocal Folds

• This outermost layer encapsulates softer,
  fluid-like tissue, somewhat like a balloon filled
  with water.
• Its purpose is to maintain the shape of the vocal
  folds, serve as an initial boundary of protection
  for the underlying tissue, and help regulate
  vocal fold hydration.

73
Histology of the Vocal Folds

• The subepithelial tissues are composed of a
  three-layered lamina propria based on the amount
  of elastin and collagen fibers.
• It is found between the epithelium and the
  muscle.
• It can conveniently be divided into three layers
  superficial, intermediate, and deep.

74
Histology of the Vocal Folds

• The superficial layer (CS) is composed of mostly
  amorphous ground substance and contains a scant
  amount of elastin with few fibroblasts.
• Superficial lamina propria is composed mostly of
  loose fibrous and elastic components in a matrix.
  

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Superficial Lamina Propria (CS)

• This layer, termed Reinkes space, adds a pliant
  cushion with its mechanical properties consistent
  with a "mass of soft gelatin."
• Reinkes space and the epithelial covering are
  responsible for the vocal fold vibration.

76
Intermedia Lamina Propria (CI)

• The intermediate layer has increased elastin
  content.
• Intermediate lamina propria adds elastic
  mechanical integrity with the consistency of a
  "bundle of soft rubber bands."

77
Deep Lamina Propria (CP)

• The deep layer has less elastin but a greater
  amount of collagen fibers. 
• Deep lamina propria is likened to a "bundle of
  cotton thread" and contributes to the durability
  of the layer.

78
Vocal Fold Muscularis

• Deep to the lamina propria is the thyroarytenoid
  muscle. 
• The thyroarytenoid muscle has both passive and
  active mechanical properties.
• Passively it has the consistency of "stiff rubber
  bands."

79
Vocal Fold Muscularis

• Since it is a muscle, it also has active
  (contractile) properties that help control
  stiffness.
• There are gradual changes in stiffness from the
  very pliable superficial layer of the lamina
  propria to the rather stiff thyroarytenoid
  muscle.

80
Histology of the Vocal Folds
81
Histology of the Vocal Folds

• From a mechanical point of view, the five layers
  can be reclassified into three sections
• the cover, consisting of the epithelium and the
  superficial layer of the lamina propria
• the transition, consisting of the vocal ligament
  and
• the body, consisting of the thyroarytenoid muscle
  itself, controls the shape of the VF and the
  degree of tonicity.
• The mechanical properties of the outer four
  layers are controlled passively, while the
  mechanical properties of the body are regulated
  both passively and actively.

82
Histology of the Vocal Folds
83
Histology of the Vocal Folds

• The soft tissue layers of the vocal folds are
  thought to have been adapted for phonation in an
  evolutionary sense.
• The ligament is thicker at the end points, where
  larger mechanical stresses occur in the fibers.
• In the middle of the vocal folds, where most of
  the head on collision occurs, the mucosa is
  thicker.

84
Histology of the Vocal Folds

• This suggest that the superficial tissue may be
  well suited to withstand direct impact, perhaps
  providing a cushion (shock absorber) for the
  ligament.
• During phonation, the cover of the fold produces
  a wave-like motion.
• The undulating wave of movement travels from the
  lower surface to the upper surface of the VF in
  each cycle of vibration.
• Indeed, the mucous membrane cover vibrates more
  than the muscle during phonation.

85
Histology of the Vocal Folds

• For clear phonation, the margins of the vocal
  folds must be mobile.
• In patients with scarred or dry vocal folds, the
  mucosa loses its mobility, and phonation is
  breathy and elevated in pitch because the VFs are
  stiff not pliable.

86
Vocal Fold Opening/Closing

• Sounds begin in the larynx by means of rapid
  repeated opening and closing of the glottis, the
  chink between the vocal folds, in response to
  tracheal air pressure.
• Rapid variation of the narrow glottis aperture to
  produce a pulsing sound source is called
  phonation.
• Phonation occurs because of the vibration of the
  vocal fold cover.
• The vocal folds are held in different postures by
  the arytenoid cartilages.

87
Vocal Fold Opening/Closing

• During breathing, the arytenoid cartilages are
  held outward, keeping the glottis open at the
  back in a wide-open position.
• When phonation is about to begin, arytenoids move
  inward to bring the vocal folds together.
• The top sections of the folds are then brought to
  touch each other and the bottom sections separate
  by a small space opening toward the trachea.

88
Theories of Vocal Fold Function

• The late 1950s saw the elaboration of a number of
  theories of vocal fold function, some highly
  controversial.
• For our purposes, we will spend time on two that
  have been upheld through the 1990s.
• Van den Berg (1958) propounded the
  myoelastic-aerodynamic theory which has been the
  cornerstone of nearly all subsequent theoretical
  developments on phonation.
• This description of vocal fold vibration invokes
  the Bernoulli effect (negative pressure in the
  glottis), tissue elasticity, and vocal fold
  collision.

89
Theories of Vocal Fold Function

• It begins with an expiration of air, setting the
  approximating vocal folds in vibration as the
  airflow passes between the folds.
• The subglottal pressure builds ups when the folds
  are approximated.
• The volume of expired air leaving the lungs is
  impeded at the level of the glottis, resulting in
  an increased velocity of airflow through the
  glottis.
• Subglottal pressure increases and the vocal folds
  are blown apart, equalizing supraglottal and
  subglottal pressure.
• The result is the opening phase of one cycle of
  vibration.

90
Theories of Vocal Fold Function

• There are two forces working to bring about the
  closing phase of the vibratory cyclethe
  Bernoulli effect and the mass of the folds.
• Specifically, if the glottis is sufficiently
  narrow, and airflow sufficiently high, and the
  medial surface of the vocal folds soft enough to
  yield, then because of the Bernoulli effectflow
  conservation lawan increase in particle velocity
  must be accompanied by a decrease in pressurethe
  glottis collapses and draws the folds together.

91
Theories of Vocal Fold Function

• Moreover, lateral movement of the vocal folds in
  the opening phase will continue until elastic
  forces in the tissue retard the motion and
  ultimately reverse it.
• With the collapse of the glottis, subglottic air
  pressure again builds, causing the folds to begin
  to move laterally (outward) and the glottis to
  open.
• The myolelastic-aerodyanamic theory, however, is
  inadequate to explain the important features of
  self-sustained oscillation when there is a
  continual energy transfer from the airstream to
  the tissue.

92
Theories of Vocal Fold Function

• Titze (1994) expanded Van den Berg's (1958)
  classic aerodynamic-myoelastic theory to describe
  the vocal folds as a flow-induced oscillating
  system, sustained across time by aerodynamic
  force provided by pulmonary air stream.
• In other words, vocal fold vibration is achieved
  through means of the physical process of
  flow-induced oscillation-a consistent stream of
  air flowing past the tissues creating a repeated
  pattern of opening and closing.

93
Theory of Vocal Fold Function

• In Titze's flow-induced oscillation model,
  respiration is the driving force that sets the
  vocal folds into motion (oscillation), and the
  interchange between pressure and flow at three
  critical sites keeps the vocal folds vibrating.
• These three critical sites are the subglottic
  region, the intraglottic space, and the
  supraglottic region.
• In the subglottic region, the area directly
  beneath the vocal folds, the "leading edge" is
  set into motion by the pulmonary airflow.

94
Theory of Vocal Fold Function

• Directly between the paired vocal folds, the
  intraglottic space is the site where the exchange
  of airflow and pressure peaks influences the
  convergent and divergent shaping of the vocal
  fold's back and forth motion.
• Immediately above the vocal folds, where the air
  molecules in the vocal tract are alternately
  compressed or rarified in a delayed response to
  the alternate pressure and flow fluctuation
  modulated by the vibrating vocal folds, the
  excitation of the supraglottic air column
  facilitates a "top down" loading effect that
  helps sustain vocal fold oscillation.

95
Theory of Vocal Fold Function

• The sequence of vocal fold oscillation is as
  follows
• The subglottic to translaryngeal flow is positive
  and blows the folds open and they move from
  midline laterally.
• As the vocal folds open, intraglottal pressure is
  also positive, but drops as flow increases.
• As the vocal folds close, intraglottal pressure
  is negative, but rises again as flow is cut off
  by the closing glottis.

96
Theory of Vocal Fold Function

• This flow and ebb of intraglottal pressures keep
  the vocal folds oscillating.
• In the supraglottic vocal tract, molecules in the
  air column are pushed and released in response to
  puffs of pressure and continued flow of pulmonary
  air released by the oscillating vocal folds.
• This "top-down" driving force transfers energy
  from the air pressure to the vocal folds and
  assists in sustaining oscillation.

97
Theory of Vocal Fold Function

• Repeated observation of the vocal folds in slow
  motion using high-speed cinematography or
  videostroboscopy has demonstrated that uniform
  tissue displacement seldom if ever occurs.
• The vocal folds do not move like solid bar
  masses.
• Vibration is maintained for phonation because
  different parts of the vocal folds move relative
  to each other.
• Air pushes through the very small space between
  the vocal folds and, in so doing, makes the
  muscosal covering of the vocal folds vibrate.

98
Theory of Vocal Fold Function

• This occurs by means of a phenomenon known as the
  venturi effect.
• As air passes through a constriction (or
  venturi), it speeds up and creates a suction in
  its wake.
• This suction draws in the pliable mucosa from
  each vocal fold.
• The mucosal cover slides over the vocal fold body
  producing a wavea mucosal wave--that moves or
  travels across the superior surface of the vocal
  fold about two-thirds of the way to the lateral
  ledge of the fold.

99
Theory of Vocal Fold Function

• The wave generally dissipates before reaching the
  inner surface of the thyroid cartilage.
• The wave is nothing more than a manifestation of
  a loose and pliable vocal fold cover that permits
  ribbon-like movement at the medial surface.
• The regularity of the mucosal wave is essential
  to the production of good voice.
• Without the mucosal wave, vocal fold vibration is
  either impeded or requires considerably greater
  pulmonary effort.

100
Theory of Vocal Fold Function

• The upper and lower portions of the vocal folds
  do not move in phase.
• Indeed, Smith (1954, 1957) put forth the
  membrane-cushion (mucosa-muscle) theory to
  explain the vertical phase differences between
  the upper and the lower borders of the vocal fold
  margins.
• The bottom of the fold moves ahead of the topit
  leads the top in the direction of overall
  movementa lead-lag relation of motion.
• In a cycle of vocal fold vibration, the lower
  parts move apart before the upper parts, and the
  movement of the lower part causes the rest of the
  fold to move

101
Theory of Vocal Fold Function

• Think of the vocal folds as consisting of a pair
  of two-section pieces, one upper and the other
  lower.
• Each section is compliant and has mass.
• The subglottal air pressure from the lungs pushes
  the lower sections, forcing them apart.
• As the lower sections move father apart, they
  begin to pull each upper section along with the
  lower section to which is attached.
• Then the upper sections separate and the glottis
  becomes open.

102
Theory of Vocal Fold Function

• Air begins to flow through the opening, causing
  the pressure between the lower folds to decrease
  rapidly and so the lower sections begin to move
  inward.
• As they come close together, the Bernoulli effect
  increase the speed of closure.
• The lower sections quickly return to their closed
  positions.
• The upper sections follow the movement of the
  lower ones and return to the position of touching
  each other.

103
Theory of Vocal Fold Function

• Then the cycle begins again with the lower
  sections being forced apart with subglottal air
  pressure.
• At the same time that the lower and upper
  sections of the vocal folds are converging and
  diverging on a vertical plane, there is also
  longitudinal (anterior-posterior) variations in
  movement.
• The amplitude of vibration is maximum in the
  middle of the ribbon and decreases gradually
  toward the end points.
• When the center of the ribbon is not vibrating,
  the end points are vibrating.
• Thus, anterior and posterior portions move in
  opposite directions.