Title: SHLD740
1SHLD740
- Voice Resonance Disorders
2Anatomy Physiology of Phonation Resonance
- A. Vocal Tract Anatomy (A Review)
3Vocal 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.
4Laryngeal 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
5Laryngeal 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
6Laryngeal 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.
7Suprahyoid muscles
- Suprahyoid muscles all are laryngeal elevators
- digastric
- geniohyoid
- mylohyoid
- stylohyoid
8Infrahyoid muscles
- Infrahyoid muscles all are laryngeal depressors
- sternohyoid
- omohyoid
- Sternothyroid
- thyrohyoid
9Laryngeal 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).
10Laryngeal Model Review
- Glottal Abductor
- Paired posterior cricoarytenoid muscle
11Laryngeal Model Review
- Glottal Adductor
- Paired interarytenoid (arytenoideus) muscle
- Paired lateral cricoarytenoid muscle
12Laryngeal Model Review
- Glottal Tensor
- Paired cricothyroid muscle
13Laryngeal Model Review
- Glottal Relaxer
- Paired thyroarytenoid muscles
14Vocal 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.
15Vocal 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.
16Vocal 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.
17Vocal 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.
18Vocal 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.
19Vocal Tract Anatomy
- Only the interarytenoid muscles receive bilateral
innervation from the recurrent laryngeal nerves.
20Vocal 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.
21Respiration
- 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.
22Respiration
- 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.
23Respiration
- 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.
24Respiration
- 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.
25Phonation
- 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.
26Phonation
- 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.
27Phonation
- 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.
28Phonation
- 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.
29Phonation
- 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.
30Phonation
- 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.
31Phonation 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
32Phonation 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
33Phonation 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.
34Resonance
- 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
35Resonance
- 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.
36Properties 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.
37Properties 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.
38Properties 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.
39Properties 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.
40Properties 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.
41Properties 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.
42Properties 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.
43Properties of Sound Waves
- Period is defined as the time interval between
repeating eventsthe time for one complete cycle.
44Properties 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.
45Properties of Sound Waves
- The distance or extent of the oscillation from
the original resting position is called the
amplitude.
46Properties 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.
47Properties 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.
48Properties 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.
49Normal 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.
50Normal 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.
51Vocal 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.
52Vocal 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.
53Vocal 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.
54Vocal 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.
55Pitch 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.
56Pitch 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.
57Vocal 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.
58Vocal 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.
59Amplitude 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.
60Vocal 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).
61Vocal 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.
62Vocal 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.
63Vocal 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.
64Multicultural 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.
65Multicultural 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
66Multicultural 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.
67Vocal 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.
68Vocal 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.
69Vocal 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.
70Histology 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.
71Histology 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.
72Histology 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.
73Histology 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.
74Histology 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.
75Superficial 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.
76Intermedia 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."
77Deep 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.
78Vocal 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."
79Vocal 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.
80Histology of the Vocal Folds
81Histology 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.
82Histology of the Vocal Folds
83Histology 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.
84Histology 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.
85Histology 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.
86Vocal 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.
87Vocal 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.
88Theories 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.
89Theories 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.
90Theories 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.
91Theories 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.
92Theories 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.
93Theory 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.
94Theory 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.
95Theory 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.
96Theory 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.
97Theory 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.
98Theory 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.
99Theory 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.
100Theory 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
101Theory 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.
102Theory 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.
103Theory 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.